NAG Library Routine Document

f02wgf  (real_gen_partialsvd)

 Contents

    1  Purpose
    7  Accuracy

1
Purpose

f02wgf returns leading terms in the singular value decomposition (SVD) of a real general matrix and computes the corresponding left and right singular vectors.

2
Specification

Fortran Interface
Subroutine f02wgf ( m, n, k, ncv, av, nconv, sigma, u, ldu, v, ldv, resid, iuser, ruser, ifail)
Integer, Intent (In):: m, n, k, ncv, ldu, ldv
Integer, Intent (Inout):: iuser(*), ifail
Integer, Intent (Out):: nconv
Real (Kind=nag_wp), Intent (Inout):: u(ldu,ncv), v(ldv,ncv), ruser(*)
Real (Kind=nag_wp), Intent (Out):: sigma(ncv), resid(ncv)
External:: av
C Header Interface
#include nagmk26.h
void  f02wgf_ ( const Integer *m, const Integer *n, const Integer *k, const Integer *ncv,
void (NAG_CALL *av)( Integer *iflag, const Integer *m, const Integer *n, const double x[], double ax[], Integer iuser[], double ruser[]),
Integer *nconv, double sigma[], double u[], const Integer *ldu, double v[], const Integer *ldv, double resid[], Integer iuser[], double ruser[], Integer *ifail)

3
Description

f02wgf computes a few, k, of the largest singular values and corresponding vectors of an m by n matrix A. The value of k should be small relative to m and n, for example kOminm,n. The full singular value decomposition (SVD) of an m by n matrix A is given by
A=UΣVT ,  
where U and V are orthogonal and Σ is an m by n diagonal matrix with real diagonal elements, σi, such that
σ1 σ2 σ minm,n 0 .  
The σi are the singular values of A and the first minm,n columns of U and V are the left and right singular vectors of A.
If Uk, Vk denote the leading k columns of U and V respectively, and if Σk denotes the leading principal submatrix of Σ, then
Ak Uk Σk VTk  
is the best rank-k approximation to A in both the 2-norm and the Frobenius norm.
The singular values and singular vectors satisfy
Avi = σi ui   and   ATui = σi vi   so that   ATA νi = σi2 νi ​ and ​ A AT ui = σ i 2 u i ,  
where ui and vi are the ith columns of Uk and Vk respectively.
Thus, for mn, the largest singular values and corresponding right singular vectors are computed by finding eigenvalues and eigenvectors for the symmetric matrix ATA. For m<n, the largest singular values and corresponding left singular vectors are computed by finding eigenvalues and eigenvectors for the symmetric matrix AAT. These eigenvalues and eigenvectors are found using routines from Chapter F12. You should read the F12 Chapter Introduction for full details of the method used here.
The real matrix A is not explicitly supplied to f02wgf. Instead, you are required to supply a routine, av, that must calculate one of the requested matrix-vector products Ax or ATx for a given real vector x (of length n or m respectively).

4
References

Wilkinson J H (1978) Singular Value Decomposition – Basic Aspects Numerical Software – Needs and Availability (ed D A H Jacobs) Academic Press

