# Singular value decomposition

## What is

A singular-value decomposition (SVD) of a high-dimensional matrix gives a low-dimensional representation of it. Given a matrix
$M$
, an
$m \times n$
matrix of rank
$r$
, we can find matrices
$U$
,
$\Sigma$
and
$V$
such that we have the decomposition
$M = U \Sigma V^t$
. These matrices respect conditions:
1. 1.
$U$
is an
$m \times r$
matrix and is column orthonormal (each column is a unit vector and the dot product of any two columns is 0);
2. 2.
$V$
is a
$n \times r$
matrix, also column orthonormal;
3. 3.
$\Sigma$
is a diagonal matrix and the elements on its diagonal are called the singular values of
$M$
.
$U$
,
$\Sigma$
and
$V$
are called the SVD components of
$M$
. The singular values represent "hidden concepts" which connect the two other matrices together. SVD is often employed in recommender systems.

### A small example, in words

If
$M$
contains the ratings given by people to movies, so that people are on the rows and movies on the columns, then
$U$
connects people to concepts,
$V$
connects movies to concepts and
$\Sigma$
gives the strength of each concept.
A person not represented in
$M$
wants to know what movies he/she would like based on the ones he/she has seen. If
$q$
is the vector representing the movies the person rated,
$q V$
maps the person to the concept space and this can be mapped back to the movie space by multiplying by
$V^t$
.
To find similar users to a given user, we can map all users to the concept space using
$V$
and then apply some similarity.

## Where is the dimensionality Reduction part

If
$M$
is a very large matrix, its SVD components will be large as well. But in real-world uses of the technique, approximate decompositions are employed instead, so to reduce the original dimensionality.
Setting the
$s$
smallest singular values to 0 and eliminating the corresponding
$s$
rows of
$U$
and
$V$
we obtain an approximate representation. This procedure works well because it can be shown that it minimises the root mean square error between
$M$
and its approximation, or the Frobenius norm of this difference.
Let
$M = PQR$
be a generic decomposition, then
$m{ij} = \sum_k \sum_l p_{ik} q_{kl} r_{lj}$
and then
$\begin{cases} ||M||^2 &= \sum_i \sum_j m_{ij}^2 \\ &= \sum_i \sum_j (\sum_k \sum_l p_{ik} q_{kl} r_{lj})^2 \\ &= \sum_i \sum_j \sum_k \sum_l \sum_n \sum_m p_{ik} q{kl} r_{lj} p_{in} q_{nm} r_{mj} \end{cases}$
Now, if
$P$
,
$Q$
and
$R$
are the SVD components of
$M$
, then it means
$Q$
is diagonal,
$P$
is column orthonormal,
$R$
is row orthonormal (note the transpose in the SVD equation), so
$||M||^2 = \sum_k q_{kk}^2$
.
Calling
$P = U$
,
$Q=\Sigma$
and
$R=V^t$
, with
$\sigma_i$
being the
$i$
-th diagonal element of
$\Sigma$
, if we keep the first
$n$
elements of
$\Sigma$
and set the remaining ones to 0, obtaining a
$\Sigma'$
, we get
$M' = U \Sigma' V^t \ ,$
which is an approximation of
$M$
. The matrix giving the resulting errors is
$M - M' = U (\Sigma - \Sigma') V^t$
, whose norm is (following from the above)
$||M - M'||^2 = \sum_k (\Sigma - \Sigma')_{kk}^2$
The matrix
$\Sigma - \Sigma'$
has 0 in the first
$\Sigma - \Sigma'$
diagonal elements and
$\sigma_i$
in the remaining ones, with
$n < i \leq r$
. So,
$||M - M'||^2$
is the sum of the squares of the elements of
$\Sigma$
which were set to 0. In order to minimise it, we pick the smallest elements of
$\Sigma$
.

## Computing the SVD of a matrix

The computation is strictly connected to the eigenvalues of the symmetric matrix
$M^tM$
and
$M M^t$
.
If
$M = U \Sigma V^t$
, then
$M^t = V \Sigma^t U^t$
and because
$\Sigma$
is diagonal, then
$\Sigma = \Sigma^t$
, so
$M^t = V \Sigma U^t$
, which then means (using the orthonormality of
$U$
and
$V$
)
$M^t M = V \Sigma U^t U \Sigma V^t = V \Sigma^2 V^t$
so
$M^t M V = V \Sigma^2$
Now,
$\Sigma^2$
is diagonal, with entries being the squares of the
$\Sigma$
's entries. This last equation says that
$V$
is the matrix of eigenvectors of
$M^tM$
and
$\Sigma^2$
is the matrix of eigenvalues. So, by computing
$M^tM$
we have the eigenvectors and the singular values, only
$U$
remains.
Now,
$M M^t = U \Sigma V^t V \Sigma U^t = U \Sigma^2 U^t$
so
$M M^t U = U \Sigma^2$
.
which means that
$U$
is the matrix of eigenvectors of
$M M^t$
.
$M M^t$
is a
$n \times m$
matrix,
$M M^t$
is
$m \times n$
and
$n, m \geq r$
. Hence,
$M^t M$
and
$M M^t$
$n-r$
$m-r$
$U$
$V$
$\Sigma$
$r$