Shreyas’ Notes

# MATH 355

• existence

• uniqueness

• coefficient matrix

• augmented matrix

Linear algebra formalizes the procedure to find all solutions to a linear system of equations when they exist, or to prove that they don’t exist.

### Linear Transformations §

$T : V \rightarrow W$ is a linear transformation if:

• For $u, v \in V$, $T(u + v) = T(u) + T(v)$
• For $v \in V$ and scalar $a$, $T(av) = a \cdot T(v)$

Properties:

• $T(0) = 0$
• any linear transformations respects linear combinations

### Onto and Into §

The image of a linear transformation $T : V \rightarrow W$ is the set $\{T(v) : v \in V\}$. The image is also the span of the columns of $A$, the std matrix of $T$.

A linear transformation $T : V \rightarrow W$ is onto if the image of $T$ is $W$.

A linear transformation $T : V \rightarrow W$ is into if for every $u, v \in V$, $T(u) = T(V) \implies u = v$.

$T$ is onto $T$ is into
$A$'s RREF has a pivot in each row $A$'s RREF has a pivot in each column
The columns of $A$ span $\mathbb{R}^m$ The columns of $A$ are LI
$Ax = w$ has a solution for each $w \in \mathbb{R}^m$ $Ax = 0$ has only the trivial solution $x = 0$
The image of $T$ is $\mathbb{R}^m$ The kernel of $T$ is $\{0\}$
Existence Uniqueness

### Matrix Multiplication §

If $S : \mathbb{R}^p \rightarrow \mathbb{R}^n$ and $T : \mathbb{R}^n \rightarrow \mathbb{R}^m$ are linear transformations, $T \circ S : \mathbb{R}^p \rightarrow \mathbb{R}^m$ is a linear transformation.

If the $m \times n$ matrix $A$ is the std matrix for $T : \mathbb{R}^n \rightarrow \mathbb{R}^m$ and the $n \times p$ matrix $B$ is the std matrix for $S : \mathbb{R}^p \rightarrow \mathbb{R}^n$, then $AB$ will be a $m \times p$ matrix which is the std matrix for $T \circ S : \mathbb{R}^p \rightarrow \mathbb{R}^m$.

For a $m \times n$ matrix $A$, $A I_n = A$ and $I_m A = A$.

### Inverses §

Consider $T: V \rightarrow W$ and $S : W \rightarrow V$. $S$ and $T$ are inverses of each other if $S(T(V)) = v \forall v \in V$ and $T(S(w)) = w \forall w \in W$.

If $S = T^{-1}$ exists, $T$ is invertible.

A linear transformation is invertible iff it is both into and onto.

Consider a $m \times n$ matrix $A$ and a $n \times m$ matrix $B$. $A$ and $B$ are inverses of each other if $AB = I_m$ and $BA = I_n$.

If $B = A^{-1}$ exists, $A$ is invertible.

### Transposes and Inverses §

$A = {(A^t)}^t$

A matrix $A$ is symmetric if $A = A^t$

If $A$ and $B$ are matrices such that $AB$ is defined, ${(AB)}^t = B^t A^t$.

If $A$ is invertible, $A^t$ is invertible. ${(A^t)}^{-1} = {(A^{-1})}^t$.

#### Invertible Matrix Theorem §

Consider a linear transformation $T : \mathbb{R}^n \rightarrow \mathbb{R}^n$ with a $n \times n$ standard matrix $A$. The following statements are equivalent:

• $T$ is into
• $\ker(T) = \{0\}$
• The columns of $A$ are linearly independent
• $Ax = 0$ has only the trivial solution
• $A$'s RREF has a pivot in every column (no free vars)
• $A$'s RREF is the identity matrix
• $A$'s RREF has a pivot in each row
• For every $v \in \mathbb{R}^n$, $Ax = v$ is consistent
• The columns of $A$ span $\mathbb{R}^n$
• $T$ is onto
• $T$ is invertible
• $A$ is invertible
• $A^t$ is invertible
• The rows of $A$ span $\mathbb{R}^n$
• The rows of $A$ are linearly independent

To calculate the matrix of a matrix, row reduce it with the identity matrix on the right side.

