Linear Algebra: Orthogonal Sets

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Questions and Answers

What characterizes the set Y as an orthogonal basis for W?

  • All vectors in Y are of equal length.
  • Y spans the entire space W. (correct)
  • The vectors in Y are linearly dependent.
  • The dot product of any two distinct vectors in Y is zero. (correct)

In the expression Spandv,..., v3 Spanxi, what does 'Span' refer to?

  • The dimension of the vector space.
  • The set of linear combinations of a given set of vectors. (correct)
  • A specific vector in the basis.
  • A unique point in the vector space.

What does it imply if W = Spandx xn3?

  • W has a dimension greater than 3.
  • W consists of all linear combinations of the vector x. (correct)
  • W is defined without reference to the vector x.
  • W is specifically spanned by a single vector x.

What is the outcome of constructing an orthogonal basis Ev r23 for W?

<p>Provides a simplification in calculations within W. (A)</p> Signup and view all the answers

When referring to the expression v = (2), what does the number 2 represent?

<p>A specific component of the vector. (C)</p> Signup and view all the answers

What condition must be met for a basis of W to be considered orthogonal?

<p>The dot product of any two distinct basis vectors must be zero. (B)</p> Signup and view all the answers

In the context of orthogonal bases, which expression properly represents the summation of vectors?

<p>$y = Up1 + Up2 + Up3$ (A)</p> Signup and view all the answers

Which interpretation can be drawn from the statement involving E and the orthogonal basis?

<p>E can be any vector in the space defined by the basis. (B)</p> Signup and view all the answers

If a basis is orthogonal, what can be said about the angles between the basis vectors?

<p>The angles between basis vectors are all right angles. (A)</p> Signup and view all the answers

Which of the following is true for an orthogonal basis concerning vector components?

<p>Each vector is solely represented by its projection onto the basis vectors. (A)</p> Signup and view all the answers

What is the relationship between the vector $y$ and the orthogonal basis $E$?

<p>Vector $y$ is a linear combination of the basis vectors in $E$. (D)</p> Signup and view all the answers

What does the term 'orthogonal projection' refer to in the context of vector spaces?

<p>The process of finding the closest point in a subspace to a given vector. (B)</p> Signup and view all the answers

Which of the following best describes the span of a set of vectors?

<p>The set of all possible linear combinations of the vectors. (C)</p> Signup and view all the answers

If $W$ is defined as Span($ ext{u}_1, ext{u}_2, ext{u}_3$), which of the following is true?

<p>Every vector in $W$ can be expressed as a combination of $ ext{u}_1$, $ ext{u}_2$, and $ ext{u}_3$. (B)</p> Signup and view all the answers

What property must the basis of $W$ have to be considered orthogonal?

<p>All pairs of vectors must be orthogonal to each other. (B)</p> Signup and view all the answers

What characterizes an orthogonal set of vectors?

<p>The dot product of each pair of distinct vectors is zero. (A)</p> Signup and view all the answers

Given the vectors $u_1 = (1, 0)$ and $u_2 = (0, 1)$, are they orthogonal?

<p>Yes, their dot product is 0. (D)</p> Signup and view all the answers

If vectors $u_i$ and $u_j$ are part of an orthogonal set, what can be inferred about their relationship?

<p>Their dot product equals zero. (C)</p> Signup and view all the answers

Which of the following sets of vectors is orthogonal?

<p>$u_1 = (2, 0)$, $u_2 = (0, 2)$ (C)</p> Signup and view all the answers

What is the result of the dot product for two orthogonal vectors?

<p>It is always 0. (D)</p> Signup and view all the answers

Which property does NOT belong to orthogonal sets of vectors?

<p>They can only exist in two-dimensional space. (D)</p> Signup and view all the answers

Which statement is true regarding orthogonal sets of vectors?

<p>The vectors can have any length. (A)</p> Signup and view all the answers

In the context of orthogonal sets, the notation $U_i . U_j = 0$ signifies what?

<p>The vectors are orthogonal. (C)</p> Signup and view all the answers

What is the value of 'u' when calculated from the equation provided in the content?

<p>40 (B)</p> Signup and view all the answers

What is the result of the expression $2(2)$ based on the content?

<p>4 (D)</p> Signup and view all the answers

If $5(i) = y - j$, what is 'j' given that 'y' is expressed as $5i$?

<p>$j = y - 5i$ (A)</p> Signup and view all the answers

What geometric concept is related to the 'Span' in the context provided?

