Differential Calculus Chapter 6 PDF

Chapter 6 Differential Calculus In this chapter, it is assumed that all linear spaces and flat spaces under consideration are finite-dimensional. 61 Differentiation of Processes Let E be a flat space with translation space V. A mapping p : I → E from some interval I ∈ Sub R to E will be calle...

Chapter 6 Differential Calculus In this chapter, it is assumed that all linear spaces and flat spaces under consideration are finite-dimensional. 61 Differentiation of Processes Let E be a flat space with translation space V. A mapping p : I → E from some interval I ∈ Sub R to E will be called a process. It is useful to think of the value p(t) ∈ E as describing the state of some physical system at time t. In special cases, the mapping p describes the motion of a particle and p(t) is the place of the particle at time t. The concept of differentiability for real-valued functions (see Sect.08) extends without difficulty to processes as follows: Definition 1: The process p : I → E is said to be differentiable at t ∈ I if the limit 1 ∂t p := lim (p(t + s) − p(t)) (61.1) s→0 s exists. Its value ∂t p ∈ V is then called the derivative of p at t. We say that p is differentiable if it is differentiable at all t ∈ I. In that case, the mapping ∂p : I → V defined by (∂p)(t) := ∂t p for all t ∈ I is called the derivative of p. Given n ∈ N× , we say that p is n times differentiable if ∂ n p : I → V can be defined by the recursion ∂ 1 p := ∂p, ∂ k+1 p := ∂(∂ k p) for all k ∈ (n − 1)]. (61.2) We say that p is of class Cn if it is n times differentiable and ∂ n p is continuous. We say that p is of class C∞ if it is of class C n for all n ∈ N×. 209 210 CHAPTER 6. DIFFERENTIAL CALCULUS As for a real-valued function, it is easily seen that a process p is contin- uous at t ∈ Dom p if it is differentiable at t. Hence p is continuous if it is differentiable, but it may also be continuous without being differentiable. In analogy to (08.34) and (08.35), we also use the notation p(k) := ∂ k p for all k ∈ (n − 1)] (61.3) when p is an n-times differentiable process, and we use p := p(1) = ∂p, p := p(2) = ∂ 2 p, p := p(3) = ∂ 3 p, (61.4) if meaningful. We use the term “process” also for a mapping p : I → D from some interval I into a subset D of the flat space E. In that case, we use poetic license and ascribe to p any of the properties defined above if p|E has that property. Also, we write ∂p instead of ∂(p|E ) if p is differentiable, etc. (If D is included in some flat F , then one can take the direction space of F rather than all of V as the codomain of ∂p. This ambiguity will usually not cause any difficulty.) The following facts are immediate consequences of Def.1, and Prop.5 of Sect.56 and Prop.6 of Sect.57. Proposition 1: The process p : I → E is differentiable at t ∈ I if and only if, for each λ in some basis of V ∗ , the function λ(p − p(t)) : I → R is differentiable at t. The process p is differentiable if and only if, for every flat function a ∈ Flf E, the function a ◦ p is differentiable. Proposition 2: Let E, E ′ be flat spaces and α : E → E ′ a flat mapping. If p : I → E is a process that is differentiable at t ∈ I, then α ◦ p : I → E ′ is also differentiable at t ∈ I and ∂t (α ◦ p) = (∇α)(∂t p). (61.5) If p is differentiable then (61.5) holds for all t ∈ I and we get ∂(α ◦ p) = (∇α)∂p. (61.6) Let p : I → E and q : I → E ′ be processes having the same domain I. Then (p, q) : I → E × E ′ , defined by value-wise pair formation, (see (04.13)) is another process. It is easily seen that p and q are both differentiable at t ∈ I if and only if (p, q) is differentiable at t. If this is the case we have ∂t (p, q) = (∂t q, ∂t q). (61.7) 61. DIFFERENTIATION OF PROCESSES 211 Both p and q are differentiable if and only if (p, q) is, and, in that case, ∂(p, q) = (∂p, ∂q). (61.8) Let p and q be processes having the same domain I and the same codomain E. Since the point-difference (x, y) 7→ x − y is a flat mapping from E × E into V whose gradient is the vector-difference (u, v) 7→ u − v from V × V into V we can apply Prop.2 to obtain Proposition 3: If p : I → E and q : I → E are both differentiable at t ∈ I, so is the value-wise difference p − q : I → V, and ∂t (p − q) = (∂t p) − (∂t q). (61.9) If p and q are both differentiable, then (61.9) holds for all t ∈ I and we get ∂(p − q) = ∂p − ∂q. (61.10) The following result generalizes the Difference-Quotient Theorem stated in Sect.08. Difference-Quotient Theorem: Let p : I → E be a process and let t1 , t2 ∈ I with t1 < t2. If p|[t1 ,t2 ] is continuous and if p is differentiable at each t ∈ ]t1 , t2 [ then p(t2 ) − p(t1 ) ∈ Clo Cxh{∂t p | t ∈ ]t1 , t2 [}. (61.11) t2 − t1 Proof: Let a ∈ Flf E be given. Then (a ◦ p) |[t1 ,t2 ] is continuous and, by Prop.1, a ◦ p is differentiable at each t ∈ ]t1 , t2 [. By the elementary Difference-Quotient Theorem (see Sect.08) we have (a ◦ p)(t2 ) − (a ◦ p)(t2 ) ∈ {∂t (a ◦ p) | t ∈ ]t1 , t2 [}. t2 − t1 Using (61.5) and (33.4), we obtain   p(t2 ) − p(t1 ) ∇a ∈ (∇a)> (S), (61.12) t2 − t1 where S := {∂t p | t ∈ ]t1 , t2 [}. Since (61.12) holds for all a ∈ Flf E we can conclude that b( p(t2t2)−p(t −t1 1) )≥0 holds for all those b ∈ Flf V that satisfy b> (S) ⊂ P. Using the Half-Space Intersection Theorem of Sect.54, we obtain the desired result (61.11). Notes 61 (1) See Note (8) to Sect.08 concerning notations such as ∂t p, ∂p, ∂ n p, p· , p(n) , etc. 212 CHAPTER 6. DIFFERENTIAL CALCULUS 62 Small and Confined Mappings Let V and V ′ be linear spaces of strictly positive dimension. Consider a mapping n from a neighborhood of zero in V to a neighborhood of zero in V ′. If n(0) = 0 and if n is continuous at 0, then we can say, intuitively, that n(v) approaches 0 in V ′ as v approaches 0 in V. We wish to make precise the idea that n is small near 0 ∈ V in the sense that n(v) approaches 0 ∈ V ′ faster than v approaches 0 ∈ V. Definition 1: We say that a mapping n from a neighborhood of 0 in V to a neighborhood of 0 in V ′ is small near 0 if n(0) = 0 and, for all norms ν and ν ′ on V and V ′ , respectively, we have ν ′ (n(u)) lim = 0. (62.1) u→0 ν(u) The set of all such small mappings will be denoted by Small(V, V ′ ). Proposition 1: Let n be a mapping from a neighborhood of 0 in V to a neighborhood of 0 in V ′. Then the following conditions are equivalent: (i) n ∈ Small(V, V ′ ). (ii) n(0) = 0 and the limit-relation (62.1) holds for some norm ν on V and some norm ν ′ on V ′. (iii) For every bounded subset S of V and every N ′ ∈ Nhd0 (V ′ ) there is a δ ∈ P× such that n(sv) ∈ sN ′ for all s ∈ ]−δ, δ[ (62.2) and all v ∈ S such that sv ∈ Dom n. Proof: (i) ⇒ (ii): This implication is trivial. (ii) ⇒ (iii): Assume that (ii) is valid. Let N ′ ∈ Nhd0 (V ′ ) and a bounded subset S of V be given. By Cor.1 to the Cell-Inclusion Theorem of Sect.52, we can choose b ∈ P× such that ν(v) ≤ b for all v ∈ S. (62.3) By Prop.3 of Sect.53 we can choose ε ∈ P× such that εbCe(ν ′ ) ⊂ N ′. (62.4) Applying Prop.4 of Sect.57 to the assumption (ii) we obtain δ ∈ P× such that, for all u ∈ Dom n, ν ′ (n(u)) < εν(u) if 0 < ν(u) < δb. (62.5) 62. SMALL AND CONFINED MAPPINGS 213 Now let v ∈ S be given. Then ν(sv) = |s|ν(v) ≤ |s|b < δb for all s ∈ ]−δ, δ[ such that sv ∈ Dom n. Therefore, by (62.5), we have ν ′ (n(sv)) < εν(sv) = ε|s|ν(v) ≤ |s|εb if sv 6= 0, and hence n(sv) ∈ sεbCe(ν ′ ) for all s ∈ ]−δ, δ[ such that sv ∈ Dom n. The desired conclusion (62.2) now follows from (62.4). (iii) ⇒ (i): Assume that (iii) is valid. Let a norm ν on V, a norm ν ′ on V ′ , and ε ∈ P× be given. We apply (iii) to the choices S := Ce(ν), N ′ := εCe(ν ′ ) and determine δ ∈ P× such that (62.2) holds. If we put s := 0 in (62.2) we obtain n(0) = 0. Now let u ∈ Dom n be given such that 0 < ν(u) < δ. If we apply (62.2) with the choices s := ν(u) and v := 1s u, we see that n(u) ∈ ν(u)εCe(ν ′ ), which yields ν ′ (n(u)) < ε. ν(u) The assertion follows by applying Prop.4 of Sect.57. The condition (iii) of Prop.1 states that 1 lim n(sv) = 0 (62.6) s→0 s for all v ∈ V and, roughly, that the limit is approached uniformly as v varies in an arbitrary bounded set. We also wish to make precise the intuitive idea that a mapping h from a neighborhood of 0 in V to a neighborhood of 0 in V ′ is confined near zero in the sense that h(v) approaches 0 ∈ V ′ not more slowly than v approaches 0 ∈ V. Definition 2: A mapping h from a neighborhood of 0 in V to a neigh- borhood of 0 in V ′ is said to be confined near 0 if for every norm ν on V and every norm ν ′ on V ′ there is N ∈ Nhd0 (V) and κ ∈ P× such that ν ′ (h(u)) ≤ κν(u) for all u ∈ N ∩ Dom h. (62.7) The set of all such confined mappings will be denoted by Conf(V, V ′ ). Proposition 2: Let h be a mapping from a neighborhood of 0 in V to a neighborhood of 0 in V ′. Then the following are equivalent: (i) h ∈ Conf(V, V ′ ). 