CLALSD(1) LAPACK routine (version 3.2) CLALSD(1)NAME
CLALSD - uses the singular value decomposition of A to solve the least
squares problem of finding X to minimize the Euclidean norm of each
column of A*X-B, where A is N-by-N upper bidiagonal, and X and B are N-
by-NRHS
SYNOPSIS
SUBROUTINE CLALSD( UPLO, SMLSIZ, N, NRHS, D, E, B, LDB, RCOND, RANK,
WORK, RWORK, IWORK, INFO )
CHARACTER UPLO
INTEGER INFO, LDB, N, NRHS, RANK, SMLSIZ
REAL RCOND
INTEGER IWORK( * )
REAL D( * ), E( * ), RWORK( * )
COMPLEX B( LDB, * ), WORK( * )
PURPOSE
CLALSD uses the singular value decomposition of A to solve the least
squares problem of finding X to minimize the Euclidean norm of each
column of A*X-B, where A is N-by-N upper bidiagonal, and X and B are N-
by-NRHS. The solution X overwrites B. The singular values of A smaller
than RCOND times the largest singular value are treated as zero in
solving the least squares problem; in this case a minimum norm solution
is returned. The actual singular values are returned in D in ascending
order. This code makes very mild assumptions about floating point
arithmetic. It will work on machines with a guard digit in add/sub‐
tract, or on those binary machines without guard digits which subtract
like the Cray XMP, Cray YMP, Cray C 90, or Cray 2. It could conceiv‐
ably fail on hexadecimal or decimal machines without guard digits, but
we know of none.
ARGUMENTS
UPLO (input) CHARACTER*1
= 'U': D and E define an upper bidiagonal matrix.
= 'L': D and E define a lower bidiagonal matrix. SMLSIZ
(input) INTEGER The maximum size of the subproblems at the bot‐
tom of the computation tree.
N (input) INTEGER
The dimension of the bidiagonal matrix. N >= 0.
NRHS (input) INTEGER
The number of columns of B. NRHS must be at least 1.
D (input/output) REAL array, dimension (N)
On entry D contains the main diagonal of the bidiagonal matrix.
On exit, if INFO = 0, D contains its singular values.
E (input/output) REAL array, dimension (N-1)
Contains the super-diagonal entries of the bidiagonal matrix.
On exit, E has been destroyed.
B (input/output) COMPLEX array, dimension (LDB,NRHS)
On input, B contains the right hand sides of the least squares
problem. On output, B contains the solution X.
LDB (input) INTEGER
The leading dimension of B in the calling subprogram. LDB must
be at least max(1,N).
RCOND (input) REAL
The singular values of A less than or equal to RCOND times the
largest singular value are treated as zero in solving the least
squares problem. If RCOND is negative, machine precision is used
instead. For example, if diag(S)*X=B were the least squares
problem, where diag(S) is a diagonal matrix of singular values,
the solution would be X(i) = B(i) / S(i) if S(i) is greater than
RCOND*max(S), and X(i) = 0 if S(i) is less than or equal to
RCOND*max(S).
RANK (output) INTEGER
The number of singular values of A greater than RCOND times the
largest singular value.
WORK (workspace) COMPLEX array, dimension (N * NRHS).
RWORK (workspace) REAL array, dimension at least
(9*N + 2*N*SMLSIZ + 8*N*NLVL + 3*SMLSIZ*NRHS + (SMLSIZ+1)**2),
where NLVL = MAX( 0, INT( LOG_2( MIN( M,N )/(SMLSIZ+1) ) ) + 1 )
IWORK (workspace) INTEGER array, dimension (3*N*NLVL + 11*N).
INFO (output) INTEGER
= 0: successful exit.
< 0: if INFO = -i, the i-th argument had an illegal value.
> 0: The algorithm failed to compute an singular value while
working on the submatrix lying in rows and columns INFO/(N+1)
through MOD(INFO,N+1).
FURTHER DETAILS
Based on contributions by
Ming Gu and Ren-Cang Li, Computer Science Division, University of
California at Berkeley, USA
Osni Marques, LBNL/NERSC, USA
LAPACK routine (version 3.2) November 2008 CLALSD(1)