<p>Linear combinations (D)</p> Signup and view all the answers

For values expressed in the format $||y - y_a|| + 2 = 5$, what does 'y_a' represent?

<p>An arbitrary point in the space (C)</p> Signup and view all the answers

What does the notation $||y - y||$ equal to?

<p>0 (A)</p> Signup and view all the answers

Which of the following expressions represents a line orthogonal to the vector 'u'?

<p>$(-7, 5)$ (B)</p> Signup and view all the answers

Given $nu = 2$ as stated in the content, what is the possible conclusion for 'nu'?

<p>Nu must be double the value of 'u' (A)</p> Signup and view all the answers

If 'y' is expressed in terms of 'z' as $y = 4z$, what does 'y' depend on?

<p>It varies directly with 'z' (D)</p> Signup and view all the answers

What does the expression $||y - L|| = x$ signify in geometric terms?

<p>Distance from point 'y' to line L (C)</p> Signup and view all the answers

What condition must a matrix U meet to have orthonormal columns?

<p>UTU = I_n (C)</p> Signup and view all the answers

If U is a mxn matrix with orthonormal columns, what is true about the inner product of two vectors x and y?

<p>(Ux)(Uy) = 0 iff x . y = 0 (C)</p> Signup and view all the answers

In the context of an mxn matrix U with orthonormal columns, which relationship correctly shows the transformation of vector x?

<p>||Ux|| = ||x|| (C)</p> Signup and view all the answers

What is a characteristic of an orthogonal square matrix U?

<p>U^T = U^-1 (C)</p> Signup and view all the answers

What is the outcome of Ux if U has orthonormal columns and x is a vector?

<p>Ux preserves the length of x. (C)</p> Signup and view all the answers

Which statement about the inner product (Ux)(Uy) is true when U has orthonormal columns?

<p>(Ux)(Uy) equals (x . y). (C)</p> Signup and view all the answers

If U is an mxn matrix with orthonormal columns, what does it imply about the columns of U?

<p>Columns are linearly independent. (C)</p> Signup and view all the answers

What happens to the vector x when multiplied by an orthonormal matrix U?

<p>The direction of x remains unchanged. (A)</p> Signup and view all the answers

Flashcards

Orthogonal Set

A set of vectors in R^n is called an orthogonal set if every pair of distinct vectors in the set is orthogonal.

Orthogonal Vectors

Two vectors are orthogonal if their dot product is zero.

Dot Product

The dot product of two vectors in R^n is calculated by multiplying corresponding components and summing the results.

Dot Product Formula

Given two vectors u = (u1, u2, ..., un) and v = (v1, v2, ..., vn), the dot product is calculated as: u · v = u1v1 + u2v2 + ... + unvn

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Vector in R^n

A vector in R^n has n components, which are real numbers.

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Dot Product of a Vector with Itself

The dot product of a vector with itself is equal to the square of its magnitude.

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Linearly Independent Vectors

A set of vectors is linearly independent if no vector in the set can be expressed as a linear combination of the other vectors.

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Proof of Orthogonality

To show that a set of vectors is orthogonal, you need to calculate the dot product of every pair of distinct vectors in the set and verify that each dot product is zero.

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Orthogonal basis

A set of linearly independent vectors that span a vector space.

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Vector in the subspace

Any linear combination of the vectors in the basis.

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Projection onto a subspace

The projection of a vector onto a subspace finds the closest point in the subspace to the original vector.

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Orthogonal component

The difference between a vector and its projection onto a subspace. It represents the component of the vector that is orthogonal to the subspace.

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Pythagorean theorem in vector spaces

The length squared of the orthogonal component is equal to the length squared of the original vector minus the length squared of its projection onto the subspace.

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Orthonormal Column Invertible Matrix

A matrix U with orthonormal columns is invertible and its inverse is simply its transpose, UT.

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Norm Preservation in Orthonormal Matrix Transformations

For any vector x in R^n, the norm of Ux is equal to the norm of x. This means the transformation preserves the length of vectors.

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Inner Product Preservation in Orthonormal Matrix Transformations

For any two vectors x and y in R^n, the inner product of Ux and Uy is equal to the inner product of x and y. This means the transformation preserves the angle between two vectors.

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Orthogonal Matrix

A matrix U is considered orthogonal if it is a square matrix with orthonormal columns and its inverse is equal to its transpose (UT).

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Invertibility of Orthogonal Matrices

Invertible matrices have a unique inverse. For orthogonal matrices, the inverse is simply the transpose. This makes calculations easier.