214 CHAPTER 6. DIFFERENTIAL CALCULUS (ii) There exists a norm ν on V, a norm ν ′ on V ′ , a neighborhood N of 0 in V, and κ ∈ P× such that (62.7) holds. (iii) For every bounded subset S of V there is δ ∈ P× and a bounded subset S ′ of V ′ such that h(sv) ∈ sS ′ for all s ∈ ]−δ, δ[ (62.8) and all v ∈ S such that sv ∈ Dom h. Proof: (i) ⇒ (ii): This is trivial. (ii) ⇒ (iii): Assume that (ii) holds. Let a bounded subset S of V be given. By Cor.1 to the Cell-Inclusion Theorem of Sect.52, we can choose b ∈ P× such that ν(v) ≤ b for all v ∈ S. By Prop.3 of Sect.53, we can determine δ ∈ P× such that δbCe(ν) ⊂ N. Hence, by (62.7), we have for all u ∈ Dom h ν ′ (h(u)) ≤ κν(u) if ν(u) < δb. (62.9) Now let v ∈ S be given. Then ν(sv) = |s|ν(v) ≤ |s|b < δb for all s ∈ ]−δ, δ[ such that sv ∈ Dom h. Therefore, by (62.9) we have ν ′ (h(sv) ≤ κν(sv) = κ|s|ν(v) < |s|κb and hence h(sv) ∈ sκbCe(ν ′ ) for all s ∈ ]−δ, δ[ such that sv ∈ Dom h. If we put S ′ := κbCe(ν ′ ), we obtain the desired conclusion (62.8). (iii) ⇒ (i): Assume that (iii) is valid. Let a norm ν on V and a norm ν ′ on V ′ be given. We apply (iii) to the choice S := Ce(ν) and determine S ′ and δ ∈ P× such that (62.8) holds. Since S ′ is bounded, we can apply the Cell- Inclusion Theorem of Sect.52 and determine κ ∈ P× such that S ′ ⊂ κCe(ν ′ ). We put N := δCe(ν) ∩ Dom h, which belongs to Nhd0 (V). If we put s := 0 in (62.8) we obtain h(0) = 0, which shows that (62.7) holds for u := 0. Now let u ∈ N × be given, so that 0 < ν(u) < δ. If we apply (62.8) with the choices s := ν(u) and v := 1s u, we see that h(u) ∈ ν(u)S ′ ⊂ ν(u)κCe(ν ′ ), which yields the assertion (62.7). The following results are immediate consequences of the definition and of the properties on linear mappings discussed in Sect.52: 62. SMALL AND CONFINED MAPPINGS 215 (I) Value-wise sums and value-wise scalar multiples of mappings that are small [confined] near zero are again small [confined] near zero. (II) Every mapping that is small near zero is also confined near zero, i.e. Small(V, V ′ ) ⊂ Conf(V, V ′ ). (III) If h ∈ Conf(V, V ′ ), then h(0) = 0 and h is continuous at 0. (IV) Every linear mapping is confined near zero, i.e. Lin(V, V ′ ) ⊂ Conf(V, V ′ ). (V) The only linear mapping that is small near zero is the zero-mapping, i.e. Lin(V, V ′ ) ∩ Small(V, V ′ ) = {0}. Proposition 3: Let V, V ′ , V ′′ be linear spaces and let h ∈ Conf(V, V ) and k ∈ Conf(V ′ , V ′′ ) be such that Dom k = Cod h. Then ′ k ◦ h ∈ Conf(V, V ′′ ). Moreover, if one of k or h is small near zero so is k ◦ h. Proof: Let norms ν, ν ′ , ν ′′ on V, V ′ , V ′′ , respectively, be given. Since h and k are confined we can find κ, κ′ ∈ P× and N ∈ Nhd0 (V), N ′ ∈ Nhd0 (V ′ ) such that ν ′′ ((k ◦ h)(u) ≤ κ′ ν ′ (h(u)) ≤ κ′ κν(u) (62.10) for all u ∈ N ∩ Dom h such that h(u) ∈ N ′ ∩ Dom k, i.e. for all u ∈ N ∩ h< (N ′ ∩ Dom k). Since h is continuous at 0 ∈ V, we have h< (N ′ ∩ Dom k) ∈ Nhd0 (V) and hence N ∩ h< (N ′ ∩ Dom k) ∈ Nhd0 (V). Thus, (62.7) remains satisfied when we replace h, κ and N by k ◦ h, κ′ κ, and N ∩ h< (N ′ ∩ Dom k), respectively, which shows that k ◦ h ∈ Conf(V, V ′′ ). Assume now, that one of k and h, say h, is small. Let ε ∈ P× be given. Then we can choose N ∈ Nhd0 (V) such that ν ′ (h(u)) ≤ κν(u) holds for all u ∈ N ∩Dom h with κ := κε′. Therefore, (62.10) gives ν ′′ ((k◦h)(u)) ≤ εν(u) for all u ∈ N ∩ h< (N ′ ∩ Dom k). Since ε ∈ P× was arbitrary this proves that ν ′′ ((k ◦ h)(u)) lim = 0, u→0 ν(u) i.e. that k ◦ h is small near zero. Now let E and E ′ be flat spaces with translation spaces V and V ′ , respec- tively.  216 CHAPTER 6. DIFFERENTIAL CALCULUS Definition 3: Let x ∈ E be given. We say that a mapping σ from a neighborhood of x ∈ E to a neighborhood of 0 ∈ V ′ is small near x if the mapping v 7→ σ(x + v) from (Dom σ) − x to Cod σ is small near 0. The set of all such small mappings will be denoted by Smallx (E, V ′ ). We say that a mapping ϕ from a neighborhood of x ∈ E to a neighborhood of ϕ(x) ∈ E ′ is confined near x if the mapping v 7→ (ϕ(x + v) − ϕ(x)) from (Dom ϕ) − x to (Cod ϕ) − ϕ(x) is confined near zero. The following characterization is immediate. Proposition 4: The mapping ϕ is confined near x ∈ E if and only if for every norm ν on V and every norm ν ′ on V ′ there is N ∈ Nhdx (E) and κ ∈ P× such that ν ′ (ϕ(y) − ϕ(x)) ≤ κν(y − x) for all y ∈ N. (62.11) We now state a few facts that are direct consequences of the definitions, the results (I)–(V) stated above, and Prop.3: (VI) Value-wise sums and differences of mappings that are small [confined] near x are again small [confined] near x. Here, “sum” can mean either the sum of two vectors or sum of a point and a vector, while “differ- ence” can mean either the difference of two vectors or the difference of two points. (VII) Every σ ∈ Smallx (E, V ′ ) is confined near x. (VIII) If a mapping is confined near x it is continuous at x. (IX) A flat mapping α : E → E ′ is confined near every x ∈ E. (X) The only flat mapping β : E → V ′ that is small near some x ∈ E is the constant 0E→V ′. (XI) If ϕ is confined near x ∈ E and if ψ is a mapping with Dom ψ = Cod ϕ that is confined near ϕ(x) then ψ ◦ ϕ is confined near x. (XII) If σ ∈ Smallx (E, V ′ ) and h ∈ Conf(V ′ , V ′′ ) with Cod σ = Dom h, then h ◦ σ ∈ Smallx (E, V ′′ ). (XIII) If ϕ is confined near x ∈ E and if σ is a mapping with Dom σ = Cod ϕ that is small near ϕ(x) then σ ◦ ϕ ∈ Smallx (E, V ′′ ), where V ′′ is the linear space for which Cod σ ∈ Nhd0 (V ′′ ). (XIV) An adjustment of a mapping that is small [confined] near x is again small [confined] near x, provided only that the concept small [confined] near x remains meaningful after the adjustment. 63. GRADIENTS, CHAIN RULE 217 Notes 62 (1) In the conventional treatments, the norms ν and ν ′ in Defs.1 and 2 are assumed to be prescribed and fixed. The notation n = o(ν), and the phrase “n is small oh of ν”, are often used to express the assertion that n ∈ Small(V, V ′ ). The notation h = O(ν), and the phrase “h is big oh of ν”, are often used to express the assertion that h ∈ Conf(V, V ′ ). I am introducing the terms “small” and “confined” here for the first time because I believe that the conventional terminology is intolerably awkward and involves a misuse of the = sign. 63 Gradients, Chain Rule Let I be an open interval in R. One learns in elementary calculus that if a function f : I → R is differentiable at a point t ∈ I, then the graph of f has a tangent at (t, f (t)). This tangent is the graph of a flat function a ∈ Flf(R). Using poetic license, we refer to this function itself as the tangent to f at t ∈ I. In this sense, the tangent a is given by a(r) := f (t) + (∂t f )(r − t) for all r ∈ R. If we put σ := f − a|I , then σ(r) = f (r) − f (t) − (∂t f )(r − t) for all r ∈ I. We have lims→0 σ(t+s) s = 0, from which it follows that σ ∈ Smallt (R, R). One can use the existence of a tangent to define differentiability at t. Such a definition generalizes directly to mappings involving flat spaces. Let E, E ′ be flat spaces with translation spaces V, V ′ , respectively. We consider a mapping ϕ : D → D′ from an open subset D of E into an open subset D′ of E ′. Proposition 1: Given x ∈ D, there can be at most one flat mapping α : E → E ′ such that the value-wise difference ϕ − α|D : D → V ′ is small near x. Proof: If the flat mappings α1 , α2 both have this property, then the value-wise difference (α2 − α1 )|D = (ϕ − α1 |D ) − (ϕ − α2 |D ) is small near 218 CHAPTER 6. DIFFERENTIAL CALCULUS x ∈ E. Since α2 − α1 is flat, it follows from (X) and (XIV) of Sect.62 that α2 − α1 is the zero mapping and hence that α1 = α2. Definition 1: The mapping ϕ : D → D′ is said to be differentiable at x ∈ D if there is a flat mapping α : E → E ′ such that ϕ − α|D ∈ Smallx (E, V ′ ). (63.1) This (unique) flat mapping α is then called the tangent to ϕ at x. The gradient of ϕ at x is defined to be the gradient of α and is denoted by ∇x ϕ := ∇α. (63.2) We say that ϕ is differentiable if it is differentiable at all x ∈ D. If this is the case, the mapping ∇ϕ : D → Lin(V, V ′ ) (63.3) defined by (∇ϕ)(x) := ∇x ϕ for all x ∈ D (63.4) is called the gradient of ϕ. We say that ϕ is of class C1 if it is differentiable and if its gradient ∇ϕ is continuous. We say that ϕ is twice differentiable if it is differentiable and if its gradient ∇ϕ is also differentiable. The gradient of ∇ϕ is then called the second gradient of ϕ and is denoted by ∇(2) ϕ := ∇(∇ϕ) : D → Lin(V, Lin(V, V ′ )) ∼ = Lin2 (V 2 , V ′ ). (63.5) We say that ϕ is of class C2 if it is twice differentiable and if ∇(2) ϕ is continuous. If the subsets D and D′ are arbitrary, not necessarily open, and if x ∈ Int D, we say that ϕ : D → D′ is differentiable at x if ϕ|EInt D is differentiable at x and we write ∇x ϕ for ∇x (ϕ|EInt D ). The differentiability properties of a mapping ϕ remain unchanged if the codomain of ϕ is changed to any open subset of E ′ that includes Rng ϕ. The gradient of ϕ remains unaltered. If Rng ϕ is included in some flat F ′ in E ′ , one may change the codomain to a subset that is open in F ′. in that case, the gradient of ϕ at a point x ∈ D must be replaced by the adjustment ∇x ϕ|U of ∇x ϕ, where U ′ is the direction space of F ′. ′ The differentiability and the gradient of a mapping at a point depend only on the values of the mapping near that point. To be more precise, let ϕ1 and ϕ2 be two mappings whose domains are neighborhoods of a given x ∈ E and whose codomains are open subsets of E ′. Assume that ϕ1 and ϕ2 agree 63. GRADIENTS, CHAIN RULE 219 on some neighborhood of x, i.e. that ϕ1 |EN = ϕ2 |EN for some N ∈ Nhdx (E). ′ ′ Then ϕ1 is differentiable at x if and only if ϕ2 is differentiable at x. If this is the case, we have ∇x ϕ1 = ∇x ϕ2. Every flat mapping α : E → E ′ is differentiable. The tangent of α at every point x ∈ E is α itself. The gradient of α as defined in this section is the constant (∇α)E→Lin(V,V ′ ) whose value is the gradient ∇α of α in the sense of Sect.33. The gradient of a linear mapping is the constant whose value is this linear mapping itself. In the case when E := V := R and when D := I is an open interval, differentiability at t ∈ I of a process p : I → D′ in the sense of Def.1 above reduces to differentiability of p at t in the sense of Def.1 of Sect.1. The gradient ∇t p ∈ Lin(R, V ′ ) becomes associated with the derivative ∂t p ∈ V ′ by the natural isomorphism from V ′ to Lin(R, V ′ ), so that ∇t p = (∂t p)⊗ and ∂t p = (∇t p)1 (see Sect.25). If I is an interval that is not open and if t is an endpoint of it, then the derivative of p : I → D′ at t, if it exists, cannot be associated with a gradient. If ϕ is differentiable at x then it is confined and hence continuous at x. This follows