# slasd2 (l) - Linux Man Pages

## NAME

SLASD2 - merges the two sets of singular values together into a single sorted set

## SYNOPSIS

SUBROUTINE SLASD2(
NL, NR, SQRE, K, D, Z, ALPHA, BETA, U, LDU, VT, LDVT, DSIGMA, U2, LDU2, VT2, LDVT2, IDXP, IDX, IDXC, IDXQ, COLTYP, INFO )

INTEGER INFO, K, LDU, LDU2, LDVT, LDVT2, NL, NR, SQRE

REAL ALPHA, BETA

INTEGER COLTYP( * ), IDX( * ), IDXC( * ), IDXP( * ), IDXQ( * )

REAL D( * ), DSIGMA( * ), U( LDU, * ), U2( LDU2, * ), VT( LDVT, * ), VT2( LDVT2, * ), Z( * )

## PURPOSE

SLASD2 merges the two sets of singular values together into a single sorted set. Then it tries to deflate the size of the problem. There are two ways in which deflation can occur: when two or more singular values are close together or if there is a tiny entry in the Z vector. For each such occurrence the order of the related secular equation problem is reduced by one.
SLASD2 is called from SLASD1.

## ARGUMENTS

NL (input) INTEGER
The row dimension of the upper block. NL >= 1.
NR (input) INTEGER
The row dimension of the lower block. NR >= 1.
SQRE (input) INTEGER
= 0: the lower block is an NR-by-NR square matrix.
= 1: the lower block is an NR-by-(NR+1) rectangular matrix. The bidiagonal matrix has N = NL + NR + 1 rows and M = N + SQRE >= N columns.
K (output) INTEGER
Contains the dimension of the non-deflated matrix, This is the order of the related secular equation. 1 <= K <=N.
D (input/output) REAL array, dimension (N)
On entry D contains the singular values of the two submatrices to be combined. On exit D contains the trailing (N-K) updated singular values (those which were deflated) sorted into increasing order.
Z (output) REAL array, dimension (N)
On exit Z contains the updating row vector in the secular equation.
ALPHA (input) REAL
Contains the diagonal element associated with the added row.
BETA (input) REAL
Contains the off-diagonal element associated with the added row.
U (input/output) REAL array, dimension (LDU,N)
On entry U contains the left singular vectors of two submatrices in the two square blocks with corners at (1,1), (NL, NL), and (NL+2, NL+2), (N,N). On exit U contains the trailing (N-K) updated left singular vectors (those which were deflated) in its last N-K columns.
LDU (input) INTEGER
The leading dimension of the array U. LDU >= N.
VT (input/output) REAL array, dimension (LDVT,M)
On entry VTaq contains the right singular vectors of two submatrices in the two square blocks with corners at (1,1), (NL+1, NL+1), and (NL+2, NL+2), (M,M). On exit VTaq contains the trailing (N-K) updated right singular vectors (those which were deflated) in its last N-K columns. In case SQRE =1, the last row of VT spans the right null space.
LDVT (input) INTEGER
The leading dimension of the array VT. LDVT >= M. DSIGMA (output) REAL array, dimension (N) Contains a copy of the diagonal elements (K-1 singular values and one zero) in the secular equation.
U2 (output) REAL array, dimension (LDU2,N)
Contains a copy of the first K-1 left singular vectors which will be used by SLASD3 in a matrix multiply (SGEMM) to solve for the new left singular vectors. U2 is arranged into four blocks. The first block contains a column with 1 at NL+1 and zero everywhere else; the second block contains non-zero entries only at and above NL; the third contains non-zero entries only below NL+1; and the fourth is dense.
LDU2 (input) INTEGER
The leading dimension of the array U2. LDU2 >= N.
VT2 (output) REAL array, dimension (LDVT2,N)
VT2aq contains a copy of the first K right singular vectors which will be used by SLASD3 in a matrix multiply (SGEMM) to solve for the new right singular vectors. VT2 is arranged into three blocks. The first block contains a row that corresponds to the special 0 diagonal element in SIGMA; the second block contains non-zeros only at and before NL +1; the third block contains non-zeros only at and after NL +2.
LDVT2 (input) INTEGER
The leading dimension of the array VT2. LDVT2 >= M.
IDXP (workspace) INTEGER array, dimension (N)
This will contain the permutation used to place deflated values of D at the end of the array. On output IDXP(2:K)
points to the nondeflated D-values and IDXP(K+1:N) points to the deflated singular values.
IDX (workspace) INTEGER array, dimension (N)
This will contain the permutation used to sort the contents of D into ascending order.
IDXC (output) INTEGER array, dimension (N)
This will contain the permutation used to arrange the columns of the deflated U matrix into three groups: the first group contains non-zero entries only at and above NL, the second contains non-zero entries only below NL+2, and the third is dense.
IDXQ (input/output) INTEGER array, dimension (N)
This contains the permutation which separately sorts the two sub-problems in D into ascending order. Note that entries in the first hlaf of this permutation must first be moved one position backward; and entries in the second half must first have NL+1 added to their values. COLTYP (workspace/output) INTEGER array, dimension (N) As workspace, this will contain a label which will indicate which of the following types a column in the U2 matrix or a row in the VT2 matrix is:
1 : non-zero in the upper half only
2 : non-zero in the lower half only
3 : dense
4 : deflated On exit, it is an array of dimension 4, with COLTYP(I) being the dimension of the I-th type columns.
INFO (output) INTEGER
= 0: successful exit.
< 0: if INFO = -i, the i-th argument had an illegal value.

## FURTHER DETAILS

Based on contributions by

Ming Gu and Huan Ren, Computer Science Division, University of
California at Berkeley, USA