## Unit2.3.6Best rank-k approximation

We are now ready to answer the question "How do we find the best rank-k approximation for a picture (or, more generally, a matrix)? " posed in Unit 2.1.1.

The proof of this theorem builds on the following insight:

###### Homework2.3.6.1.

Given $A \in \C^{m \times n}\text{,}$ let $A = U \Sigma V^H$ be its SVD. Show that

\begin{equation*} A v_j = \sigma_j u_j \mbox{ for } 0 \leq j \lt \min( m, n ), \end{equation*}

where $u_j$ and $v_j$ equal the columns of $U$ and $V$ indexed by $j \text{,}$ and $\sigma_j$ equals the diagonal element of $\Sigma$ indexed with $j \text{.}$

Solution

W.l.o.g. assume $n \leq m \text{.}$ Rewrite $A = U \Sigma V^H$ as $A V = U \Sigma \text{.}$ Then

\begin{equation*} \begin{array}{l} A V = U \Sigma ~~~=~~~~\lt \mbox{ partition }\gt \\ A \left( \begin{array}{c | c | c } v_0 \amp \cdots \amp v_{n-1} \end{array} \right) \\ ~~~~~~~= \left( \begin{array}{c | c | c | c | c | c} u_0 \amp \cdots \amp u_{n-1} \amp u_n \amp \cdots \amp u_{m-1} \end{array} \right) \left( \begin{array}{c | c | c } \sigma_0 \amp \cdots \amp 0 \\ \hline \vdots \amp \ddots \amp \vdots \\ \hline 0 \amp \cdots \amp \sigma_{n-1} \\ \hline 0 \amp \cdots \amp 0 \\ \hline \vdots \amp \amp \vdots \\ \hline 0 \amp \amp 0 \end{array} \right) \\ ~~~=~~~~\lt \mbox{ multiply out } \gt \\ \left( \begin{array}{c | c | c } A v_0 \amp \cdots \amp A v_{n-1} \end{array} \right) = \left( \begin{array}{c | c | c } \sigma_{0} u_0 \amp \cdots \amp \sigma_{n-1} u_{n-1} \end{array} \right). \end{array} \end{equation*}

Hence $A v_j = \sigma_j u_j$ for $0 \leq j \lt n \text{.}$

First, if $B$ is as defined, then $\| A - B \|_2 = \sigma_{k} \text{:}$

\begin{equation*} \begin{array}{l} \| A - B \|_2 \\ ~~~=~~~~ \lt \mbox{ multiplication with unitary matrices preserves 2-norm } \gt \\ \| U^H ( A - B ) V \|_2 \\ ~~~=~~~~ \lt \mbox{ distribute } \gt \\ \| U^H A V - U^H B V \|_2 \\ ~~~=~~~~ \lt \mbox{ use SVD of } A \mbox{ and partition } \gt \\ \left\| \Sigma - \FlaOneByTwo{U_L}{U_R}^H B \FlaOneByTwo{V_L}{V_R} \right\|_2 \\ ~~~=~~~~ \lt \mbox{ how } B \mbox{ was chosen } \gt \\ \left\| \FlaTwoByTwo{\Sigma_{TL}}{0}{0}{\Sigma_{BR}} - \FlaTwoByTwo{\Sigma_{TL}}{0}{0}{0} \right\|_2 \\ ~~~=~~~~ \lt \mbox{ partitioned subtraction } \gt \\ \left\| \FlaTwoByTwo{0}{0}{0}{\Sigma_{BR}} \right\|_2 \\ ~~~=~~~~ \lt \gt \\ \left\| {\Sigma_{BR}} \right\|_2 \\ ~~~=~~~~ \lt \Sigma_{TL} \mbox{ is } k \times k \gt \\ \sigma_k \end{array} \end{equation*}

(Obviously, this needs to be tidied up for the case where $k \gt \rank( A ) \text{.}$)

Next, assume that $C$ has rank $r \leq k$ and $\| A - C \|_2 \lt \| A - B \|_2 \text{.}$ We will show that this leads to a contradiction.

• The null space of $C$ has dimension at least $n - k$ since $\dim( \Null( C ) ) = n - r \text{.}$

• If $x \in \Null( C )$ then

\begin{equation*} \| A x \|_2 = \| ( A - C ) x \|_2 \leq \| A - C \|_2 \| x \|_2 \lt \sigma_{k} \| x \|_2. \end{equation*}
• Partition $U = \left( \begin{array}{c | c | c} u_0 \amp \cdots \amp u_{m-1} \end{array} \right)$ and $V = \left( \begin{array}{c | c | c} v_0 \amp \cdots \amp v_{n-1} \end{array} \right) \text{.}$ Then $\| A v_j \|_2 = \| \sigma_j u_j \|_2 = \sigma_j \geq \sigma_{k}$ for $j = 0, \ldots, k \text{.}$