from the fact that its tangent α, being flat, is confined near x and that the difference ϕ − α|D , being small near x, is confined near x. The converse is not true. For example, it is easily seen that the absolute-value function (t 7→ |t|) : R → R is confined near 0 ∈ R but not differentiable at 0. The following criterion is immediate from the definition: Characterization of Gradients: The mapping ϕ : D → D′ is dif- ferentiable at x ∈ D if and only if there is an L ∈ Lin(V, V ′ ) such that n : (D − x) → V, defined by n(v) := (ϕ(x + v) − ϕ(x)) − Lv for all v ∈ D − x, (63.6) is small near 0 in V. If this is the case, then ∇x ϕ = L. Let D, D1 , D2 be open subsets of flat spaces E, E1 , E2 with translation spaces V, V1 , V2 , respectively. The following result follows immediately from the definitions if we use the term-wise evaluations (04.13) and (14.12). Proposition 2: The mapping (ϕ1 , ϕ2 ) : D → D1 × D2 is differentiable at x ∈ D if and only if both ϕ1 and ϕ2 are differentiable at x. If this is the case, then ∇x (ϕ1 , ϕ2 ) = (∇x ϕ1 , ∇x ϕ2 ) ∈ Lin(V, V1 ) × Lin(V, V2 ) ∼ = Lin(V, V1 × V2 ) (63.7) General Chain Rule: Let D, D′ , D′′ be open subsets of flat spaces E, E ′ , E ′′ with translation spaces V, V ′ , V ′′ , respectively. If ϕ : D → D′ is 220 CHAPTER 6. DIFFERENTIAL CALCULUS differentiable at x ∈ D and if ψ : D′ → D′′ is differentiable at ϕ(x), then the composite ψ ◦ ϕ : D → D′′ is differentiable at x. The tangent to the compos- ite ψ ◦ ϕ at x is the composite of the tangent to ϕ at x and the tangent to ψ at ϕ(x) and we have ∇x (ψ ◦ ϕ) = (∇ϕ(x) ψ)(∇x ϕ). (63.8) If ϕ and ψ are both differentiable, so is ψ ◦ ϕ, and we have ∇(ψ ◦ ϕ) = (∇ψ ◦ ϕ)(∇ϕ), (63.9) where the product on the right is understood as value-wise composition. Proof: Let α be the tangent to ϕ at x and β the tangent to ψ at ϕ(x). Then σ := ϕ − α|D ∈ Smallx (E, V ′ ), τ := ψ − β|D′ ∈ Smallϕ(x) (E ′ , V ′′ ). We have ψ ◦ ϕ = (β + τ ) ◦ (α + σ) = β ◦ (α + σ) + τ ◦ (α + σ) = β ◦ α + (∇β) ◦ σ + τ ◦ (α + σ) where domain restriction symbols have been omitted to avoid clutter. It follows from (VI), (IX), (XII), and (XIII) of Sect.62 that (∇β) ◦ σ + τ ◦ (α + σ) ∈ Smallx (E, V ′′ ), which means that ψ ◦ ϕ − β ◦ α ∈ Smallx (E, V ′′ ). If follows that ψ ◦ ϕ is differentiable at x with tangent β ◦ α. The assertion (63.8) follows from the Chain Rule for Flat Mappings of Sect.33. Let ϕ : D → E ′ and ψ : D → E ′ both be differentiable at x ∈ D. Then the value-wise difference ϕ − ψ : D → V ′ is differentiable at x and ∇x (ϕ − ψ) = ∇x ϕ − ∇x ψ. (63.10) This follows from Prop.2, the fact that the point-difference (x′ , y ′ ) 7→ (x′ −y ′ ) is a flat mapping from E ′ × E ′ into V ′ , and the General Chain Rule. When the General Chain Rule is applied to the composite of a vector- valued mapping with a linear mapping it yields 63. GRADIENTS, CHAIN RULE 221 Proposition 3: If h : D → V ′ is differentiable at x ∈ D and if L ∈ Lin(V ′ , W), where W is some linear space, then Lh : D → W is differentiable at x ∈ D and ∇x (Lh) = L(∇x h). (63.11) Using the fact that vector-addition, transpositions of linear mappings, and the trace operations are all linear operations, we obtain the following special cases of Prop.3: (I) Let h : D → V ′ and k : D → V ′ both be differentiable at x ∈ D. Then the value-wise sum h + k : D → V ′ is differentiable at x and ∇x (h + k) = ∇x h + ∇x k. (63.12) (II) Let W and Z be linear spaces and let F : D → Lin(W, Z) be dif- ferentiable at x. If F⊤ : D → Lin(Z ∗ , W ∗ ) is defined by value-wise transposition, then F⊤ is differentiable at x ∈ D and ∇x (F⊤ )v = ((∇x F)v)⊤ for all v ∈ V. (63.13) In particular, if I is an open interval and if F : I → Lin(W, Z) is differentiable, so is F⊤ : I → Lin(Z ∗ , W ∗ ), and (F⊤ ) = (F )⊤. (63.14) (III) Let W be a linear space and let F : D → Lin(W) be differentiable at x. If trF : D → R is the value-wise trace of F, then trF is differentiable at x and (∇x (trF))v = tr((∇x F)v) for all v ∈ V. (63.15) In particular, if I is an open interval and if F : I → Lin(W) is differ- entiable, so is trF : I → R, and (trF) = tr(F ). (63.16) We note three special cases of the General Chain Rule: Let I be an open interval and let D and D′ be open subsets of E and E ′ , respectively. If p : I → D and ϕ : D → D′ are differentiable, so is ϕ ◦ p : I → D′ , and (ϕ ◦ p) = ((∇ϕ) ◦ p)p. (63.17) 222 CHAPTER 6. DIFFERENTIAL CALCULUS If ϕ : D → D′ and f : D′ → R are differentiable, so is f ◦ ϕ : D → R, and ∇(f ◦ ϕ) = (∇ϕ)⊤ ((∇f ) ◦ ϕ) (63.18) (see (21.3)). If f : D → I and p : I → D′ are differentiable, so is p ◦ f , and ∇(p ◦ f ) = (p ◦ f ) ⊗ ∇f. (63.19) Notes 63 (1) Other common terms for the concept of “gradient” of Def.1 are “differential”, “Fréchet differential”, “derivative”, and “Fréchet derivative”. Some authors make an artificial distinction between “gradient” and “differential”. We cannot use “derivative” because, for processes, “gradient” and “derivative” are distinct though related concepts. (2) The conventional definitions of gradient depend, at first view, on the prescription of a norm. Many texts never even mention the fact that the gradient is a norm- invariant concept. In some contexts, as when one deals with genuine Euclidean spaces, this norm-invariance is perhaps not very important. However, when one deals with mathematical models for space-time in the theory of relativity, the norm- invariance is crucial because it shows that the concepts of differential calculus have a “Lorentz-invariant” meaning. (3) I am introducing the notation ∇x ϕ for the gradient of ϕ at x because the more conventional notation ∇ϕ(x) suggests, incorrectly, that ∇ϕ(x) is necessarily the value at x of a gradient-mapping ∇ϕ. In fact, one cannot define the gradient- mapping ∇ϕ without first having a notation for the gradient at a point (see 63.4). (4) Other notations for ∇x ϕ in the literature are dϕ(x), Dϕ(x), and ϕ′ (x). (5) I conjecture that the “Chain” of “Chain Rule” comes from an old terminology that used “chaining” (in the sense of “concatenation”) for “composition”. The term “Composition Rule” would need less explanation, but I retained “Chain Rule” because it is more traditional and almost as good. 64 Constricted Mappings In this section, D and D′ denote arbitrary subsets of flat spaces E and E ′ with translation spaces V and V ′ , respectively. Definition 1: We say that the mapping ϕ : D → D′ is constricted if for every norm ν on V and every norm ν ′ on V ′ there is κ ∈ P× such that ν ′ (ϕ(y) − ϕ(x)) ≤ κν(y − x) (64.1) 64. CONSTRICTED MAPPINGS 223 holds for all x, y ∈ D. The infimum of the set of all κ ∈ P× for which (64.1) holds for all x, y ∈ D is called the striction of ϕ relative to ν and ν ′ ; it is denoted by str(ϕ; ν, ν ′ ). It is clear that  ′ ν (ϕ(y) − ϕ(x))  ′ str(ϕ; ν, ν ) = sup x, y ∈ D, x 6= y. (64.2) ν(y − x) Proposition 1: For every mapping ϕ : D → D′ , the following are equivalent: (i) ϕ is constricted. (ii) There exist norms ν and ν ′ on V and V ′ , respectively, and κ ∈ P× such that (64.1) holds for all x, y ∈ D. (iii) For every bounded subset C of V and every N ′ ∈ Nhd0 (V ′ ) there is ρ ∈ P× such that x − y ∈ sC =⇒ ϕ(x) − ϕ(y) ∈ sρN ′ (64.3) for all x, y ∈ D and s ∈ P×. Proof: (i) ⇒ (ii): This implication is trivial. (ii) ⇒ (iii): Assume that (ii) holds, and let a bounded subset C of V and N ′ ∈ Nhd0 (V ′ ) be given. By Cor.1 of the Cell-Inclusion Theorem of Sect.52, we can choose b ∈ P× such that ν(u) ≤ b for all u ∈ C. (64.4) By Prop.3 of Sect.53, we can choose σ ∈ P× such that σCe(ν ′ ) ⊂ N ′. Now let x, y ∈ D and s ∈ P× be given and assume that x − y ∈ sC. Then 1 1 s (x − y) ∈ C and hence, by (64.4), we have ν( s (x − y)) ≤ b, which gives ′ ν(x − y) ≤ sb. Using (64.1) we obtain ν (ϕ(y) − ϕ(x)) ≤ sκb. If we put ρ := κb σ , this means that ϕ(y) − ϕ(x) ∈ sρσCe(ν ′ ) ⊂ sρN ′ , i.e. that (64.3) holds. (iii) ⇒ (i): Assume that (iii) holds and let a norm ν on V and a norm ν ′ on V ′ be given. We apply (iii) with the choices C := Bdy Ce(ν) and N ′ := Ce(ν ′ ). Let x, y ∈ D with x 6= y be given. If we put s := ν(x − y) 224 CHAPTER 6. DIFFERENTIAL CALCULUS then x−y ∈ sBdy Ce(ν) and hence, by (64.3), ϕ(x)−ϕ(y) ∈ sρCe(ν ′ ), which implies ν ′ (ϕ(x) − ϕ(y)) < sρ = ρν(x − y). Thus, if we put κ := ρ, then (64.1) holds for all x, y ∈ D. If D and D′ are open sets and if ϕ : D → D′ is constricted, it is confined near every x ∈ D, as is evident from Prop.4 of Sect.62. The converse is not true. For example, one can show that the function f : ]−1, 1[→ R defined by t sin( 1t ) if t ∈ ]0, 1[   f (t) := (64.5) 0 if t ∈ ]−1, 0] is not constricted, but is confined near every t ∈ ]−1, 1[. Every flat mapping α : E → E ′ is constricted and str(α; ν, ν ′ ) = ||∇α||ν,ν ′ , where || ||ν,ν ′ is the operator norm on Lin(V, V ′ ) corresponding to ν and ν ′ (see Sect.52). Constrictedness and strictions remain unaffected by a change of codo- main. If the domain of a constricted mapping is restricted, then it remains constricted and the striction of the restriction is less than or equal to the striction of the original mapping. (Pardon the puns.) Proposition 2: If ϕ : D → D′ is constricted then it is uniformly continuous. Proof: We use condition (iii) of Prop.1. Let N ′ ∈ Nhd0 (V ′ ) be given. We choose a bounded neighborhood C of 0 in V and determine ρ ∈ P× according to (iii). Putting N := ρ1 C ∈ Nhd0 (V) and s := ρ1 , we see that (64.3) gives x−y ∈N =⇒ ϕ(x) − ϕ(y) ∈ N ′ for all x, y ∈ D. Pitfall: The converse of Prop.2 is not valid. A counterexample is the √ square-root function : P → P, which is uniformly continuous but not constricted. Another counterexample is the function defined by (64.5). The following result is the most useful criterion for showing that a given mapping is constricted. Striction Estimate for Differentiable