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Orthonormal Column Condition for Orthogonal Matrices

The condition UTU = In (where In is the identity matrix) holds true if and only if U has orthonormal columns. This is a key property of orthogonal matrices.

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Orthogonal projection

The projection of a vector onto a subspace is the closest vector in that subspace to the original vector.

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Span

The span of a set of vectors is the set of all possible linear combinations of those vectors.

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Component of a vector

The component of a vector y along a vector u is the scalar multiple of u that best approximates y in the direction of u.

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Decomposition of a vector

The vector y can be written as the sum of its orthogonal projections onto the basis vectors of W.

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Gram-Schmidt Process

The process of finding a set of orthogonal vectors that span the same space as a given set of vectors.

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Linear Combination

A linear combination of vectors is a sum of scalar multiples of those vectors.

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Spanning Set

A set of vectors that can be used to represent any other vector in the same space.

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Normal Vector

A vector perpendicular to a line or a plane. It helps determine the shortest distance from a point to a line or plane.

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Distance from Point to Line

The distance between a point y and a line L. It's the length of the perpendicular line segment connecting y to L.

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Projection of a Vector

The projection of a vector y onto a subspace spanned by a set of vectors. It's the part of the vector y that lies within the subspace.

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Vector Length

The length of a vector. It represents the distance from the origin to the endpoint of the vector.

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Study Notes

Orthogonal Sets

  • A set of vectors {u₁, u₂, ..., up} in ℝn is called an orthogonal set if each pair of distinct vectors from the set is orthogonal. This means ui ⋅ uj = 0 whenever i ≠ j.

Example

  • Show that {u₁, u₂, u₃} is an orthogonal set, where u₁ = [3, -1, 1], u₂ = [1, 2, 2], and u₃ = [-1, -2, 7/2].

  • u₁ ⋅ u₂ = 3(-1) + 1(2) + 1(2) = -3 + 2 + 2 = 1 ≠ 0, which means the set is not orthogonal.

Theorem

  • If S = {u₁, u₂, ..., up} is an orthogonal set of nonzero vectors in ℝn, then S is linearly independent. This means the vectors are not scalar multiples of each other.

  • Hence, S is a basis for the subspace spanned by S.

Definition: Orthogonal Basis

  • An orthogonal basis for a subspace W of ℝn is a basis for W that is also an orthogonal set.

Theorem (weights in a linear combination)

  • Let {u₁, u₂, ..., up} be an orthogonal basis for a subspace W of ℝn. For each y ∈ W, the weights in the linear combination y = c₁u₁ + c₂u₂ + ... + cpup are given by cj = (y ⋅ uj) / (uj ⋅ uj) for j = 1, ..., p.

Example

  • The set {u₁, u₂, u₃}, where u₁ = [3, -1, -1/2], u₂ = [1, 2, 1], and u₃ = [1, -2, 7/2], is an orthogonal basis for ℝ³.

  • Express the vector y = [6, 1, -8] as a linear combination of the vectors in S.

Example Solution

  • Calculate the dot products: y ⋅ u₁ = 11 y ⋅ u₂ = -12 y ⋅ u₃ = -33 u₁ ⋅ u₁ = 1 , u₂ ⋅ u₂ = 6 , u₃ ⋅ u₃ = 33/2
  • Thus, c₁ =(y ⋅ u₁)/ (u₁ ⋅ u₁) = 11/11 =1, and c₂ =(-12/6) = −2 and c₃ =(-33 / (33/2)) = -2.
  • y = 11u₁ - 12u₂ - 33u₃/11 (The actual calculations/ formula are part of this solution.
  • Note: There were issues with the provided example vectors. The set of vectors given for u₁, u₂ and u₃ are not mutually orthogonal.

Orthogonal Projections

  • Given a vector y ∈ ℝn and a vector u ∈ ℝn, the orthogonal projection of y onto u is given by ŷ = (y⋅u)/(u⋅u) ⋅ u.

  • ŷ is a vector in the subspace spanned by u.

  • The component of y orthogonal to u is y - ŷ.

Theorem

  • An m × n matrix U has orthonormal columns if and only if UTU = In

  • ||Ux|| = ||x|| if UTU = In

  • (Ux)⋅(Uy) = x⋅y if U has orthogonal columns

  • (Ux) ⋅ (Uy) = 0 if and only if x ⋅ y = 0

Gram-Schmidt Process

  • An algorithm for producing an orthogonal/normal basis for any subspace of ℝn.

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