• Now, let $y$ be any linear combination of $v_0, \ldots, v_k \text{:}$ $y = \alpha_0 v_0 + \cdots + \alpha_k v_k \text{.}$ Notice that

\begin{equation*} \| y \|_2^2 = \| \alpha_0 v_0 + \cdots + \alpha_k v_k \|_2^2 = \vert \alpha_0 \vert^2 + \cdots \vert \alpha_k \vert^2 \end{equation*}

since the vectors $v_j$ are orthonormal. Then

\begin{equation*} \begin{array}{l} \| A y \|_2^2 \\ ~~~=~~~~ \lt y =\alpha_0 v_0 + \cdots + \alpha_k v_k \gt \\ \| A ( \alpha_0 v_0 + \cdots + \alpha_k v_k ) \|_2^2 \\ ~~~=~~~~ \lt {\rm distributivity} \gt \\ \| \alpha_0 A v_0 + \cdots + \alpha_k A v_k \|_2^2 \\ ~~~=~~~~ \lt A v_j = \sigma_j u_j \gt \\ \| \alpha_0 \sigma_0 u_0 + \cdots + \alpha_k \sigma_k u_k \|_2^2 \\ ~~~= ~~~~\lt \mbox{ this works because the } u_j \mbox{ are orthonormal } \gt \\ \| \alpha_0 \sigma_0 u_0 \|_2^2 + \cdots + \| \alpha_k \sigma_k u_k \|_2^2 \\ ~~~=~~~~ \lt \mbox{ norms are homogeneous and } \| u_j \|_2 = 1 \gt \\ \vert \alpha_0 \vert^2 \sigma_0^2 + \cdots + \vert \alpha_k \vert^2 \sigma_k^2 \\ ~~~ \geq ~~~~ \lt \sigma_0 \geq \sigma_1 \geq \cdots \geq \sigma_k \geq 0 \gt \\ ( \vert \alpha_0 \vert^2 + \cdots + \vert \alpha_k \vert^2 ) \sigma_k^2 \\ ~~~ = ~~~~ \lt \| y \|_2^2 = \vert \alpha_0 \vert^2 + \cdots + \vert \alpha_k \vert^2 \gt \\ \sigma_k^2 \| y \|_2^2. \end{array} \end{equation*}

so that $\| A y \|_2 \geq \sigma_k \| y \|_2 \text{.}$ In other words, vectors in the subspace of all linear combinations of $\{ v_0, \ldots , v_k \}$ satisfy $\| A x \|_2 \geq \sigma_k \| x \|_2 \text{.}$ The dimension of this subspace is $k+1$ (since $\{ v_0, \cdots , v_{k} \}$ form an orthonormal basis).

• Both these subspaces are subspaces of $\Cn \text{.}$ One has dimension $k+ 1$ and the other $n - k \text{.}$ This means that if you take a basis for one (which consists of $n-k$ linearly independent vectors) and add it to a basis for the other (which has $k + 1$ linearly independent vectors), you end up with $n + 1$ vectors. Since these cannot all be linearly independent in $\Cn \text{,}$ there must be at least one nonzero vector $z$ that satisfies both $\| A z \|_2 \lt \sigma_k \| z \|_2$ and $\| A z \|_2 \geq \sigma_k \| z \|_2 \text{,}$ which is a contradiction.

Theorem 2.3.6.1 tells us how to pick the best approximation to a given matrix of a given desired rank. In Section Unit 2.1.1 we discussed how a low rank matrix can be used to compress data. The SVD thus gives the best such rank-k approximation. Let us revisit this.

Let $A \in \Rmxn$ be a matrix that, for example, stores a picture. In this case, the $i,j$ entry in $A$ is, for example, a number that represents the grayscale value of pixel $(i,j) \text{.}$

###### Homework2.3.6.2.

In Assignments/Week02/matlab execute

IMG = imread( 'Frida.jpg' );
A = double( IMG( :,:,1 ) );
imshow( uint8( A ) )
size( A )


to generate the picture of Mexican artist Frida Kahlo Although the picture is black and white, it was read as if it is a color image, which means a $m \times n \times 3$ array of pixel information is stored. Setting A = IMG( :,:,1 ) extracts a single matrix of pixel information. (If you start with a color picture, you will want to approximate IMG( :,:,1), IMG( :,:,2), and IMG( :,:,3) separately.)

Next, compute the SVD of matrix $A$

[ U, Sigma, V ] = svd( A );


and approximate the picture with a rank-k update, starting with $k = 1 \text{:}$

k = 1
B = uint8( U( :, 1:k ) * Sigma( 1:k,1:k ) * V( :, 1:k )' );
imshow( B ); Repeat this with increasing $k \text{.}$

r = min( size( A ) )
for k=1:r
imshow( uint8( U( :, 1:k ) * Sigma( 1:k,1:k ) * V( :, 1:k )' ) );
input( strcat( num2str( k ), "    press return" ) );
end


To determine a reasonable value for $k \text{,}$ it helps to graph the singular values:

figure
r = min( size( A )  );
plot( [ 1:r ], diag( Sigma ), 'x' );


Since the singular values span a broad range, we may want to plot them with a log-log plot

loglog( [ 1:r ], diag( Sigma ), 'x' );


For this particular matrix (picture), there is no dramatic drop in the singular values that makes it obvious what $k$ is a natural choice.

Solution