Mappings: Assume that D is an open convex subset of E, that ϕ : D → D′ is differentiable and that the gradient ∇ϕ : D → Lin(V, V ′ ) has a bounded range. Then ϕ is constricted and str(ϕ; ν, ν ′ ) ≤ sup{||∇z ϕ||ν,ν ′ | z ∈ D} (64.6) 64. CONSTRICTED MAPPINGS 225 for all norms ν, ν ′ on V, V ′ , respectively. Proof: Let x, y ∈ D. Since D is convex, we have tx + (1 − t)y ∈ D for all t ∈ [0, 1] and hence we can define a process p : [0, 1] → D′ by p(t) := ϕ(tx + (1 − t)y). By the Chain Rule, p is differentiable at t when t ∈ ]0, 1[ and we have ∂t p = (∇z ϕ)(x − y) with z := tx + (1 − t)y ∈ D. Applying the Difference-Quotient Theorem of Sect.61 to p, we obtain ϕ(x) − ϕ(y) = p(1) − p(0) ∈ Clo Cxh{(∇z ϕ)(x − y) | z ∈ D}. (64.7) If ν, ν ′ are norms on V, V ′ then, by (52.7), ν ′ ((∇z ϕ)(x − y)) ≤ ||∇z ϕ||ν,ν ′ ν(x − y) (64.8) for all z ∈ D. To say that ∇ϕ has a bounded range is equivalent, by Cor.1 to the Cell-Inclusion Theorem of Sect.52, to κ := sup{||∇z ϕ||ν,ν ′ | z ∈ D} < ∞. (64.9) It follows from (64.8) that ν ′ ((∇z ϕ)(x − y) ≤ κν(x − y) for all z ∈ D, which can be expressed in the form (∇z ϕ)(x − y) ∈ κν(x − y)Ce(ν ′ ) for all z ∈ D. Since the set on the right is closed and convex we get Clo Cxh{(∇z ϕ)(y − x) | z ∈ D} ⊂ κν(x − y)Ce(ν ′ ) and hence, by (64.7), ϕ(x) − ϕ(y) ∈ κν(x − y)Ce(ν ′ ). This may be expressed in the form ν ′ (ϕ(x) − ϕ(y)) ≤ κν(x − y). Since x, y ∈ D were arbitrary it follows that ϕ is constricted. The definition (64.9) shows that (64.6) holds. Remark: It is not hard to prove that the inequality in (64.6) is actually an equality. Proposition 3: If D is a non-empty open convex set and if ϕ : D → D′ is differentiable with gradient zero, then ϕ is constant. Proof: Choose norms ν, ν ′ on V, V ′ , respectively. The assumption ∇ϕ = 0 gives ||∇z ϕ||ν,ν ′ = 0 for all z ∈ D. Hence, by (64.6), we have str(ϕ, ν, ν ′ ) = 226 CHAPTER 6. DIFFERENTIAL CALCULUS 0. Using (64.2), we conclude that ν ′ (ϕ(y)−ϕ(x)) = 0 and hence ϕ(x) = ϕ(y) for all x, y ∈ D. Remark: In Prop.3, the condition that D be convex can be replaced by the weaker one that D be “connected”. This means, intuitively, that every two points in D can be connected by a continuous curve entirely within D. Proposition 4: Assume that D and D′ are open subsets and that ϕ : D → D′ is of class C 1. Let k be a compact subset of D. For every norm ν on V, every norm ν ′ on V ′ , and every ε ∈ P× there exists δ ∈ P× such that k + δCe(ν) ⊂ D and such that, for every x ∈ k, the function nx : δCe(ν) → V ′ defined by nx (v) := ϕ(x + v) − ϕ(x) − (∇x ϕ)v (64.10) is constricted with str(nx ; ν, ν ′ ) ≤ ε. (64.11) Proof: Let a norm ν on V be given. By Prop.6 of Sect.58, we can obtain δ1 ∈ P× such that k + δ1 Ce(ν) ⊂ D. For each x ∈ k, we define mx : δ1 Ce(ν) → V ′ by mx (v) := ϕ(x + v) − ϕ(x) − (∇x ϕ)v. (64.12) Differentiation gives ∇v mx = ∇ϕ(x + v) − ∇ϕ(x) (64.13) for all x ∈ k and all v ∈ δ1 Ce(ν). Since k + δ1 Ce(ν) is compact by Prop.6 of Sect.58 and since ∇ϕ is continuous, it follows by the Uniform Continuity Theorem of Sect.58 that ∇ϕ|k+δ1 Ce(ν) is uniformly continuous. Now let a norm ν ′ on V ′ and ε ∈ P× be given. By Prop.4 of Sect.56, we can then determine δ2 ∈ P× such that ν(y − x) < δ2 =⇒ ||∇ϕ(y) − ∇ϕ(x)||ν,ν ′ < ε for all x, y ∈ k + δ1 Ce(ν). In view of (64.13), it follows that v ∈ δ2 Ce(ν) =⇒ ||∇v mx ||ν,ν ′ < ε for all x ∈ k and all v ∈ δ1 Ce(ν). If we put δ := min {δ1 , δ2 } and if we define nx := mx |δCe(ν) for every x ∈ k, we see that {||∇v nx ||ν,ν ′ | v ∈ δCe(ν)} is bounded by ε for all x ∈ k. By (64.12) and the Striction Estimate for Differentiable Mappings, the desired result follows. 64. CONSTRICTED MAPPINGS 227 Definition 2: Let ϕ : D → D be a constricted mapping from a set D into itself. Then str(ϕ) := inf{str(ϕ; ν, ν) | ν a norm on V} (64.14) is called the absolute striction of ϕ. If str(ϕ) < 1 we say that ϕ is a contraction. Contraction Fixed Point Theorem: Every contraction has at most one fixed point and, if its domain is closed and not empty, it has exactly one fixed point. Proof: Let ϕ : D → D be a contraction. We choose a norm ν on V such that κ := str(ϕ; ν, ν) < 1 and hence, by (64.1), ν(ϕ(x) − ϕ(y)) ≤ κν(x − y) for all x, y ∈ D. (64.15) If x and y are fixed points of ϕ, so that ϕ(x) = x, ϕ(y) = y, then (64.15) gives ν(x − y)(1 − κ) ≤ 0. Since 1 − κ > 0 this is possible only when ν(x − y) = 0 and hence x = y. Therefore, ϕ can have at most one fixed point. We now assume D 6= ∅, choose s0 ∈ D arbitrarily, and define sn := ϕ◦n (s0 ) for all n ∈ N× (see Sect.03). It follows from (64.15) that ν(sm+1 − sm ) = ν(ϕ(sm ) − ϕ(sm−1 )) ≤ κν(sm − sm−1 ) for all m ∈ N×. Using induction, one concludes that ν(sm+1 − sm ) ≤ κm ν(s1 − s0 ) for all m ∈ N. Now, if n ∈ N and r ∈ N, then X sn+r − sn = (sn+k+1 − sn+k ) k∈r [ and hence X κn ν(sn+r − sn ) ≤ κn+k ν(s1 − s0 ) ≤ ν(s1 − s0 ). 1−κ k∈r [ Since κ < 1, we have lim n→∞ κn = 0, and it follows that for every ε ∈ P× there is an m ∈ N such that ν(sn+r −sn ) < ε whenever n ∈ m+N, r ∈ N. By 228 CHAPTER 6. DIFFERENTIAL CALCULUS the Basic Convergence Criterion of Sect.55 it follows that s := (sn | n ∈ N) converges. Since ϕ(sn ) = sn+1 for all n ∈ N, we have ϕ ◦ s = (sn+1 | n ∈ N), which converges to the same limit as s. We put x := lim s = lim (ϕ ◦ s). Now assume that D is closed. By Prop.6 of Sect.55 it follows that x ∈ D. Since ϕ is continuous, we can apply Prop.2 of Sect.56 to conclude that lim (ϕ ◦ s) = ϕ(lim s), i.e. that ϕ(x) = x. Notes 64 (1) The traditional terms for “constricted” and “striction” are “Lipschitzian” and “Lipschitz number (or constant)”, respectively. I am introducing the terms “con- stricted” and “striction” here because they are much more descriptive. (2) It turns out that the absolute striction of a lineon coincides with what is often called its “spectral radius”. (3) The Contraction Fixed Point Theorem is often called the “Contraction Mapping Theorem” or the “Banach Fixed Point Theorem”. 65 Partial Gradients, Directional Derivatives Let E1 , E2 be flat spaces with translation spaces V1 , V2. As we have seen in Sect.33, the set-product E := E1 × E2 is then a flat space with translation space V := V1 × V2. We consider a mapping ϕ : D → D′ from an open subset D of E into an open subset D′ of a flat space E ′ with translation ′ space V. Given any x2 ∈ E2 , we define (·, x2 ) : E1 → E according to (04.21), put D(·,x2 ) := (·, x2 )< (D) = {z ∈ E1 | (z, x2 ) ∈ D}, (65.1) which is an open subset of E1 because (·, x1 ) is flat and hence continuous, and define ϕ(·, x2 ) : D(·,x2 ) → D′ according to (04.22). If ϕ(·, x2 ) is dif- ferentiable at x1 for all (x1 , x2 ) ∈ D we define the partial 1-gradient ∇(1) ϕ : D → Lin(V1 , V ′ ) of ϕ by ∇(1) ϕ(x1 , x2 ) := ∇x1 ϕ(·, x2 ) for all (x1 , x2 ) ∈ D. (65.2) In the special case when E1 := R, we have V1 = R, and the partial 1-derivative ϕ,1 : D → V ′ , defined by ϕ,1 (t, x2 ) := (∂ϕ(·, x2 ))(t) for all (t, x2 ) ∈ D, (65.3) 65. PARTIAL GRADIENTS, DIRECTIONAL DERIVATIVES 229 is related to ∇(1) ϕ by ∇(1) ϕ = ϕ,1 ⊗ and ϕ,1 = (∇(1) ϕ)1 (value-wise). Similar definitions are employed to define D(x1 ,·) , the mapping ϕ(x1 , ·) : D(x1 ,·) → D′ , the partial 2-gradient ∇(2) ϕ : D → Lin(V2 , V ′ ) and the partial 2-derivative ϕ,2 : D → V ′. The following result follows immediately from the definitions. Proposition 1: If ϕ : D → D′ is differentiable at x := (x1 , x2 ) ∈ D, then the gradients ∇x1 ϕ(·, x2 ) and ∇x2 ϕ(x1 , ·) exist and (∇x ϕ)v = (∇x1 ϕ(·, x2 ))v1 + (∇x2 ϕ(x1 , ·))v2 (65.4) for all v := (v1 , v2 ) ∈ V. If ϕ is differentiable, then the partial gradients ∇(1) ϕ and ∇(2) ϕ exist and we have ∇ϕ = ∇(1) ϕ ⊕ ∇(2) ϕ (65.5) where the operation ⊕ on the right is understood as value-wise application of (14.13). Pitfall: The converse of Prop.1 is not true: A mapping can have partial gradients without being differentiable. For example, the mapping ϕ : R × R → R, defined by  st  s2 +t2 if (s, t) 6= (0, 0) ϕ(s, t) := , 0 if (s, t) = (0, 0) has partial derivatives at (0, 0) since both ϕ(·, 0) and ϕ(0, ·) are equal to the constant 0. Since ϕ(t, t) = 12 for t ∈ R× but ϕ(0, 0) = 0, it is clear that ϕ is not even continuous at (0, 0), let alone differentiable. The second assertion of Prop.1 shows that if ϕ is of class C1 , then ∇(1) ϕ and ∇(2) ϕ exist and are continuous. The converse of this statement is true, but the proof is highly nontrivial: Proposition 2: Let E1 , E2 , E ′ be flat spaces. Let D be an open subset of E1 × E2 and D′ an open subset of E ′. A mapping ϕ : D → D′ is of class C 1 if (and only if ) the partial gradients ∇(1) ϕ and ∇(2) ϕ exist and are continuous. Proof: Let (x1 , x2 ) ∈ D. Since D − (x1 , x2 ) is a neighborhood of (0, 0) in V1 × V2 , we may choose M1 ∈ Nhd0 (V1 ) and M2 ∈ Nhd0 (V2 ) such that M := M1 × M2 ⊂ (D − (x1 , x2 )). We define m : M → V ′ by m(v1 , v2 ) := ϕ((x1 , x2 ) + (v1 , v2 )) − ϕ(x1 , x2 ) − (∇(1) ϕ(x1 , x2 )v1 + ∇(2) ϕ(x1 , x2 )v2 ). 