SVD - Singular Value Decomposition

{excerpt}Singular Value Decomposition is a form of product decomposition of a matrix in which a rectangular matrix A is decomposed into a product U s V’ where U and V are orthonormal and s is a diagonal matrix.{excerpt} The values of A can be real or complex, but the real case dominates applications in machine learning. The most prominent properties of the SVD are:

The decomposition of any real matrix has only real values
The SVD is unique except for column permutations of U, s and V
If you take only the largest n values of s and set the rest to zero, you have a least squares approximation of A with rank n. This allows SVD to be used very effectively in least squares regression and makes partial SVD useful.
The SVD can be computed accurately for singular or nearly singular matrices. For a matrix of rank n, only the first n singular values will be non-zero. This allows SVD to be used for solution of singular linear systems. The columns of U and V corresponding to zero singular values define the null space of A.
The partial SVD of very large matrices can be computed very quickly using stochastic decompositions. See http://arxiv.org/abs/0909.4061v1 for details. Gradient descent can also be used to compute partial SVD’s and is very useful where some values of the matrix being decomposed are not known.

In collaborative filtering and text retrieval, it is common to compute the partial decomposition of the user x item interaction matrix or the document x term matrix. This allows the projection of users and items (or documents and terms) into a common vector space representation that is often referred to as the latent semantic representation. This process is sometimes called Latent Semantic Analysis and has been very effective in the analysis of the Netflix dataset.

Dimension Reduction in Mahout:

https://cwiki.apache.org/MAHOUT/dimensional-reduction.html