# Lecture 004

## Vectors

Planer Vector: $\mathbb{R}^2: \begin{bmatrix} x\\ y\\ \end{bmatrix} \in \mathbb{R}^2$ 3-Dim Vector: $\mathbb{R}^n: \begin{bmatrix} x_1\\ ...\\ x_n\\ \end{bmatrix} \in \mathbb{R}^n$

Linear Combination: for $v_1, ..., v_k \in \mathbb{R}^k$, $c_1, ..., c_k \in \mathbb{R}^k$, $c_1v_1 + ... + c_kv_k \in \mathbb{R}^k$ is a linear combination of $v_1, ..., v_k \in \mathbb{R}^k$.

• linear combination of x is the result of map elements of x with a constant in front.

• typically element-wise multiplication

$\overrightarrow{u} \cdot \overrightarrow{v} = \overrightarrow{u} \cdot \overrightarrow{w} \iff (\overrightarrow{v} - \overrightarrow{w}) \perp \overrightarrow{u}$

A matrix times a column vector is the linear combination of columns of the matrix (split into columns).

• Example 1: $\begin{pmatrix} 2&0\\ 1&3\\ \end{pmatrix} \cdot \begin{pmatrix} 1\\ 2\\ \end{pmatrix} = 1\begin{pmatrix} 2\\ 1\\ \end{pmatrix} + 2\begin{pmatrix} 0\\ 3\\ \end{pmatrix} = \begin{pmatrix} 2 \cdot 1 + 0 \cdot 2\\ 1 \cdot 1 + 3 \cdot 2\\ \end{pmatrix} = \begin{pmatrix} 2\\ 7\\ \end{pmatrix}$

• Example 2: $\begin{bmatrix} 1&1\\ 2&3\\ 3&4\\ \end{bmatrix} \langle{c, d}\rangle = c \begin{bmatrix} 1\\ 2\\ 3\\ \end{bmatrix} + d \begin{bmatrix} 1\\ 3\\ 4\\ \end{bmatrix}$

## Matrix

Let $A \in \mathbb{R}^{M\times N}$ (denotes $B = (b_{ij})_{MN}$) and $A \in \mathbb{R}^{M\times E}$, then $AB \in \mathbb{R}^{M \times E}$.

Matrix Multiplication Identity:

• Associative: $ABC = (AB)C = A(BC)$

• Distributive: $A(B+C) = AB + AC$

• Note: $AB = 0$ does not imply $A = 0 \lor B = 0$

### Matrix Multiplication

Definition 1: $C = (c_{ij})_{ij}$ where $c_{ij}$ is the i-th row of A times j-th column of B = $\sum_{l=1}^n a_{il} b_{lj}$

Definition 2: j-th column of $AB$ is $A \cdot \text{ j-th column of }B$

• Result: every column of AB is a linear combination of columns of A

Definition 3: let $v$ be vector, $A$ be matrix. $vA$ is a linear combination of ROWS of A ($va = v_1A[:,1] + v_2A[:,2] +...+v_mA[:,m]$) (by $(vA)_j = v\cdot A[:, j]$)

• Result: every row of AB is a linear combination of rows of B

Corollary: Columns of $AB$ is a linear combination of the columns of $A$

Corollary: Rows of $AB$ are linear combinations of the rows of $B$ (by $C_i = A_i \times B$)

### Special Matrix

$I_n$: identity matrix of $n \times n$

• Multiplication Identity: $AI = IA = A$

### Elementary Matrices

Elementary Matrices: matrix describe row operation

Diagonal Matrices ($D$): only diagonal non-zero, row multiply of identity matrix.

Permutation Matrices ($P$): row swap of identity matrix (every row and column there is exactly one 1 and others are 0)

### Inverse Matrices

Inverse (square matrix only): let $A, B \in R^{n \times n}$, if $AB = BA = I$, then $B = A^{-1}$

• Note: not all matrices have an inverse

• $\exists A^{-1} \implies A^{-1} \text{ is unique}$

• $AB = I \implies BA = I$

• if $A$ has an inverse, $(A^{-1})^{-1} = A$

Left Inverse: $B$ is left inverse of $A$ iff $BA = I$ Right Inverse: $B$ is left inverse of $A$ iff $AB = I$

• Left Inverse = Right Inverse

• If left inverse exists, right inverse exists ($AB = I \implies BAB = BI \implies BAB = IB \implies BA = I$)

Invertible: if the inverse of a matrix exists

• $\begin{pmatrix} 0 & 0\\ 0 & 0\\ \end{pmatrix}$ is not invertible because $AB = BA = 0 \neq I$

• for the inverse of matrix $\begin{pmatrix} a & b\\ c & d\\ \end{pmatrix}$ is $\frac{1}{ad-bc} \begin{pmatrix} d & -b\\ -c & a\\ \end{pmatrix}$ if $ad - bc \neq 0$

• Product: if both $A, B \in \mathbb{R}^{n \times n}$ are invertible, then $AB$ is invertible and $(AB)^{-1} = B^{-1}A^{-1}$ // WARNING: be careful

Uniqueness: if a inverse of a matrix exists, it is unique.