230 CHAPTER 6. DIFFERENTIAL CALCULUS It suffices to show that m ∈ Small(V1 × V2 , V ′ ), for if this is the case, then the Characterization of Gradients of Sect.63 tells us that ϕ is differentiable at (x1 , x2 ) and that its gradient at (x1 , x2 ) is given by (65.4). In addition, since (x1 , x2 ) ∈ D is arbitrary, we can then conclude that (65.5) holds and, since both ∇(1) ϕ and ∇(2) ϕ are continuous, that ∇ϕ is continuous. We note that m := h + k, when h : M → V ′ and k : M → V ′ are defined by  h(v1 , v2 ) := ϕ(x1 , x2 + v2 ) − ϕ(x1 , x2 ) − ∇(2) ϕ(x1 , x2 )v2 , k(v1 , v2 ) := ϕ(x1 + v1 , x2 + v2 ) − ϕ(x1 , x2 + v2 ) − ∇(1) ϕ(x1 , x2 )v1. (65.6) The differentiability of ϕ(x1 , ·) at x2 insures that h belongs to Small(V1 × V2 , V ′ ) and it only remains to be shown that k belongs to Small(V1 × V2 , V ′ ). We choose norms ν1 , ν2 , and ν ′ on V1 , V2 and V ′ , respectively. Let ε ∈ × P be given. Since ∇(1) ϕ is continuous at (x1 , x2 ), there are open convex neighborhoods N1 and N2 of 0 in V1 and V2 , respectively, such that N1 ⊂ M1 , N2 ⊂ M2 , and ||∇(1) ϕ((x1 , x2 ) + (v1 , v2 )) − ∇(1) ϕ(x1 , x2 ) ||ν1 ,ν ′ < ε for all (v1 , v2 ) ∈ N1 × N2. Noting (65.6), we see that this is equivalent to ||∇v1 k(·, v2 ) ||ν1 ,ν ′ < ε for all v1 ∈ N1 , v2 ∈ N2. Applying the Striction Estimate of Sect.64 to k(·, v2 )|N1 , we infer that str(k(·, v2 )|N1 ; ν1 , ν ′ ) ≤ ε. It is clear from (65.6) that k(0, v2 ) = 0 for all v2 ∈ N2. Hence we can conclude, in view of (64.1), that ν ′ (k(v1 , v2 )) ≤ εν1 (v1 ) ≤ ε max {ν1 (v1 ), ν2 (v2 )} for all v1 ∈ N1 , v2 ∈ N2. Since ε ∈ P× was arbitrary, it follows that ν ′ (k(v1 , v2 )) lim = 0, (v1 ,v2 )→(0,0) max {ν1 (v1 ), ν2 (v2 )} which, in view of (62.1) and (51.23), shows that k ∈ Small(V1 × V2 , V ′ ) as required. Remark: In examining the proof above one observes that one needs merely the existence of the partial gradients and the continuity of one of them in order to conclude that the mapping is differentiable. We now generalize Prop.2 to the case when E := × (Ei | i ∈ I) is the set-product of a finite family (Ei | i ∈ I) of flat spaces. This product E is a 65. PARTIAL GRADIENTS, DIRECTIONAL DERIVATIVES 231 flat space whose translation space is the set-product V := × (Vi | i ∈ I) of the translation spaces Vi of the Ei , i ∈ I. Given any x ∈ E and j ∈ I, we define the mapping (x.j) : Ej → E according to (04.24). We consider a mapping ϕ : D → D′ from an open subset D of E into an open subset D′ of E ′. Given any x ∈ D and j ∈ I, we put D(x.j) := (x.j)< (D) ⊂ Ej (65.7) and define ϕ(x.j) : D(x.j) → D′ according to (04.25). If ϕ(x.j) is differen- tiable at xj for all x ∈ D, we define the partial j-gradient ∇(j) ϕ : D → Lin(Vj , V ′ ) of ϕ by ∇(j) ϕ(x) := ∇xj ϕ(x.j) for all x ∈ D. In the special case when Ej := R for some j ∈ I, the partial j- derivative ϕ,j : D → V ′ , defined by ϕ,j (x) := (∂ϕ(x.j))(xj ) for all x ∈ D, (65.8) is related to ∇(j) ϕ by ∇(j) ϕ = ϕ,j ⊗ and ϕ,j = (∇(j) ϕ)1. In the case when I := {1, 2}, these notations and concepts reduce to the ones explained in the beginning (see (04.21)). The following result generalizes Prop.1. Proposition 3: If ϕ : D → D′ is differentiable at x = (xi | i ∈ I) ∈ D, then the gradients ∇xj ϕ(x.j) exist for all j ∈ I and X (∇x ϕ)v = (∇xj ϕ(x.j))vj (65.9) j∈I for all v = (vi | i ∈ I) ∈ V. If ϕ is differentiable, then the partial gradients ∇(j) ϕ exist for all j ∈ I and we have M ∇ϕ = (∇(j) ϕ), (65.10) j∈I L where the operation on the right is understood as value-wise application of (14.18). In the case when E := RI , we can put Ei := R for all i ∈ I and (65.10) reduces to ∇ϕ = lnc(ϕ,i | i∈I) , (65.11) where the value at x ∈ D of the right side is understood to be the linear- combination mapping of the family (ϕ,i (x) | i ∈ I) in V ′. If moreover, E ′ is also a space of the form E ′ := RK with some finite index set K, then 232 CHAPTER 6. DIFFERENTIAL CALCULUS lnc(ϕ,i | i∈I) can be identified with the function which assigns to each x ∈ D the matrix (ϕk,i (x) | (k, i) ∈ K × I) ∈ RK×I ∼ = Lin(RI , RK ). Hence ∇ϕ can be identified with the matrix ∇ϕ = (ϕk,i | (k, i) ∈ K × I). (65.12) of the partial derivatives ϕk,i : D → R of the component functions. As we have seen, the mere existence of these partial derivatives is not sufficient to insure the existence of ∇ϕ. Only if ∇ϕ is known a priori to exist is it possible to use the identification (65.12). Using the natural isomorphism between ×(E | i ∈ I) i and Ej × ( ×(E | i ∈ I \ {j})) i and Prop.2, one easily proves, by induction, the following generalization. Partial Gradient Theorem: Let I be a finite index set and let Ei , i ∈ I and E ′ be flat spaces. Let D be an open subset of E := ′ ′ ′ × (Ei | i ∈ I) and D an open subset of E. A mapping ϕ : D → D is of class C 1 if and only if the partial gradients ∇(i) ϕ exist and are continuous for all i ∈ I. The following result is an immediate corollary: Proposition 4: Let I and K be finite index sets and let D and D′ be open subsets of RI and RK , respectively. A mapping ϕ : D → D′ is of class C 1 if and only if the partial derivatives ϕk,i : D → R exist and are continuous for all k ∈ K and all i ∈ I. Let E, E ′ be flat spaces with translation spaces V and V ′ , respectively. Let D, D′ be open subsets of E and E ′ , respectively, and consider a mapping ϕ : D → D′. Given any x ∈ D and v ∈ V × , the range of the mapping (s 7→ (x + sv)) : R → E is a line through x whose direction space is Rv. Let Sx,v := {s ∈ R | x + sv ∈ D} be the pre-image of D under this mapping and x + 1Sx,v v : Sx,v → D a corresponding adjustment of the mapping. Since D is open, Sx,v is an open neighborhood of zero in R and ϕ ◦ (x + 1Sx,v v) : Sx,v → D′ is a process. The derivative at 0 ∈ R of this process, if it exists, is called the directional derivative of ϕ at x and is denoted by ϕ(x + sv) − ϕ(x) (ddv ϕ)(x) := ∂0 (ϕ ◦ (x + 1Sx,v v)) = lim. (65.13) s→0 s 65. PARTIAL GRADIENTS, DIRECTIONAL DERIVATIVES 233 If this directional derivative exists for all x ∈ D, it defines a function ddv ϕ : D → V ′ which is called the directional v-derivative of ϕ. The following result is immediate from the General Chain Rule: Proposition 5: If ϕ : D → D′ is differentiable at x ∈ D then the directional v-derivative of ϕ at x exists for all v ∈ V and is given by (ddv ϕ)(x) = (∇x ϕ)v. (65.14) Pitfall: The converse of this Proposition is false. In fact, a mapping can have directional derivatives in all directions at all points in its domain without being differentiable. For example, the mapping ϕ : R2 → R defined by ( ) s2 t s4 +t2 if (s, t) 6= (0, 0) ϕ(s, t) := 0 if (s, t) = (0, 0) has the directional derivatives ( ) α2 β if β 6= 0 (dd(α,β) ϕ)(0, 0) = 0 if β = 0 at (0, 0). Using Prop.4 one easily shows that ϕ|R2 \{(0,0)} is of class C1 and hence, by Prop.5, ϕ has directional derivatives in all directions at all (s, t) ∈ R2. Nevertheless, since ϕ(s, s2 ) = 12 for all s ∈ R× , ϕ is not even continuous at (0, 0), let alone differentiable. Proposition 6: Let b be a set basis of V. Then ϕ : D → D′ is of class C if and only if the directional b-derivatives ddb ϕ : D → V ′ exist and are 1 continuous for all b ∈ b. Proof: PWe choose q ∈ D and define α : Rb → E by α(λ) := q + b∈b λb b for all λ ∈ Rb. Since b is a basis, α is a flat iso- morphism. If we define D := α< (D) ⊂ Rb and ϕ := ϕ ◦ α|D D : D → D′ , we see that the directional b-derivatives of ϕ correspond to the partial deriva- tives of ϕ. The assertion follows from the Partial Gradient Theorem, applied to the case when I := b and Ei := R for all i ∈ I. Combining Prop.5 and Prop.6, we obtain Proposition 7: Assume that S ∈ Sub V spans V. The mapping ϕ : D → D′ is of class C 1 if and only if the directional derivatives ddv ϕ : D → V ′ exist and are continuous for all v ∈ S. 234 CHAPTER 6. DIFFERENTIAL CALCULUS Notes 65 (1) The notation ∇(i) for the partial i-gradient is introduced here for the first time. I am not aware of any other notation in the literature. (2) The notation ϕ,i for the partial i-derivative of ϕ is the only one that occurs fre- quently in the literature and is not objectionable. Frequently seen notations such as ∂ϕ/∂xi or ϕxi for ϕ,i are poison to me because they contain dangling dummies (see Part D of the Introduction). (3) Some people use the notation ∇v for the directional derivative ddv. I am intro- ducing ddv because (by Def.1 of Sect.63) ∇v means something else, namely the gradient at v. 66 The General Product Rule The following result shows that bilinear mappings (see Sect.24) are of class C1 and hence continuous. (They are not uniformly continuous except when zero.) Proposition 1: Let V1 , V2 and W be linear spaces. Every bi- linear mapping B : V1 × V2 → W is of class C 1 and its gradient ∇B : V1 × V2 → Lin(V1 × V2 , W) is the linear mapping given by ∇B(v1 , v2 ) = (B∼ v2 ) ⊕ (Bv1 ) (66.1) for all v1 ∈ V1 , v2 ∈ V2. Proof: Since B(v1 , ·) = Bv1 : V2 → W (see (24.2)) is linear for each v1 ∈ V, the partial 2-gradient of B exists and is given by ∇(2) B(v1 , v2 ) = Bv1 for all (v1 , v2 ) ∈ V1 × V2. It is evident that ∇(2) B : V1 × V2 → Lin(V2 , W) is linear and hence continuous. A similar argument shows that ∇(1) B is given by ∇(1) B(v1 , v2 ) = B∼ v2 for all (v1 , v2 ) ∈ V1 ×V2 and hence is also linear and continuous. By the Partial Gradient Theorem of Sect.65, it follows that B is of class C1. The formula (66.1) is a consequence of (65.5). Let D be an open set in a flat space E with translation space V. If h1 : D → V1 , h2 : D → V2 and B ∈ Lin2 (V1 × V2 , W) we write B(h1 , h2 ) := B ◦ (h1 , h2 ) : D → W so that B(h1 , h2 )(x) = B(h1 (x), h2 (x)) for all x ∈ D. 66. THE GENERAL PRODUCT RULE 235 The following theorem, which follows directly from Prop.1 and the Gen- eral Chain Rule of Sect.63, is called “Product Rule” because the term “prod- uct” is often used for the bilinear forms to which the theorem is applied. General Product Rule: Let V1 , V2 and W be linear spaces and let B ∈ Lin2 (V1 × V2 , W) be given. Let D be an open subset of a flat space and let mappings h1 : D → V1 and h2 : D → V2 be given. If h1 and h2 are both differentiable at x ∈ D so is B(h1 , h2 ), and we have ∇x B(h1 , h2 ) = (Bh1 (x))∇x h2 + (B∼ h2 (x))∇x h1. (66.2) If h1 and h2 are both differentiable [of class C 1 ], so is B(h1 , h2 ), and we have ∇B(h1 , h2 ) = (Bh1 )∇h2 + (B∼ h2 )∇h1 , (66.3) where the products on the right are understood as value-wise compositions. We now apply the General Product Rule to special bilinear mappings, namely to the ordinary product in R and to the four examples given in Sect.24. In the following list of results D denotes an open subset of a flat space having V as its translation space; W, W ′ and W ′′ denote linear spaces. (I) If f : D → R and g : D → R are differentiable [of class C1 ], so is the value-wise product f g : D → R and we have ∇(f g) = f ∇g + g∇f. (66.4) (II) If f : D → R and h : D → W are differentiable [of class C1 ], so is the value-wise scalar multiple f h : D → W, and we have ∇(f h) = h ⊗ ∇f + f ∇h. (66.5) (III) If h : D → W and η : D → W ∗ are differentiable [of class C1 ], so is the function ηh : D → R defined by (ηh)(x) := η(x)h(x) for all x ∈ D, and we have ∇(ηh) = (∇h)⊤ η + (∇η)⊤ h. (66.6) (IV) If F : D → Lin(W, W ′ ) and h : D → W are differentiable at x ∈ D, so is Fh (defined by (Fh)(y) := F(y)h(y) for all y ∈ D) and we have ∇x (Fh)v = ((∇x F)v)h(x) + (F(x)∇x h)v (66.7) for all v ∈ V. If F and h are differentiable [of class C1 ], so is Fh and (66.7) holds for all x ∈ D, v ∈ V. 236 CHAPTER 6. DIFFERENTIAL CALCULUS (V) If F : D → Lin(W, W ′ ) and G : D → Lin(W ′ , W ′′ ) are differentiable at x ∈ D, so is GF, defined by value-wise composition, and we have ∇x (GF)v = ((∇x G)v)F(x) + G(x)((∇x F)v) (66.8) for all v ∈ V. If F and G are differentiable [of class C1 ], so is GF, and (66.8) holds for all x ∈ D, v ∈ V. If W is an inner-product space, then W ∗ can be identified with W, (see Sect.41) and (III) reduces to the following result. (VI) If h, k : D → W are differentiable [of class C1 ], so is their value-wise inner product h · k and ∇(h · k) = (∇h)⊤ k + (∇k)⊤ h. (66.9) In the case when D reduces to an interval in R, (66.4) becomes the Product Rule (08.32)2 of elementary calculus. The following for- mulas apply to differentiable processes f, h, η, F, and G with values in R, W, W ∗ , Lin(W, W ′ ), and Lin(W ′ , W ′′ ), respectively: (f h) = f h + f h , (66.10) (ηh) = η h + ηh , (66.11) (Fh) = F h + Fh , (66.12) (GF) = G F + GF. (66.13) If h and k are differentiable processes with values in an inner product space, then (h · k) = h · k + h · k. (66.14) Proposition 2: Let W be an inner-product space and let R : D → LinW be given. If R is differentiable at x ∈ D and if Rng R ⊂ OrthW then R(x)⊤ ((∇x R)v) ∈ SkewW for all v ∈ V. (66.15) Conversely, if R is differentiable, if D is convex, if (66.15) holds for all x ∈ D, and if R(q) ∈ OrthW for some q ∈ D then Rng R ⊂ OrthW. Proof: Assume that R is differentiable at x ∈ D. Using (66.8) with the choice F := R and G := R⊤ we find that ∇x (R⊤ R)v = ((∇x R⊤ )v)R(x) + R⊤ (x)((∇x R)v) 66. THE GENERAL PRODUCT RULE 237 holds for all v ∈ V. By (63.13) we obtain ∇x (R⊤ R)v = W⊤ + W with W := R⊤ (x)((∇x R)v) (66.16) for all v ∈ V. Now, if Rng R ⊂ OrthV then R⊤ R = 1W is constant (see Prop.2 of Sect.43). Hence the left side of (66.16) is zero and W must be skew for all v ∈ V, i.e. (66.15) holds. Conversely, if (66.15) holds for all x ∈ D, then, by (66.16) ∇x (R⊤ R) = 0 for all x ∈ D. Since D is convex, we can apply Prop.3 of Sect.64 to conclude that R⊤ R must be constant. Hence, if R(q) ∈ OrthW, i.e. if (R⊤ R)(q) = 1W , then R⊤ R is the constant 1W , i.e. Rng R ⊂ OrthW. Remark: In the second part of Prop.2, the condition that D be con- vex can be replaced by the one that D be “connected” as explained in the Remark after Prop.3 of Sect.64. Corollary: Let I be an open interval and let R : I → LinW be a differentiable process. Then Rng R ⊂ OrthW if and only if R(t) ∈ OrthW for some t ∈ I and Rng (R⊤ R ) ⊂ SkewW. The following result shows that quadratic forms (see Sect.27) are of class 1 C and hence continuous. Proposition 3: Let V be a linear space. Every quadratic form Q : V → R is of class C 1 and its gradient ∇Q : V → V ∗ is the linear mapping ∇Q = 2 Q , (66.17) where Q is identified with the symmetric bilinear form Q ∈ Sym2 (V 2 , R) ∼ = Sym(V, V ∗ ) associated with Q (see (27.14)). Proof: By (27.14), we have Q(u) = Q (u, u) = (Q (1V , 1V ))(u) for all u ∈ V. Hence, if we apply the General Product Rule with the choices ∼ B := Q and h1 := h2 := 1V , we obtain ∇Q = Q + Q. Since Q is symmetric, this reduces to (66.17). Let V be a linear space. The lineonic nth power pown : LinV → LinV on the algebra LinV of lineons (see Sect.18) is defined by pown (L) := Ln for all n ∈ N. (66.18) The following result is a generalization of the familiar differentiation rule (ιn ) = nιn−1. 238 CHAPTER 6. DIFFERENTIAL CALCULUS Proposition 4: For every n ∈ N the lineonic nth power pown on LinV defined by (66.18) is of class C 1 and, for each L ∈ LinV, its gradi- ent ∇L pown ∈ Lin(LinV) at L ∈ LinV is given by X (∇L pown )M = Lk−1 MLn−k for all M ∈ LinV. (66.19) k∈n] Proof: For n = 0 the assertion is trivial. For n = 1, we have pow1 = 1LinV and hence ∇L pow1 = 1LinV for all L ∈ LinV, which is consistent with (66.19). Also, since ∇pow1 is constant, pow1 is of class C1. Assume, then, that the assertion is valid for a given n ∈ R. Since pown+1 = pow1 pown holds in terms of value-wise composition, we can apply the result (V) above to the case when F := pown , G := pow1 in order to conclude that pown+1 is of class C1 and that (∇L pown+1 )M = ((∇L pow1 )M)pown (L) + pow1 (L)((∇L pown )M) for all M ∈ LinV. Using (66.18) and (66.19) we get X (∇L pown+1 )M = MLn + L( Lk−1 MLn−k ), k∈n] which shows that (66.19) remains valid when n is replaced by n + 1. The desired result follows by induction. If M commutes with L, then (66.19) reduces to (∇L pown )M = (nLn−1 )M. Using Prop.4 and the form (63.17) of the Chain Rule, we obtain Proposition 5: Let I be an interval and let F : I → LinV be a process. If F is differentiable [of class C 1 ], so is its value-wise nth power Fn : I → LinV and we have X (Fn ) = Fk−1 F Fn−k for all n ∈ N. (66.20) k∈n] 67 Divergence, Laplacian In this section, D denotes an open subset of a flat space E with translation space V, and W denotes a linear space. 67. DIVERGENCE, LAPLACIAN 239 If h : D → V is differentiable at x ∈ D, we can form the trace (see Sect.26) of the gradient ∇x h ∈ LinV. The result divx h := tr(∇x h) (67.1) is called the divergence of h at x. If h is differentiable we can define the divergence div h : D → R of h by (div h)(x) := divx h for all x ∈ D. (67.2) Using the product rule (66.5) and (26.3) we obtain Proposition 1: Let h : D → V and f : D → R be differentiable. Then the divergence of f h : D → V is given by div (f h) = (∇f )h + f div h. (67.3) Consider now a mapping H : D → Lin(V ∗ , W) that is differentiable at x ∈ D. For every ω ∈ W ∗ we can form the value-wise composite ωH : D → Lin(V ∗ , R) = V ∗∗ ∼ = V (see Sect.22). Since ωH is differen- ∗ tiable at x for every ω ∈ W (see Prop.3 of Sect.63) we may consider the mapping (ω 7→ tr(∇x (ωH))) : W ∗ → R. ∼ W. It is clear that this mapping is linear and hence an element of W ∗∗ = ∗ Definition 1: Let H : D → Lin(V , W) be differentiable at x ∈ D. Then the divergence of H at x is defined to be the (unique) element divx H of W which satisfies ω(divx H) = tr(∇x (ωH)) for all ω ∈ W ∗. (67.4) If H is differentiable, then its divergence div H : D → W is defined by (div H)(x) := divx H for all x ∈ D. (67.5) In the case when W := R, we also have W ∗ ∼ = R and Lin(V ∗ , W) = V ∗∗ ∼ = V. Thus, using (67.4) with ω := 1 ∈ R, we see that the definition just given is consistent with (67.1) and (67.2). If we replace h and L in Prop.3 of Sect.63 by H and (K 7→ ωK) ∈ Lin(Lin(V ∗ , W), V), respectively, we obtain ∇x (ωH) = ω∇x H for all ω ∈ W ∗ , (67.6) 240 CHAPTER 6. DIFFERENTIAL CALCULUS where the right side must be interpreted as the composite of ∇x H ∈ Lin2 (V × V ∗ , W) with ω ∈ Lin(W, R). This composite, being an ele- ment of Lin2 (V ×V ∗ , R), must be reinterpreted as an element of LinV via the identifications Lin2 (V × V ∗ , R) ∼ = Lin(V, Lin(V ∗ , R)) = Lin(V, V ∗∗ ) ∼ = LinV to make (67.6) meaningful. Using (67.6), (67.4), and (67.1) we see that div H satisfies ω(divx H) = divx (ωH) = tr(ω∇x H) (67.7) for all ω ∈ W ∗. The following results generalize Prop.1. Proposition 2: Let H : D → Lin(V ∗ , W) and ρ : D → W ∗ be differentiable. Then the divergence of the value-wise composite ρH : D → Lin(V ∗ , R) ∼ = V is given by div (ρH) = ρdiv H + tr(H⊤ ∇ρ), (67.8) where value-wise evaluation and composition are understood. Proof: Let x ∈ D and v ∈ V be given. Using (66.8) with the choices G := ρ and F := H we obtain ∇x (ρH)v = ((∇x ρ)v)H(x) + ρ(x)((∇x H)v). (67.9) Since (∇x ρ)v ∈ W ∗ , (21.3) gives ((∇x ρ)v)H(x) = (H(x)⊤ ∇x ρ)v. On the other hand, we have ρ(x)((∇x H)v) = (ρ(x)(∇x H))v if we interpret ∇x H on the right as an element of Lin2 (V × V ∗ , W). Hence, since v ∈ V was arbitrary, (67.9) gives ∇x (ρH) = ρ(x)∇x H + H(x)⊤ ∇x ρ. Taking the trace, using (67.1), and using (67.7) with ω := ρ(x), we get divx (ρH) = ρ(x)divx H + tr(H(x)⊤ ∇x ρ). Since x ∈ D was arbitrary, the desired result (67.8) follows. Proposition 3: Let h : D → V and k : D → W be differentiable. The divergence of k ⊗ h : D → Lin(V ∗ , W), defined by taking the value-wise tensor product (see Sect.25), is then given by div (k ⊗ h) = (∇k)h + (div h)k. (67.10) 67. DIVERGENCE, LAPLACIAN 241 Proof: By (67.7) we have ω divx (k ⊗ h) = divx (ω(k ⊗ h)) = divx ((ω k)h) for all x ∈ D and all ω ∈ W ∗. Hence, by Prop.1, ω div (k ⊗ h) = (∇(ωk))h + (ωk)div h = ω((∇k)h + k div h) holds for all ω ∈ W ∗ , and (67.10) follows. Proposition 4: Let H : D → Lin(V ∗ , W) and f : D → R be differen- tiable. The divergence of f H : D → Lin(V ∗ , W) is then given by div (f H) = f div H + H(∇f ). (67.11) Proof: Let ω ∈ W ∗ be given. If we apply Prop.2 to the case when ρ := f ω we obtain div (ω(f H)) = ω(f div H) + tr(H⊤ ∇(f ω)). (67.12) Using ∇(f ω) = ω ⊗ ∇f and using (25.9), (26.3), and (22.3), we obtain tr(H⊤ ∇(f ω)) = tr(H⊤ (ω ⊗ ∇f )) = ∇f (H⊤ ω) = ω(H∇f ). Hence (67.12) and (67.7) yield ω div (f H) = ω(f div H + H∇f ). Since ω ∈ W ∗ was arbitrary, (67.11) follows. From now on we assume that E has the structure of a Euclidean space, so