Finding Inverse Matrices

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Questions and Answers

Which of the following operations is NOT considered an elementary row operation?

  • Swapping two rows.
  • Multiplying a row by a constant.
  • Squaring all elements in a row. (correct)
  • Adding a multiple of one row to another.

What is a key property of all elementary matrices?

  • They are symmetric.
  • They are diagonal.
  • They are invertible. (correct)
  • They are not invertible.

According to the Invertible Matrix Theorem, if a square matrix A is invertible, what must be true about the equation $Ax = b$?

  • It has either no solution or infinitely many solutions.
  • It has a unique solution for every b. (correct)
  • It has no solution for some b.
  • It has infinitely many solutions for every b.

If a matrix can be expressed as the product of elementary matrices, what can be concluded about the matrix?

<p>It is row-equivalent to the identity matrix. (C)</p> Signup and view all the answers

Why is finding the determinant of a matrix useful?

<p>To quickly determine if the matrix is invertible. (D)</p> Signup and view all the answers

What is the determinant of a 1x1 matrix equal to?

<p>The single entry in the matrix. (C)</p> Signup and view all the answers

What geometric quantity does the absolute value of the determinant of a 2x2 matrix represent, when the columns of the matrix are considered as vectors?

<p>The area of a parallelogram. (D)</p> Signup and view all the answers

What are the smaller sub-matrices called when computing determinants by expanding along rows or columns?

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

In the context of determinants, what is a 'minor'?

<p>The determinant of a submatrix formed by deleting a row and a column. (A)</p> Signup and view all the answers

What is the relationship between minors and cofactors in determinant calculation?

<p>Cofactors are signed minors. (B)</p> Signup and view all the answers

If you swap two rows of a matrix, how does it affect the determinant?

<p>The determinant is multiplied by -1. (A)</p> Signup and view all the answers

What does multiplying a row of a matrix by a scalar multiple do to the determinant?

<p>It multiplies the determinant by the scalar. (A)</p> Signup and view all the answers

What happens to the determinant if you add a multiple of one row to another row in a matrix?

<p>The determinant is unchanged. (D)</p> Signup and view all the answers

How is the adjoint of a matrix related to its cofactors?

<p>The adjoint is the transpose of the matrix of cofactors. (C)</p> Signup and view all the answers

Which of the following is a condition for a matrix to be invertible, according to the Invertible Matrix Theorem?

<p>It is row-equivalent to the identity matrix. (D)</p> Signup and view all the answers

What does Cramer's Rule provide?

<p>A formula for solving systems of linear equations using determinants. (A)</p> Signup and view all the answers

In Cramer's Rule, if you are solving for $x_i$ in the system $Ax = b$, what do you do to the matrix A?

<p>Replace the i-th column of A with b. (B)</p> Signup and view all the answers

What characterizes a 'sparse matrix'?

<p>It has 50% or more zero entries. (A)</p> Signup and view all the answers

Why might Cramer's Rule be particularly useful, despite potentially high computational cost?

<p>It is useful for proving further theorems and is efficient with sparse matrices. (A)</p> Signup and view all the answers

What two operations must be defined for a set of objects to be considered a vector space?

<p>Addition and scalar multiplication. (A)</p> Signup and view all the answers

What does it mean for a set to be 'closed' under an operation in the context of vector spaces?

<p>The operation results in an element within the set. (A)</p> Signup and view all the answers

In the vector space $R^3$, what is a general vector written as a linear combination of?

<p>Three unit vectors. (B)</p> Signup and view all the answers

Which of the following is NOT a condition that must be satisfied for a set to be considered a vector space?

<p>Existence of a multiplicative inverse. (D)</p> Signup and view all the answers

Which set can be classified to follow the basic conditions for a Vector Space?

<p>The set of 2 x 2 matrices. (A)</p> Signup and view all the answers

Which of the following sets, with the usual operations of addition and scalar multiplication, forms a vector space?

<p>The set of all 2x2 matrices with real entries. (C)</p> Signup and view all the answers

Given that polynomials with real coefficients follow Vector Space rules, which additional piece of information is needed to prove this?

<p>There is a zero polynomial that satisfies being the additive identity. (A)</p> Signup and view all the answers

For any vector space V over a field F, and for any vector v in V, what must be true about the zero vector?

