Matrix factorization is a class of algorithms used for recommendation systems in machine learning. Matrix factorization algorithms work by decomposing dimensionality.

$$\begin{array}{c|cc} & m_1 & m_2 & m_3 & m_4 \\ \hline u_1 & & w_{12}^{um} & &\\ u_2 & w_{21}^{um} & & &\\ u_3 & & & w_{32}^{um} & \end{array}$$

How do we compute matrix factorizations?

Possible approach: $$\min _{W, Z}\left\|W Z^{T}-X\right\|_{\mathrm{Fro}}^{2}$$

Here $||\cdot||_{Fro}$ denotes the so-called Frobenius norm, i.e.:

$$\|X\|_{\mathrm{Fro}} :=\sqrt{\sum_{i=1}^{s} \sum_{j=1}^{n}\left|x_{i j}\right|^{2}}$$

Then we can add regularization to the model for $W$ and $Z$:

$$\min _{W, Z}\left\|W Z^{T}-X\right\|_{\mathrm{Fro}}^{2}+R_{1}(W)+R_{2}(Z)$$

Here we need to choose suitable regularization functions $R_1$ and $R_2$.

Since $W\cdot Z^{T}$ is equal to a value with more than one solution, thus this optimization problem is not a convex problem. We could use coordinate descent also called alternating minimization to solve this problem.

## Coordinate Descent / Alternating Minimization

keep one fixed. 其基本思路是一次优化一个参数（坐标），轮流循环，将复杂优化问题分解为简单的子问题

Example:

$$W^{k+1}=\arg \min _{W}\left\|W\left(Z^{k}\right)^{T}-X\right\|^{2}$$ $$Z^{k+1}=\arg \min _{Z}\left\|W^{k+1} Z^{T}-X\right\|^{2}$$$$\Rightarrow \left\{ \begin{array}{lr} W^{k+1}=X Z^{k}\left(\left(Z^{k}\right)^{T} Z^{k}\right)^{-1} & \\ Z^{k+1}=\left(\left(\left(W^{k+1}\right)^{T} W^{k+1}\right)^{-1}\left(W^{k+1}\right)^{T} X\right)^{T} & \end{array} \right.$$

## Singular Value Decomposition

We can obtain other kinds of matrix factorizations. A very popular one is the singular value decomposition:

Every matrix $X \in \mathbb{R}^{s \times n}$ can be written as

$$X=U \Sigma V^{T}$$

$U \in \mathbb{R}^{s \times s}$ is unitary matrix, i.e. $U^{T} U=I$

$V \in \mathbb{R}^{n \times n}$ is unitary matrix, i.e. $V^{T} V=I$

$$\Sigma= \left( \begin{array}{lllllll}{ \sigma_{1}} & {0} & {\ldots} & {0} & {0} & {\ldots} & {0} \\ {0} & \sigma_{2} & {\ldots} & {0} & {0} & {\ldots} & {0} \\ {\vdots} & {\vdots} & {\ddots} & {0} & {0} & {\ldots} & {0} \\ {0} & {0} & {\ldots} & \sigma_{s} & {0} & {\ldots} & {0} \\ \end{array} \right) \in \mathbb{R}^{s \times n}=\text { diagonal } \quad \text{for s less than n}$$

$\sigma_1$ is the largest, $\sigma_s$ is the smallest

$$\Sigma= \left( \begin{array}{llll}{ \sigma_{1}} & {0} & {\ldots} & {0} \\ {0} & {\sigma_{2}} & {\ldots} & {0} \\ {0} & {0} & {\ddots} & {0} \\ {0} & {0} & {\ldots} & {\sigma_{n}} \\ {0} & {0} & {\ldots} & {0} \\ {\vdots} & {\vdots} & {\ddots} & {\vdots} \\ {0} & {0} & {\ldots} & {0} \end{array} \right) \in \mathbb{R}^{s \times n}=\text { diagonal } \quad \text{for} s>n$$

Not for unitary matrices, we have:

$$\|U x\|^{2}=\langle U x, U x\rangle=\left\langle x, {U}^{T} U x\right\rangle=\langle x, x\rangle=\|x\|^{2}$$

i.e., rotations do not affect the Euclidean norm (Frobenius norm)

$$\|X V w\|^{2}=\left\|U \Sigma V^{T} V w\right\|^{2}=\langle U \Sigma w, U \Sigma w\rangle==\langle\Sigma w, \Sigma w\rangle=\sum_{i=1}^{S} \sigma_{i}^{2}\left|w_{i}\right|^{2}$$

## Principal Component Analysis

Principal Component Analysis is another matrix factorization algorithm which is similar to SVD.