that V becomes an inner-product space and we can use the identification V∗ ∼= V (see Sect.41). Thus, if k : D → W is twice differentiable, we can consider the divergence of ∇k : D → Lin(V, W) ∼ = Lin(V ∗ , W). Definition 2: Let k : D → W be twice differentiable. Then the Lapla- cian of k is defined to be ∆k := div (∇k). (67.13) If ∆k = 0, then k is called a harmonic function. Remark: In the case when E is not a genuine Euclidean space, but one whose translation space has index 1 (see Sect.47), the term D’Alembertian or Wave-Operator and the symbol are often used instead of Laplacian and ∆. The following result is a direct consequence of (66.5) and Props.3 and 4. Proposition 5: Let k : D → W and f : D → R be twice differentiable. The Laplacian of f k : D → W is then given by ∆(f k) = (∆f )k + f ∆k + 2(∇k)(∇f ). (67.14) 242 CHAPTER 6. DIFFERENTIAL CALCULUS The following result follows directly from the form (63.19) of the Chain Rule and Prop.3. Proposition 6: Let I be an open interval and let f : D → I and g : I → W both be twice differentiable. The Laplacian of g ◦ f : D → W is then given by ∆(g ◦ f ) = (∇f )·2 (g ◦ f ) + (∆f )(g ◦ f ). (67.15) We now discuss an important application of Prop.6. We consider the case when f : D → I is given by f (x) := (x − q)·2 for all x ∈ D, (67.16) where q ∈ E is given. We wish to solve the following problem: How can D, I and g : I → W be chosen such that g ◦ f is harmonic? It follows directly from (67.16) and Prop.3 of Sect.66, applied to Q := sq, that ∇x f = 2(x−q) for all x ∈ D and that hence (∇f ) 2 = 4f. Also, ∇(∇f ) is the constant with value 21V and hence, by (26.9), ∆f = div (∇f ) = 2tr1V = 2 dim V = 2 dim E. Substitution of these results into (67.15) gives ∆(g ◦ f ) = 4f (g ◦ f ) + 2(dim E)g ◦ f. (67.17) Hence, g ◦ f is harmonic if and only if (2ιg + ng ) ◦ f = 0 with n := dim E, (67.18) where ι is the identity mapping of R, suitably adjusted (see Sect.08). Now, if we choose I := Rng f , then (67.18) is satisfied if and only if g satisfies the ordinary differential equation 2ιg + ng = 0. It follows that g must be of the form  −( n −1)  aι 2 + b if n 6= 2 g= , (67.19) a log +b if n = 2 where a, b ∈ W, provided that I is an interval. If E is a genuine Euclidean space, we may take D := E \ {q} and I := Rng f = P×. We then obtain the harmonic function  −(n−2)  ar + b if n 6= 2 h= , (67.20) a(log ◦ r) + b if n = 2 where a, b ∈ W and where r : E \ {q} → P× is defined by r(x) := |x − q|. 68. LOCAL INVERSION, IMPLICIT MAPPINGS 243 If the Euclidean space E is not genuine and V is double-signed we have two possibilities. For all n ∈ N× we may take D := {x ∈ E | (x − q) 2 > 0} and I := P×. If n is even and n 6= 2, we may instead take D := {x ∈ E | (x − q) 2 < 0} and I := −P×. Notes 67 (1) In some of the literature on “Vector Analysis”, the notation ∇ h instead of div h is used for the divergence of a vector field h, and the notation ∇2 f instead of ∆f for the Laplacian of a function f. These notations come from a formalistic and erroneous understanding of the meaning of the symbol ∇ and should be avoided. 68 Local Inversion, Implicit Mappings In this section, D and D′ denote open subsets of flat spaces E and E ′ with ′ translation spaces V and V , respectively. To say that a mapping ϕ : D → D′ is differentiable at x ∈ D means, roughly, that ϕ may be approximated, near x, by a flat mapping α, the tangent of ϕ at x. One might expect that if the tangent α is invertible, then ϕ itself is, in some sense, “locally invertible near x”. To decide whether this expectation is justified, we must first give a precise meaning to “locally invertible near x”. Definition: Let a mapping ϕ : D → D′ be given. We say that ψ is a local inverse of ϕ if ψ = (ϕ|N ′ ← N ) for suitable open subsets N = Cod ψ = ′ ′ Rng ψ and N = Dom ψ of D and D , respectively. We say that ψ is a local inverse of ϕ near x ∈ D if x ∈ Rng ψ. We say that ϕ is locally invertible near x ∈ D if it has some local inverse near x. To say that ψ is a local inverse of ϕ means that ψ ◦ ϕ|N ϕ|N ′ ′ N = 1N and N ◦ ψ = 1N ′ (68.1) for suitable open subsets N = Cod ψ and N = Dom ψ of D and D′ , respec- ′ tively. If ψ1 and ψ2 are local inverses of ϕ and M := (Rng ψ1 ) ∩ (Rng ψ2 ), then ψ1 and ψ2 must agree on ϕ> (M) = ψ1< (M) = ψ2< (M), i.e. ψ1 |M M ϕ> (M) = ψ2 |ϕ> (M) if M := (Rng ψ1 ) ∩ (Rng ψ2 ). (68.2) Pitfall: A mapping ϕ : D → D′ may be locally invertible near every point in D without being invertible, even if it is surjective. In fact, if ψ is a local inverse of ϕ, ϕ< (Dom ψ) need not be included in Rng ψ. 244 CHAPTER 6. DIFFERENTIAL CALCULUS For example, the function f : ]−1, 1[× → ]0, 1[ defined by f (s) := |s| for all s ∈ ]−1, 1[× is easily seen to be locally invertible, surjective, and even continuous. The identity function 1]0,1[ of ]0, 1[ is a local inverse of f , and we have f < (Dom 1]0,1[ ) = f < ( ]0, 1[ ) = ]−1, 1[× 6⊂ ]0, 1[= Rng 1[0,1]. The function h : ]0, 1[ → ]−1, 0[ defined by h(s) = −s for all s ∈ ]0, 1[ is another local inverse of f. If dim E ≥ 2, one can even give counter-examples of mappings ϕ : D → D′ of the type described above for which D is convex (see Sect.74). The following two results are recorded for later use. Proposition 1: Assume that ϕ : D → D′ is differentiable at x ∈ D and locally invertible near x. Then, if some local inverse near x is differentiable at ϕ(x), so are all others and all have the same gradient, namely (∇x ϕ)−1. Proof: We choose a local inverse ψ1 of ϕ near x such that ψ1 is differ- entiable at ϕ(x). Applying the Chain Rule to (68.1) gives (∇ϕ(x) ψ1 )(∇x ϕ) = 1V and (∇x ϕ)(∇ϕ(x) ψ1 ) = 1V ′ , which shows that ∇x ϕ is invertible and ∇ϕ(x) ψ1 = (∇x ϕ)−1. Let now a local inverse ψ2 of ϕ near x be given. Let M be defined as in (68.2). Since M is open and since ψ1 , being differentiable at x, is continuous at x, it follows that ϕ> (M) = ψ1< (M) is a neighborhood of ϕ(x) in E ′. By (68.2), ψ2 agrees with ψ1 on the neighborhood ϕ> (M) of ϕ(x). Hence ψ2 must also be differentiable at ϕ(x), and ∇ϕ(x) ψ2 = ∇ϕ(x) ψ1 = (∇x ϕ)−1. Proposition 2: Assume that ϕ : D → D′ is continuous and that ψ is a local inverse of ϕ near x. (i) If M′ is an open neighborhood of ϕ(x) and M′ ⊂ Dom ψ, then ψ> (M′ ) = ϕ< (M′ ) ∩ Rng ψ is an open neighborhood of x; hence the ψ (M′ ) adjustment ψ|M>′ is again a local inverse of ϕ near x. (ii) Let G be an open subset of E with x ∈ G. If ψ is continuous at ϕ(x) then there is an open neighborhood M of x with M ⊂ Rng ψ ∩ G such that ψ < (M) = ϕ> (M) is open; hence the adjustment ψ|M ψ < (M) is again a local inverse of ϕ near x. Proof: Part (i) is an immediate consequence of Prop.3 of Sect.56. To prove (ii), we observe first that ψ < (Rng ψ ∩ G) must be a neighborhood of ϕ(x) = ψ ← (x). We can choose an open subset M′ of ψ < (Rng ψ ∩ G) with 68. LOCAL INVERSION, IMPLICIT MAPPINGS 245 ϕ(x) ∈ M′ (see Sect.53). Application of (i) gives the desired result with M := ψ> (M′ ). Remark: It is in fact true that if ϕ : D → D′ is continuous, then every local inverse of ϕ is also continuous, but the proof is highly non-trivial and goes beyond the scope of this presentation. Thus, in Part (ii) of Prop.2, the requirement that ψ be continuous at ϕ(x) is, in fact, redundant. Pitfall: The expectation mentioned in the beginning is not justified. A continuous mapping ϕ : D → D′ can have an invertible tangent at x ∈ D without being locally invertible near x. An example is the function f : R → R defined by 2t sin 1t + t if t ∈ R×  2
 f (t) :=. (68.3) 0 if t = 0 It is differentiable and hence continuous. Since f (0) = 1, the tangent to f at 0 is invertible. However, f is not monotone, let alone locally invertible, near 0, because one can find numbers s arbitrarly close to 0 such that f (s) < 0. Let I be an open interval and let f : I → R be a function of class C1. If the tangent to f at a given t ∈ I is invertible, i.e. if f (t) 6= 0, one can easily prove, not only that f is locally invertible near t, but also that it has a local inverse near t that is of class C1. This result generalizes to mappings ϕ : D → D′ , but the proof is far from easy. Local Inversion Theorem: Let D and D′ be open subsets of flat spaces, let ϕ : D → D′ be of class C 1 and let x ∈ D be such that the tangent to ϕ at x is invertible. Then ϕ is locally invertible near x and every local inverse of ϕ near x is differentiable at ϕ(x). Moreover, there exists a local inverse ψ of ϕ near x that is of class C 1 and satisfies ∇y ψ = (∇ψ(y) ϕ)−1 for all y ∈ Dom ψ. (68.4) Before proceeding with the proof, we state two important results that are closely related to the Local Inversion Theorem. Implicit Mapping Theorem: Let E, E ′ , E ′′ be flat spaces, let A be an open subset of E × E ′ and let ω : A → E ′′ be a mapping of class C 1. Let (xo , yo ) ∈ A, zo := ω(xo , yo ), and assume that ∇yo ω(xo , ) is invertible. Then there exist an open neighborhood D of xo and a mapping ϕ : D → E ′ , differentiable at xo , such that ϕ(xo ) = yo and Gr(ϕ) ⊂ A, and ω(x, ϕ(x)) = zo for all x ∈ D. (68.5) Moreover, D and ϕ can be chosen such that ϕ is of class C 1 and ∇ϕ(x) = −(∇(2) ω(x, ϕ(x)))−1 ∇(1) ω(x, ϕ(x)) for all x ∈ D. (68.6) 246 CHAPTER 6. DIFFERENTIAL CALCULUS Remark: The mapping ϕ is determined implicitly by an equation in this sense: for any given x ∈ D, ϕ(x) is a solution of the equation ? y ∈ E ′, ω(x, y) = zo. (68.7) In fact, one can find a neighborhood M of (xo , yo ) in E × E ′ such that ϕ(x) is the only solution of ? y ∈ M(x, ) , ω(x, y) = zo , (68.8) where M(x, ) := {y ∈ E ′ | (x, y) ∈ M}. Differentiation Theorem for Inversion Mappings: Let V and V ′ be linear spaces of equal dimension. Then the set Lis(V, V ′ ) of all linear isomorphorisms from V onto V ′ is a (non-empty) open subset of Lin(V, V ′ ), the inversion mapping