<p>The zero vector must be unique. (C)</p> Signup and view all the answers

If $c$ is a scalar from the field F and $0$ is the zero vector in vector space V, then what is the result of $c * 0$?

<p>0 (The Zero Vector) (B)</p> Signup and view all the answers

Under what condition, for scalar $c$ in field $F$, and vector $v$ in vector space $V$ can $cv = 0$?

<p>If $c = 0$ <em>or</em> if $v = 0$. (B)</p> Signup and view all the answers

What is a subspace?

<p>A subset of a vector space V that is itself a vector space under the same operations as V. (D)</p> Signup and view all the answers

Which condition must be met for a subset W of a vector space V to be considered a subspace of V?

<p>All of the above. (D)</p> Signup and view all the answers

What is a key difference between any subset of a vector space and being closed under two separate operations?

<p>Being a subset does not mean that the subset will comply under vector space rules. (D)</p> Signup and view all the answers

What is one of the first things you can check to see if a subset is actually a subspace of another?

<p>Checking non-emptiness. (C)</p> Signup and view all the answers

What is the null space of a matrix A?

<p>The set of all vectors $x$ such that $Ax = 0$. (B)</p> Signup and view all the answers

According to Theorem 4.3.12, what statement is correct about the null space?

<p>Consider A as a matrix. The solution set of the homogenous linear system is a subspace of (or ) called the null space. (A)</p> Signup and view all the answers

If a subset $S$ of a vector space $V$ does not contain the zero vector, what can be concluded?

<p>$S$ is definitely not a subspace of $V$. (A)</p> Signup and view all the answers

What is a sufficient check to determine if a solution to an equation is a subspace for (or)?

<p>Zero Vector Check (A)</p> Signup and view all the answers

If a matrix A has an inverse, and that inverse is matrix B, what is the result of multiplying matrix A by matrix B?

<p>The identity matrix (D)</p> Signup and view all the answers

How does the invertibility of an elementary matrix relate to its row operations?

<p>Since elementary matrices correspond to elementary row operations, they are <em>always</em> invertible. (A)</p> Signup and view all the answers

According to the Invertible Matrix Theorem, which of the following conditions ensures that a square matrix A is invertible?

<p>The columns of A span $R^n$. (A)</p> Signup and view all the answers

Why is it useful to know if a matrix can be expressed as the product of elementary matrices?

<p>It guarantees that the matrix is invertible. (A)</p> Signup and view all the answers

If the columns of a 2x2 matrix are viewed as vectors forming a parallelogram, and the determinant of this matrix is zero, what can be said about the area of the parallelogram?

<p>The area is zero. (D)</p> Signup and view all the answers

In calculating the determinant using cofactor expansion, how do you determine the sign (positive or negative) associated with each cofactor?

<p>The sign alternates based on a checkerboard pattern of + and - starting with + in the upper left. (C)</p> Signup and view all the answers

How does the determinant of a matrix $A$ relate to the determinant of its adjoint, adj$(A)$, when $A$ is a $n \times n$ matrix?

<p>det(adj$(A)$) = det$(A)^{n-1}$ (A)</p> Signup and view all the answers

According to the Invertible Matrix Theorem, which of the following statements is true about the determinant of an invertible matrix A?

<p>The determinant of A must be non-zero. (A)</p> Signup and view all the answers

In using Cramer's Rule to solve a system of linear equations, what happens if the determinant of the coefficient matrix A is zero?

<p>Cramer's Rule cannot be applied because the system either has no solution or infinitely many solutions. (D)</p> Signup and view all the answers

What is the primary advantage of using cofactor expansion to find the determinant of a sparse matrix?

<p>It can significantly reduce computations if expansion is done along a row or column with many zeros. (C)</p> Signup and view all the answers

If a set of vectors in $R^n$ fails to meet the condition of closure under scalar multiplication, what can be concluded about the set?

<p>It cannot be a vector space. (B)</p> Signup and view all the answers

In the context of vector spaces, what does it mean for a vector space to be 'closed' under vector addition?

<p>The sum of any two vectors in the space is also within that space. (A)</p> Signup and view all the answers

Consider a set $S$ of all 2x2 matrices with real entries where the determinant of each matrix is equal to 1. With standard matrix addition, and scalar multiplication, is this set closed under addition?

<p>No, because the determinant of the sum of two such matrices may not be 1. (B)</p> Signup and view all the answers

What distinguishes a subspace from any arbitrary subset of a vector space?

