# 4. Singular Value Decomposition

The singular value decomposition makes use of the observation that matrices are essentially operations on a unit hypersphere. If $A$ has rank $r$, then exactly $r$ lengths of the hyperellipse, $\sigma_i$, are nonzero. If $m\geq n$, then at most $n$ will be nonzero.

Definition 4.1.1 (Singular values). Suppose we have the unit sphere $S$ and $A \in \R^{m\times n}$. The singular values of $A$, written $\sigma_1, \dots, \sigma_n$, are the lengths of the $n$ principal semiaxes of $AS$.

Definition 4.1.2 (Left singular values). The left singular values of $A$ are the unit vectors ${u_1, \dots, u_n}$ in the directions of the principal semiaxes of $AS$. The vector $\sigma_i u_i$ is the $i$th largest principal semiaxis of AS.

Definition 4.1.3 (Right singular values). The right singular values of $A$, ${v_1, \dots, v_n}$ are the preimages of the principal semiaxes of $AS$.

$Av_i = \sigma_iu_i.$

Theorem 4.1.4 (Singular value decomposition). The collection of vectors that make the principal semiaxes of $AS$ can be written as the matrix equation

$AV = U\Sigma\\A = U\Sigma V^T$

where $U\in \R^{m\times m}$ and $V\in \R^{n\times n}$. Both $V$ and $U$ are orthonormal (unitary) and $\Sigma$ is diagonal along the first $n$ rows. The values of $\Sigma$ are the singular values. In this case, $A$ does not need to have full rank.

## Compact/Reduced/Thin SVD

The reduced SVD of $A$ is a similar transformation where $\hat \Sigma$ is $n\times n$ and $\hat U$ is $m\times n$. Here, $\hat U$ has less columns than $U$ since it has rank $n$.

$A = \hat U \hat \Sigma V^T.$

## SVD and Norms

Using SVD, the norm of the matrix is equal to its largest singular value.

$\|A\|_2 \|U\Sigma V^T\| = \|\Sigma \|_2 = \sigma_1.$

## Relationship with Eigendecomposition

Theorem. The nonzero singular values of $A$ are the square roots of the nonzero eigenvalues of $A^TA$ or $AA^T$:

$A^TA = V\Sigma U^TU\Sigma V^T = V\Sigma^2V^T.$

From this, we see that $|A|_2 = \sqrt{\lambda_{max}(A^TA)} = \sqrt{\sigma^2} = \sigma$. The values of $\Sigma^2$ are the eigenvalues.

Theorem. If $A = A^T$ ($A$ is symmetric), then the singular values of $A$ are the absolute values of the eigenvalues of $A$.

$A = X\Lambda X^T = X|\Lambda|\text{sign}(\Lambda|) X^T$

where $X$ is orthonormal. There is no guarantee that the eigenvalues are non negative.

## Solving Linear Systems

SVD can be used to solve equations. Suppose

$Ax = b.$

By the SVD,

$U\Sigma V^Tx = b\\ \Sigma V^Tx = U^Tb\\ \Sigma y = U^Tb\\ y = \Sigma^{-1}U^Tb\\ x = V\Sigma^{-1}U^Tb.$

## Least Squares

Suppose w want to solve the equation

$\min_x\|Ax-b\|_2.$

By SVD, and since the 2 norm is invariant under orthogonal transformations,

$\begin{gather} \|Ax-b\|_2\\ = \|U\Sigma V^Tx - b\|_2\\ = \|\Sigma V^Tx - U^Tb\|_2.\\ = \|\begin{bmatrix}\Sigma\\0\end{bmatrix}V^Tx - \begin{bmatrix}U_1^Tb\\U_2^Tb\end{bmatrix}\|_2\\ \|\Sigma V^Tx - U_1^Tb\|_2\\ = \Sigma V^T x = U_1^Tb\\ x = V\Sigma^{-1}U_1^Tb. \end{gather}$

## Rank/Range/Null Space

Theorem. The rank of $A$ is $r$, the number of nonzero singular values.

Proof. $U$ and $V^T$ have full rank since they are orthogonal. Therefore,

$\text{rank}(A) = \text{rank}(\Sigma).$

Since $\Sigma$ is diagonal, the range of $\Sigma$ is just the span of its singular values. Therefore, its rank is the number of nonzero singular values.

Theorem. The range of $A$ is the span of the vectors of $U$. The null space of $A$ is the span of the vectors of $V$.

Proof.

$\text{range}(A) = \{Ax \mid x\in \R^n\}\\ = \{U\Sigma V^T x \mid x\in \R^n\}.$

Since $V^T$ has the same number of pivotal columns as $A$, they have the same span.

$\{U\Sigma V^T x \mid x\in \R^n\} = \{U\Sigma y \mid y \in \R^n\}.$

Since $\Sigma$ is diagonal, each term in $\Sigma y$ is equal to $\sigma_iy_i$. Therefore, the span of $A$ is the span of $U$.

Since $U$ has full rank, it has a trivial null space. Therefore, the null space of $A$ is equal to the null space of $V^T$.

## SVD as low rank approximation

Theorem. Another interpretation of the singular value decomposition is that $A$ is the sum of $r$ rank 1 matrices:

$A = \sum_{i=1}^r\sigma_iu_iv_i^T.$

Demo. An image can be compressed by representing its matrix as rank 1 matrices. For a matrix with high ranks, $\sigma_n$ is much greater than the other singular values. These can be discarded.

using PyPlot
using Images
using LinearAlgebra