SVD_UPDATE - Singular Value Decomposition of a 
  rank-l update matrix with MATLAB (eigs)
    [U, S, V]=SVD_UPDATE(A, X, Y, K) computes the 
    K-factor SVD of A-X*Y, using the eigs function of MATLAB.
 
  Copyright 2011 Dimitrios Zeimpekis, Eugenia Maria Kontopoulou, 
                 Efstratios Gallopoulos