<p>A subspace must contain the zero vector, be also closed under vector addition and scalar multiplication. (B)</p> Signup and view all the answers

If W is a subspace of V, and V is a vector space, which of the following operations when applied to elements of W is guaranteed to result in an element that is also in W?

<p>Multiplying a vector in W by any scalar. (A)</p> Signup and view all the answers

What condition regarding the zero vector is necessary for a subset $W$ of a vector space $V$ to potentially be a subspace of $V$?

<p>$W$ must contain the zero vector. (D)</p> Signup and view all the answers

If $W$ is the null space of matrix $A$, then for any vector $x$ in $W$, what is the result of $Ax$?

<p>The zero vector (C)</p> Signup and view all the answers

Theorems regarding the Null Space detail that the set of solutions to $Ax = 0$ form a...

<p>a subspace (C)</p> Signup and view all the answers

If a non-empty subset $S$ of a vector space $V$ does NOT satisfy the condition that for all vectors $u$ and $v$ in $S$, the sum $u + v$ is also in $S$, what can be definitively concluded?

<p>$S$ is not a subspace of $V$. (A)</p> Signup and view all the answers

To determine if the solution set of a nonhomogeneous linear system $Ax = b$ (where $b \neq 0$) is a subspace, what is the most efficient first step?

<p>Check if the zero vector is in the solution set. (A)</p> Signup and view all the answers

Flashcards

Elementary Row Operations

Elementary row operations include swapping rows, multiplying a row by a constant, and adding a multiple of one row to another.

Invertibility of Elementary Matrices

All elementary matrices are invertible, and the inverse of an elementary matrix is another elementary matrix.

Invertible Matrix Theorem

If A is invertible, then Ax = b has a unique solution for every b.

Rank and Row Reduction

If A has rank n, it can be row reduced to the identity matrix using elementary row operations.

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Determinant and Invertibility

A square matrix A is invertible if and only if its determinant is not zero.

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What is a Determinant?

The determinant is a single number calculated from the entries in a matrix.

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Determinant as Area

The area of a parallelogram formed by vectors is given by determinant.

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Minors

A minor is the determinant of a smaller sub-matrix.

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Method of Minors

Minors involves cofactor expansion to compute determinant.

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Determinants and Row Operations

Row operations affect the determinant of a matrix. Swapping rows changes the sign, multiplying a row by a scalar multiplies the determinant by the same scalar, and adding a multiple of a row to another row does not change the determinant.

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Cofactor Definition

The minor with a sign is the cofactor

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Cramer's Rule

Cramer's Rule provides a method to solve a system of linear equations using determinants.

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Vector Space

Vector spaces consist of a set of objects and two operations (addition and scalar multiplication) that satisfy ten specific conditions.

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Closure Property

A vector space must be closed under addition and scalar multiplication. So, the result must reside in the vector space.

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Vector Space Properties

Includes Commutativity, Associativity, Additive Identity, Existence of Additive Inverses Unit Property, associativity of scalar multiplication.

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3D Vector Representation

A general 3D vector can be expressed as a linear combination of the three unit vectors.

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Subspace

Let W be a nonempty subset of a vector space V. If W is a vector space under the same operations of addition and scalar multiplication as used in V, then W is a subspace of V.

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Zero Vector Check

If a subset of a vector space does not contain the zero vector, it cannot be a subspace.

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Study Notes

  • Day 13 discusses finding inverse matrices

Announcements

  • Today involves groups
  • Invertible Matrix Theorem is in Section 2.8
  • Determinants can be found in Section 3.1
  • Grading is in progress
  • In-class exams grading is back
  • Old discussion board posts are not graded

Elementary Matrices

  • Section 2.7 covers elementary matrices
  • Elementary row operations include swapping, multiplying by a constant, and adding a multiple of one row to another

Elementary Matrices (cont.)

  • All elementary matrices are invertible and their inverses are also elementary matrices
  • Row reduction can be expressed by pre-multiplying elementary matrices by the matrix

The Invertible Matrix Theorem

  • If a matrix is invertible, the equation has a unique solution
  • If an equation has a unique solution then it only has a trivial solution
  • If a matrix has a rank, row reduction can be done using elementary row operations

Determinants

  • Chapter 3 briefly mentions determinants
  • Determinants are a way to determine if a matrix is invertible
  • Determinants are a single number calculated from entries in a matrix
  • A square matrix is invertible if and only if its determinant meets a certain condition, to be proved later

Determinants by Cases

  • Case 1: a 1 x 1 matrix
  • Case 2: a 2x2 matrix with row reduction

Geometry of Determinants

  • Area of a parallelogram is equal to base x perpendicular height
  • The definition of cross product is mentioned

Determinants by Cases (cont.)