### Elementary Matrices §

#### Scaling a row §

$E_1 = \left[ \begin{array}{ccccccc} 1 & & & & & & \\ & \ddots & & & & & \\ & & 1& & & & \\ & & & c & & & \\ & & & & 1 & & \\ & & & & & \ddots & \\ & & & & & & 1 \end{array} \right]$

#### Adding $c$ times a row to another row §

$E_2 = \left[ \begin{array}{ccccccc} 1 & & & & & & \\ & \ddots & & & & & \\ & & 1& & & & \\ & & & \ddots & & & \\ & & c & & 1 & & \\ & & & & & \ddots & \\ & & & & & & 1 \end{array} \right]$

#### Swapping two rows §

$E_3 = \left[ \begin{array}{ccccccccccc} 1 & & & & & & & & & & \\ & \ddots & & & & & & & & & \\ & & & 0 & & & & 1 & & & \\ & & & & 1 & & & & & & \\ & & & & & \ddots & & & & & \\ & & & & & & 1 & & & & \\ & & & 1 & & & & 0 & & & \\ & & & & & & & & & \ddots & \\ & & & & & & & & & & 1 \end{array} \right]$

### Dimension §

If $V$ is spanned by a finite list of vectors, the dimension of $V$ is the number of vectors in a basis. If $V$ is nto spanned by a finite set, $V$ is infinite-dimensional.

Any two bases of $V$ have the same size.

Any set of more than $n$ vectors is linearly dependent.

Consider a $n$-dimensional vector space $V$:

• if $\{v_1, \cdots, v_n\}$ are LI, they also span $V$ and hence form a basis
• if $\{v_1, \cdots, v_n\}$ span $V$, they are also LI and hence form a basis

## Matrices Relative to Bases §

Let $T: V \rightarrow W$. $B = \{b_1, \dots, b_n\}$ is a basis for $V$ and $C$ is a basis for $W$. If $A$ is the matrix for $T$ relative to $B$ and $C$, $A {[v]}_B = {[T(v)]}_C$ for all $v \in V$.

$A$ has the columns: ${[T(b_1)]}_C, \dots, {[T(b_n)]}_C$.

$T$ is invertible iff $A$ is invertible.

$A^{-1}$ is the matrix for $T^{-1}$ relative to $C$ and $B$.

## Similarity and Diagonalization §

A square $n \times n$ matrix $A$ is diagonalizable if it is similar to some diagonal matrix $D$.

If $A$ has $n$ linearly independent eigenvectors $\{v_1, \dots, v_n\}$, $A$ is diagonalizable and $A = P D P^{-1}$, where $P$ has columns $v_1, \dots, v_n$ and $D$ has $\lambda_1, \dots, \lambda_n$ along its diagonal.

## Eigentheory §

Any list of eigenvectors for a matrix $A$ with distinct eigenvalues must be linearly independent.

A $n \times n$ matrix has at most $n$ distinct eigenvalues.

If a $n \times n$ matrix has $n$ distinct eigenvalues, it has an eigenbasis and is therefore diagonalizable.

If $A$ is a square matrix and $\lambda$ is a scalar, $\ker (A - \lambda I)$ (plus $\vec 0$) is the $\lambda$-eigenspace of $A$.

The characteristic polynomial of $A$ in $\lambda$ is $|A - \lambda I|$. A scalar is an eigenvalue of $A$ iff it is a solution to $|A - \lambda I = 0$.

If $\lambda$ is an eigenvalue for $A$, the algebraic multiplicity of $A$ is the number of times $\lambda$ appears as a root of the characteristic poly (aka the exponent of the corresponding term). The geometric multiplicity of $\lambda$ is the dimension of the $\lambda$-eigenspace ($\dim A - \lambda I$).

## Complex Numbers §

Let $z = a + bi$. $\Re(z) = a = \frac{z + \overline{z}}{2}$ and $\Im(z) = b = \frac{z - \overline{z}}{2i}$.

Fundamental theorem of algebra: Any non-constant polynomial with real or complex coefficients factors completely into linear factors over $\mathbb{C}$.

If $f$ is a polynomial with real coefficients, and $f(z) = 0$, $f(\overline{z})$ is also zero.

Norm: $|z| = \sqrt{a^2 + b^2}$

Argument: $\tan^{-1} \frac{b}{a}$

$|zw| = |z| \cdot |w|$ and $\arg(zw) = \arg(z) + \arg(w)$

## Complex Eigenvalues §

If $v$ is an eigenvector ofor a real matrix with eigenvalue $\lambda$, then $\overline v$ is also an eigenvector with eigenvalue $\overline \lambda$.