5
Arguments

1:     m – IntegerInput
On entry: m, the number of rows of the matrix A.
Constraint: m0.
If m=0, an immediate return is effected.
2:     n – IntegerInput
On entry: n, the number of columns of the matrix A.
Constraint: n0.
If n=0, an immediate return is effected.
3:     k – IntegerInput
On entry: k, the number of singular values to be computed.
Constraint: 0<k<minm,n-1.
4:     ncv – IntegerInput
On entry: the dimension of the arrays sigma and resid and the second dimension of the arrays u and v as declared in the (sub)program from which f02wgf is called. This is the number of Lanczos basis vectors to use during the computation of the largest eigenvalues of ATA (mn) or AAT (m<n).
At present there is no a priori analysis to guide the selection of ncv relative to k. However, it is recommended that ncv2×k+1. If many problems of the same type are to be solved, you should experiment with varying ncv while keeping k fixed for a given test problem. This will usually decrease the required number of matrix-vector operations but it also increases the internal storage required to maintain the orthogonal basis vectors. The optimal ‘cross-over’ with respect to CPU time is problem dependent and must be determined empirically.
Constraint: k<ncvminm,n.
5:     av – Subroutine, supplied by the user.External Procedure
av must return the vector result of the matrix-vector product Ax or ATx, as indicated by the input value of iflag, for the given vector x.
av is called from f02wgf with the argument iuser and ruser as supplied to f02wgf. You are free to use these arrays to supply information to av.
The specification of av is:
Fortran Interface
Subroutine av ( iflag, m, n, x, ax, iuser, ruser)
Integer, Intent (In):: m, n
Integer, Intent (Inout):: iflag, iuser(*)
Real (Kind=nag_wp), Intent (In):: x(*)
Real (Kind=nag_wp), Intent (Inout):: ax(*), ruser(*)
C Header Interface
#include nagmk26.h
void  av ( Integer *iflag, const Integer *m, const Integer *n, const double x[], double ax[], Integer iuser[], double ruser[])
1:     iflag – IntegerInput/Output
On entry: if iflag=1, ax must return the m-vector result of the matrix-vector product Ax.
If iflag=2, ax must return the n-vector result of the matrix-vector product ATx.
On exit: may be used as a flag to indicate a failure in the computation of Ax or ATx. If iflag is negative on exit from av, f02wgf will exit immediately with ifail set to iflag.
2:     m – IntegerInput
On entry: the number of rows of the matrix A.
3:     n – IntegerInput
On entry: the number of columns of the matrix A.
4:     x* – Real (Kind=nag_wp) arrayInput
On entry: the vector to be pre-multiplied by the matrix A or AT.
5:     ax* – Real (Kind=nag_wp) arrayOutput
On exit: if iflag=1, contains the m-vector result of the matrix-vector product Ax.
If iflag=2, contains the n-vector result of the matrix-vector product ATx.
6:     iuser* – Integer arrayUser Workspace
7:     ruser* – Real (Kind=nag_wp) arrayUser Workspace
av is called with the arguments iuser and ruser as supplied to f02wgf. You should use the arrays iuser and ruser to supply information to av.
av must either be a module subprogram USEd by, or declared as EXTERNAL in, the (sub)program from which f02wgf is called. Arguments denoted as Input must not be changed by this procedure.
Note: av should not return floating-point NaN (Not a Number) or infinity values, since these are not handled by f02wgf. If your code inadvertently does return any NaNs or infinities, f02wgf is likely to produce unexpected results.
6:     nconv – IntegerOutput
On exit: the number of converged singular values found.
7:     sigmancv – Real (Kind=nag_wp) arrayOutput
On exit: the nconv converged singular values are stored in the first nconv elements of sigma.
8:     ulduncv – Real (Kind=nag_wp) arrayOutput
On exit: the left singular vectors corresponding to the singular values stored in sigma.
The ith element of the jth left singular vector uj is stored in uij, for i=1,2,,m and j=1,2,,nconv.
9:     ldu – IntegerInput
On entry: the first dimension of the array u as declared in the (sub)program from which f02wgf is called.
Constraint: ldumax1,m.
10:   vldvncv – Real (Kind=nag_wp) arrayOutput
On exit: the right singular vectors corresponding to the singular values stored in sigma.
The ith element of the jth right singular vector vj is stored in vij, for i=1,2,,n and j=1,2,,nconv.
11:   ldv – IntegerInput
On entry: the first dimension of the array v as declared in the (sub)program from which f02wgf is called.
Constraint: ldvmax1,n.
12:   residncv – Real (Kind=nag_wp) arrayOutput
On exit: the residual A vj - σj uj , for mn, or AT uj - σj vj , for m<n, for each of the converged singular values σj and corresponding left and right singular vectors uj and vj.
13:   iuser* – Integer arrayUser Workspace
14:   ruser* – Real (Kind=nag_wp) arrayUser Workspace
iuser and ruser are not used by f02wgf, but are passed directly to av and may be used to pass information to this routine.
15:   ifail – IntegerInput/Output
On entry: ifail must be set to 0, -1​ or ​1. If you are unfamiliar with this argument you should refer to Section 3.4 in How to Use the NAG Library and its Documentation for details.
For environments where it might be inappropriate to halt program execution when an error is detected, the value -1​ or ​1 is recommended. If the output of error messages is undesirable, then the value 1 is recommended. Otherwise, if you are not familiar with this argument, the recommended value is 0. When the value -1​ or ​1 is used it is essential to test the value of ifail on exit.
On exit: ifail=0 unless the routine detects an error or a warning has been flagged (see Section 6).
f02wgf returns with ifail=0 if at least k singular values have converged and the corresponding left and right singular vectors have been computed.

6
Error Indicators and Warnings

If on entry ifail=0 or -1, explanatory error messages are output on the current error message unit (as defined by x04aaf).
Errors or warnings detected by the routine:
ifail=1
On entry, m=value.
Constraint: m0.
ifail=2
On entry, n=value.
Constraint: n0.
ifail=3
On entry, k=value.
Constraint: k>0.
ifail=4
On entry, k=value, ncv=value, m=value and n=value.
Constraint: k<ncvminm,n.
ifail=5
On entry, ldu=value and m=value.
Constraint: ldum.
ifail=6
On entry, ldv=value and n=value.
Constraint: ldvn.
ifail=8
The maximum number of iterations has been reached. The maximum number of iterations =value. The number of converged eigenvalues =value.
ifail=9
No shifts could be applied during a cycle of the implicitly restarted Lanczos iteration.
ifail=10
Could not build a full Lanczos factorization.
ifail=11
The number of eigenvalues found to sufficient accuracy is zero.
ifail=20
An error occurred during an internal call. Consider increasing the size of ncv relative to k.
ifail<0
On output from user-defined routine av, iflag was set to a negative value, iflag=value.
ifail=-99
An unexpected error has been triggered by this routine. Please contact NAG.
See Section 3.9 in How to Use the NAG Library and its Documentation for further information.
ifail=-399
Your licence key may have expired or may not have been installed correctly.
See Section 3.8 in How to Use the NAG Library and its Documentation for further information.
ifail=-999
Dynamic memory allocation failed.
See Section 3.7 in How to Use the NAG Library and its Documentation for further information.

7
Accuracy

See Section 2.14.2 in the F08 Chapter Introduction.

8
Parallelism and Performance

f02wgf is threaded by NAG for parallel execution in multithreaded implementations of the NAG Library.
f02wgf makes calls to BLAS and/or LAPACK routines, which may be threaded within the vendor library used by this implementation. Consult the documentation for the vendor library for further information.
Please consult the X06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this routine. Please also consult the Users' Note for your implementation for any additional implementation-specific information.

9
Further Comments

None.

10
Example

This example finds the four largest singular values (σ) and corresponding right and left singular vectors for the matrix A, where A is the m by n real matrix derived from the simplest finite difference discretization of the two-dimensional kernel ks,tdt where
ks,t = st-1 if ​0st 1 ts-1 if ​0t<s1 .  

10.1
Program Text

Program Text (f02wgfe.f90)

10.2
Program Data

Program Data (f02wgfe.d)

10.3
Program Results

Program Results (f02wgfe.r)

© The Numerical Algorithms Group Ltd, Oxford, UK. 2017