• Left inverse is unique: $XA = I \land YA = I \implies X = Y$

• Right inverse is unique: $AX = I \land AY = I \implies X = Y$

Matrix inverse in System: if $Ax = b$ for $A \in \mathbb{R}^{n \times n}, b \in \mathbb{R}^n$ and $A$ is invertible, then $x = A^{-1}b$ is the unique solution

• if a matrix has a unique solution, then it is invertible

Power of Matrix: $A^n = AAAA....A$ with $n$ many $A$s.

• if $A \in \mathbb{R}^{n \times n}$ is invertible, then $A^k$ is invertible and $(A^k)^{-1} = (A^{-1})^k = A^{-k}$

Transpose: transpose of $A \in \mathbb{R}^{n \times n}$ is $A^\intercal$ (by reflect on main diagonal)

• B is the transpose of A if the i-th column of A has the same entries in the same order as the i-th row of B

• $A \text{ is invertible } \implies A^\intercal \text{ is invertible}$

• $(AB)^T = B^TA^T$

Theorem: $A \in \mathbb{R}^{n \times n}$, the following are equivalent

• $A$ is invertible

• $Ax = b$ has a unique solution for every $b \in \mathbb{R}^n$ ($A$ is Non-Singular Matrix)

• The only solution for $Ax = 0$ is $x = 0$

• Using row operation on $Ax = 0$ can reach $\begin{pmatrix} x_1 & 0 & 0\\ 0 & x_2 & 0\\ 0 & 0 & x_3\\ \end{pmatrix} = 0$

• Using row operation on $Ax = b$ can reach $\begin{pmatrix} x_1 & 0 & 0\\ 0 & x_2 & 0\\ 0 & 0 & x_3\\ \end{pmatrix} = \text{ something}$

• $A$ is a product of elementary matrices

Gauss Jordan Method:

• If the matrix is not invertible, then you will loose a pivot and find a row of zero.

Lower Triangular Matrix: $\begin{pmatrix} \circ & 0 & 0 & 0\\ x & \circ & 0 & 0\\ x & x & \circ & 0\\ x & x & x & \circ\\ \end{pmatrix}$

Upper Triangular Matrix: $\begin{pmatrix} \circ & x & x & x\\ 0 & \circ & x & x\\ 0 & 0 & \circ & x\\ 0 & 0 & 0 & \circ\\ \end{pmatrix}$

$LU$ factorization: Every invertible matrix $A$ can be written as a product of a lower and an upper triangular matrix $A = LU$

• $E_{a, b}$ is elementary matrix that uses row $b$ to modify row $a$.

• $A = LU$ ($L^{-1}A = U$) where $L^{-1} = E_0E_1E_2...E_{n-1}$ and $U$ is upper triangular

• Using $LU$ to find $x$: to solve $A \overrightarrow{x} = \overrightarrow{b}$, $\overrightarrow{a} = U^{-1}(L^{-1}\overrightarrow{b})$

• LU factorization is not unique

• To Find LU:

• When doing Gaussian Elimination, keep extended matrix started from identity matrix on the left
• $R_i = R_i - (n)R_j$: write $n$ in $I_{i, j}$
• result will be LU
• // WARNING: this process is not the same as finding $L^1$ (Gauss Jordan Method), this method find $L$ directly.
• // WARNING: to find $L^{-1}$, $I$ do the same operation with $A$ doing Gaussian Elimination
• To solve $Ax = b$, find $Ux$ in $L(Ux) = b$, then find $x$ in $Ux = (Ux)$.

Matrix Optimization using LU:

• finding $LU$ costs $n^3$

• finding $L^{-1}\overrightarrow{b}$ costs $n^2$

• finding $\overrightarrow{a}$ costs $n^2$

• if there are $k$ similar $A$ with different $b$, then it cost $n^3 + 2kn^2$ in total compared to $kn^3$

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