Suppose $A$ is a real $2 \times 2$ matrix. Suppose that $\lambda = a + bi$ is a non-real complex eigenvalue for $A$, with a corresponding eigenvector $v$. Let $x = \Re(v)$ and $y = -\Im(v)$. Then $B = \{x; y\}$ is a basis of $\mathbb{R}^2$, and the matrix for $A$ with respect to $B$ is $\begin{bmatrix}a & -b \\ b & a\end{bmatrix}$.

## Inner Products §

For two vectors $v = [a_1, \dots, a_n]$ and $w = [b_1, \dots, b_n]$ in $\mathbb{R}^n$, $v \dot w = \langle v, w \rangle = a_1 b_1 + \dots + a_n b_n$.

• $\langle v, w \rangle = \langle w, v \rangle$
• $\langle av, w \rangle = a \cdot \langle v, w \rangle$
• $\langle v, v \rangle \geq 0$
• $\langle u + v, w \rangle = \langle u, w \rangle + \langle v, w \rangle$

Norm/Magnitude $||v|| = \sqrt{\langle v, v \rangle}$

The distance between two vectors $v$ and $w$ is $||v - w||$.

For two vectors $v$, $w$ in a vector space $V$ that has an inner product, $v$ and $w$ are orthogonal if $\langle v, w \rangle = 0$. Additionally, $v$ and $w$ are orthogonal iff ${||v + w||}^2 = {||v||}^2 + {||w||}^2$.

A list $\{v_1, \dots, v_n\}$ is orthogonal iff $\langle v_i, v_j \rangle = 0$ for $i \neq j$. If the list is orthogonal and $||v_i|| = 1$ for all $i$, it is also orthonormal.

A list of non-zero orthogonal vectors is linearly independent.

## Orthogonal Projections §

For a $v \in V$; $W$, a subspace of $V$; $\{w_1, \dots, w_k\}$, an orthonormal basis of $W$; $\hat v = \sum_{i = 1}^k \langle v, w_i \rangle w_i$.

1. $\hat v \in W$
2. $v - \hat v$ is perpendicular to all vectors in $W$
3. $\hat v$ is the only vector that satisfies (1) and (2)
4. $\hat v$ is the closest vector to $v$ in $W$

If $W$ has an orthogonal basis $\{u_1, \dots, u_k\}$, $\hat v = \sum_{i = 1}^k \langle v, w_i \rangle \frac{w_i}{\langle w_i, w_i \rangle}$.

$\langle v, w \rangle = v^t w$

If $A$ is an $n \times n$ matrix and the columns of $A$ form an ONB of $\mathbb{R}^n$, $A^t A = I_n$ and $A^t = A^{-1}$.

## Orthogonal Decomposition §

$V$ is the direct sum of $U$ and $W$ if $U + W = V$ and $U \cap W = 0$. $V = U \oplus W$.

Let $W$ be a subspace of $V$. $W^\perp$, the orthogonal complement of $W$, is $\{v \in V : \langle v, w \rangle = 0 \forall w \in W\}$. $W^\perp$ is a subspace of $V$. Additionally, $V = W \oplus W^\perp$.

${(W^\perp)}^\perp = W$

${(\ker A)}^\perp = im A^t$ and ${(im A)}^\perp = \ker A^t$

## Least Squares §

Let $A$ be a $m \times n$ matrix and $v \in \mathbb{R}^m$. $A^t A x = A^t v$ is consistent, and its solutions are least-squares solutions to $Ax = v$.

## Orthogonal Matrices §

If $A$ is symmetric, eigenvectors with distinct eigenvalues are orthogonal.

If $A$ is a real, symmetric matrix, $A$ is orthogonally diagonalizable.

For an $n \times n$ matrix $A$, the following are equivalent:

• $A$ is an orthogonal matrix
• The columns form an orthonormal basis for $\mathbb{R}^n$
• $A^t = A^{-1}$
• $A$ preserves inner products: $\langle Av, Aw \rangle = \langle v, w \rangle$
• $A$ preserves norms: $||Av|| = ||v||$

## Spectral Theorem §

$A$ is symmetric if and only if $A$ is orthogonally diagonalizable.

Spectral theorem: For a symmetric matrix $A$:

• All roots of $|A - \lambda I|$ are real
• The geometric multiplicity of each eigenvalue equals its algebraic multiplicity
• Eigenspaces with distinct eigenvalues are mutually orthogonal
• $A$ is orthogonally diagonalizable