inv : Lis(V, V ′ ) → Lin(V ′ , V) defined by inv(L) := L−1 is of class C 1 , and its gradient is given by (∇L inv)M = −L−1 ML−1 for all M ∈ Lin(V, V ′ ). (68.9) The proof of these three theorems will be given in stages, which will be designated as lemmas. After a basic preliminary lemma, we will prove a weak version of the first theorem and then derive from it weak versions of the other two. The weak version of the last theorem will then be used, like a bootstrap, to prove the final version of the first theorem; the final versions of the other two theorems will follow. Lemma 1: Let V be a linear space, let ν be a norm on V, and put B := Ce(ν). Let f : B → V be a mapping with f (0) = 0 such that f − 1B⊂V is constricted with κ := str(f − 1B⊂V ; ν, ν) < 1. (68.10) Then the adjustment f |(1−κ)B (see Sect.03) of f has an inverse and this inverse is confined near 0. Proof: We will show that for every w ∈ (1 − κ)B, the equation ? z ∈ B, f (z) = w (68.11) has a unique solution. To prove uniqueness, suppose that z1 , z2 ∈ B are solutions of (68.11) for a given w ∈ V. We then have (f − 1B⊂V )(z1 ) − (f − 1B⊂V )(z2 ) = z2 − z1 68. LOCAL INVERSION, IMPLICIT MAPPINGS 247 and hence, by (68.10), ν(z2 − z1 ) ≤ κν(z2 − z1 ), which amounts to (1 − κ)ν(z2 − z1 ) ≤ 0. This is compatible with κ < 1 only if ν(z1 − z2 ) = 0 and hence z1 = z2. To prove existence, we assume that w ∈ (1 − κ)B is given and we put α := ν(w) 1−κ , so that α ∈ [0, 1[. For every v ∈ αB, we have ν(v) ≤ α and hence, by (68.10) ν((f − 1B⊂V )(v)) ≤ κν(v) ≤ κα, which implies that ν(w − (f (v) − v)) ≤ ν(w) + ν((f − 1B−V )(v)) ≤ (1 − κ)α + κα = α. Therefore, it makes sense to define hw : αB → αB by hw (v) := w − (f (v) − v) for all v ∈ αB. (68.12) Since hw is the difference of a constant with value w and a restriction of f − 1B−V , it is constricted and has an absolute striction no greater than κ. Therefore, it is a contraction, and since its domain αB is closed, the Contraction Fixed Point Theorem states that hw has a fixed point z ∈ αB. It is evident from (68.12) that a fixed point of hw is a solution of (68.11). We proved that (68.11) has a unique solution z ∈ B and that this solution satisfies ν(w) ν(z) ≤ α :=. (68.13) 1−κ If we define g : (1 − κ)B → f < ((1 − κ)B) in such a way that g(w) is this solution of (68.11), then g is the inverse we are seeking. It follows from 1 (68.13) that ν(g(w)) ≤ 1−κ ν(w) for all w ∈ (1 − κ)B. In view of part (ii) of Prop.2 of Sect.62, this proves that g is confined. Lemma 2: The assertion of the Local Inversion Theorem, except possi- bly the statement starting with “Moreover”, is valid. Proof: Let α : E → E ′ be the (invertible) tangent to ϕ at x. Since D is open, we may choose a norming cell B such that x + B ⊂ D. We define f : B → V by f := (α← |D′ ◦ ϕ|x+B ◦ (x + 1V )|B x+B ) − x. (68.14) Since ϕ is of class C1 , so is f. Clearly, we have f (0) = 0, ∇0 f = 1V. (68.15) 248 CHAPTER 6. DIFFERENTIAL CALCULUS Put ν := noB , so that B = Ce(ν). Choose ε ∈ ]0, 1[ (ε := 12 would do). If we apply Prop.4 of Sect.64 to the case when ϕ there is replaced by f and k by {0}, we see that there is δ ∈ ]0, 1] such that κ := str(f |δB − 1δB⊂V ; ν, ν) ≤ ε. (68.16) Now, it is clear from (64.2) that a striction relative to ν and ν ′ remains unchanged if ν and ν ′ are multiplied by the same strictly positive factor. Hence, (68.16) remains valid if ν there is replaced by 1δ ν. Since Ce( 1δ ν) = δCe(ν) = δB (see Prop.6 of Sect.51), it follows from (68.16) that Lemma 1 can be applied when ν, B, and f there are replaced by 1δ ν, δB and f |δB , respectively. We infer that if we put Mo := (1 − κ)δB, No := f < (Mo ), then f |M No has an inverse g : Mo → No that is confined near zero. Note o that Mo and No are both open neighborhoods of zero, the latter because it is the pre-image of an open set under a continuous mapping. We evidently have g|V − 1Mo ⊂V = (1No ⊂V − f |No ) ◦ g. In view of (68.15), 1No ⊂V −f |No is small near 0 ∈ V. Since g is confined near 0, it follows by Prop.3 of Sect.62 that g|V −1Mo ⊂V is small near zero. By the Characterization of Gradients of Sect.63, we conclude that g is differentiable at 0 with ∇0 g = 1V. We now define N := x + No , N ′ := ϕ(x) + (∇α)> (Mo ). These are open neighborhoods of x and ϕ(x), respectively. A simple calcu- lation, based on (68.14) and g = (f |M o ← No ) , shows that Mo ψ := (x + 1V )|N ← No ◦ g ◦ (α − x)|N ′ (68.17) is the inverse of ϕ|N ′ N. Using the Chain Rule and the fact that g is dif- ferentiable at 0, we conclude from (68.17) that ψ is differentiable at ϕ(x). Lemma 3: The assertion of the Implicit Mapping Theorem, except pos- sibly the statement starting with “Moreover”, is valid. Proof: We define ω̃ : A → E × E ′′ by ω̃(x, y) := (x, ω(x, y)) for all (x, y) ∈ A. (68.18) 68. LOCAL INVERSION, IMPLICIT MAPPINGS 249 Of course, ω̃ is of class C1. Using Prop.2 of Sect.63 and (65.4), we see that (∇(xo ,yo ) ω̃)(u, v) = (u, (∇xo ω(·, yo ))u + (∇yo ω(xo , ·))v) ′ for all (u, v) ∈ V × V. Since ∇yo ω(xo , ·) is invertible, so is ∇(xo ,yo ) ω̃. In- deed,we have (∇(xo ,yo ) ω̃)−1 (u, w) = (u, (∇yo ω(xo , ·))−1 (w − (∇xo ω(·, yo ))u)) ′′ for all (u, w) ∈ V ×V. Lemma 2 applies and we conclude that ω̃ has a local inverse ρ with (xo , yo ) ∈ Rng ρ that is differentiable at ω̃(xo , yo ) = (xo , zo ). In view of Prop.2, (i), we may assume that the domain of ρ is of the form D × D′′ , where D and D′′ are open neighborhoods of xo and zo , respectively. Let ψ : D × D′′ → E and ψ ′ : D × D′′ → E ′ be the value-wise terms of ρ, so that ρ(x, z) = (ψ(x, z), ψ ′ (x, z)) for all x ∈ D, z ∈ D′′. Then, by (68.18), ω̃(ρ(x, y)) = (ψ(x, z), ω(ψ(x, z), ψ ′(x, z))) = (x, z) and hence ψ(x, z) = x, ω(x, ψ ′ (x, z)) = z for all x ∈ D, z ∈ D′′. (68.19) Since ρ(xo , zo ) = (ψ(xo , zo ), ψ ′ (xo , zo )) = (xo , yo ), we have ψ ′ (xo , zo ) = yo. Therefore, if we define ϕ : D → E ′′ by ϕ(x) := ψ ′ (x, zo ), we see that ϕ(xo ) = yo and (68.5) are satisfied. The differentiability of ϕ at xo follows from the differentiablity of ρ at (xo , zo ). If we define M := Rng ρ, it is immediate that ϕ(x) is the only solution of (68.8). Lemma 4: Let V and V ′ be linear spaces of equal dimension. Then ′ ′ Lis(V, V ) is an open subset of Lin(V, V ) and the inversion mapping inv : Lis(V, V ) → Lin(V , V) defined by inv(L) := L−1 is differentiable. ′ ′ Proof: We apply Lemma 3 with E replaced by Lin(V, V ), E ′ by ′ Lin(V , V), and E ′′ by LinV. For ω of Lemma 3 we take the mapping ′ ′ ′ (L, M) 7→ ML from Lin(V, V ) × Lin(V , V) to LinV. Being bilinear, this mapping is of class C1 (see Prop.1 of Sect.66). Its partial 2-gradient at (L, M) does not depend on M. It is the right-multiplication RiL ∈ Lin(Lin(V , V), LinV) defined by RiL (K) := KL for all K ∈ Lin(V, V ′ ). ′ It is clear that RiL is invertible if and only if L is invertible. (We have (RiL )−1 = RiL−1 if this is the case.) Thus, given Lo ∈ Lis(V, V ), we can ′ 250 CHAPTER 6. DIFFERENTIAL CALCULUS apply Lemma 3 with Lo , L−1o , and 1V playing the roles of xo , yo , and zo , respectively. We conclude that there is an open neighborhood D of Lo in ′ Lin(V, V ) such that the equation ′ ? M ∈ Lin(V , V), LM = 1V has a solution for every L ∈ D. Since this solution can only be L−1 = inv(L), it follows that the role of the mapping ϕ of Lemma 3 is played by inv|D. ′ Therefore, we must have D ⊂ Lis(V, V ) and inv must be differentiable at ′ ′ Lo. Since Lo ∈ Lis(V, V ) was arbitrary, it follows that Lis(V, V ) is open and that inv is differentiable. Completion of Proofs: We assume that hypotheses of the Local Inver- sion Theorem are satisfied. The assumption that ϕ has an invertible tangent ′ at x ∈ D is equivalent to ∇x ϕ ∈ Lis(V, V ). Since ∇ϕ is continuous and ′ ′ since Lis(V, V ) is an open subset of Lin(V, V ) by Lemma 4, it follows that ′ G := (∇ϕ)< (Lis(V, V )) is an open subset of E. By Lemma 2, we can choose a local inverse of ϕ near x which is differentiable and hence continuous at ϕ(x). By Prop.2, (ii), we can adjust this local inverse such that its range is included in G. Let ψ be the local inverse so adjusted. Then ϕ has an invertible tangent at every z ∈ Rng ψ. Let z ∈ Rng ψ be given. Applying Lemma 2 to z, we see that ϕ must have a local inverse near z that is differ- entiable at ϕ(z). By Prop.1, it follows that ψ must also be differentiable at ϕ(z). Since z ∈ Rng ψ was arbitrary, it follows that ψ is differentiable. The ′ ′ formula (68.4) follows from Prop.1. Since inv : Lis(V, V ) → Lin(V , V) is differentiable and hence continuous by Lemma 4, since ∇ϕ is continuous by assumption, and since ψ is continuous because it is differentiable, it follows ′ Lis(V,V ) that the composite inv ◦ ∇ϕ|Rng ψ ◦ ψ is continuous. By (68.4), this com- posite is none other than ∇ψ, and hence the proof of the Local Inversion Theorem is complete. Assume, now, that the hypotheses of the Implicit Mapping Theorem are satisfied. By the Local Inversion Theorem, we can then choose a local inverse ρ of ω̃, as defined by (68.18), that is of class C1. It then follows that the function ϕ : D → E ′ defined in the proof of Lemma 3 is also of class C1. The formula (68.6) is obtained easily by differentiating the constant x 7→ ω(x, ϕ(x)), using Prop.1 of Sect.65 and the Chain Rule. To prove the Differentiation Theorem for Inversion Mappings, one applies the Implicit Mapping Theorem in the same way as Lemma 3 was applied to obtain Lemma 4. Pitfall: Let I and I ′ be intervals and let f : I → I ′ be of class C1 and surjective. If f has an invertible tangent at every t ∈ I, then f is not only 69. EXTREME VALUES, CONSTRAINTS 251 locally invertible near every t ∈ I but (globally) invertible, as is easily seen. This conclusion does not generalize to the case when I and I ′ are replaced by connected open subsets of flat s