  • Case 1: a 1 x 1 matrix
  • Case 2: a 2x2 matrix
  • Case 3 is labeled "oh no"

Determinants: 3 x 3: Sarrus' Rule

  • Sarrus' Rule covers finding determinants for 3x3 matrices

Determinants: 3 x 3 (cont.)

  • The determinant calculation becomes complicated
  • Smaller sub-matrices are called minors

Groups

  • Groups and spring break plans are related to finding determinants and invertibility

Method of Minors (Groups)

  • What did you do over spring break?

Determinants and Row Reduction

  • Matrices with lots of zeros are considered "nice"
  • Swapping two rows has an impact, think about the 2x2 case
  • Multiplying a row by a scalar multiple has an impact
  • Adding a multiple of one row to another has an impact

Minors vs. Cofactors

  • The method of minors is the same as "cofactor expansion"
  • The cofactor is the determinant of the minor times a sign (-1)^(i+j)
  • The adjoint is the transpose of the matrix, but each element has been replaced by its cofactor

Cramer's Rule

  • If a system has a unique solution
  • Then is the matrix with a column replaced

Cramer's Rule Example

  • Solve a system using the usual way as well as Cramer's Rule

Why Use Cramer's Rule and Cofactor Expansion?

  • Cramer’s Rule is useful for proving further theorems
  • Cofactor expansion is useful when there are a lot of zeros
  • Sparse Matrix: 50% or more zero entries
  • People developed some of these methods for numerical stability

Real Vector Space

  • A vector space is defined by a set of objects and two operations
  • Objects in this case are vectors
  • Operations are addition and scalar multiplication by a real number
  • The set is closed under the operations; the result is another vector

Real Vector Space (cont)

  • Scalar multiplication of vectors has certain properties
  • Discusses vector multiplication, cross products or dot products

Familiar Special Case in 3D space

  • Focuses on "unit vectors" and a "standard basis"
  • A general vector can be written as a linear combination of three unit vectors

Vector Spaces: Properties

  • A non-empty set must have closure under addition and closure under scalar multiplication
  • It must follow commutativity and associativity of addition
  • It must have a zero vector (additive identity) and existence of additive inverses
  • Includes a unit property or multiplicative identity
  • Associativity of scalar multiplication over vector addition is necessary
  • Distributive property of scalar multiplication over vector addition
  • Distributive property of scalar multiplication over scalar addition is required

Vector Spaces: Examples

  • Consider the set of 2 x 2 matrices
  • Or consider the set of complex numbers

Differential Equations Context

  • Discusses the most basic spring-mass system
  • Vector Spaces - Not Just for Vectors Anymore

More examples of vector spaces

  • Real or complex vectors
  • Matrices with real or complex entries
  • The set of all polynomials of degree with real coefficients
  • Set of all real-valued functions continuous on the interval
    • Have at least derivatives all continuous on the interval

Vector Space of Polynomials

  • Let be a non-empty set ,the set of all polynomials of degree with real coefficients
  • Define 2 operations: addition and scalar multiplication
  • Scalar in or
  • Ten conditions must be satisfied

Nice Properties of Vector Spaces

  • Theorem 4.2.7: Let be a vector space. Then ( is either or )
  • The zero vector is unique
  • and If is a scalar and such that then either or.

Polynomial Vector Space Example

  • Closure under addition and closure under scalar multiplication

Subspaces of a Vector Space

  • A nonempty subset of a vector space is a subspace if itself is a vector space under the operations
  • This is closed under the operations of addition and scalar multiplication in

Theorem 4.3.2: "Thank Goodness"

  • Let be a nonempty subset of a vector space. Then a closed subspace meets this theorem

Solutions of a Linear System

  • Solving a linear system
  • Zero Vector Check

The Null Space Theorem 4.3.12

  • The solution set of the homogenous linear system is a subspace of (or) called the null space
  • If there are two vectors that are solutions, and, so +, and so it’s closed under +, the null space criteria has been met

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