DLAMSH(l) ) DLAMSH(l)NAME
DLAMSH - send multiple shifts through a small (single node) matrix to
see how consecutive small subdiagonal elements are modified by subse‐
quent shifts in an effort to maximize the number of bulges that can be
sent through
SYNOPSIS
SUBROUTINE DLAMSH ( S, LDS, NBULGE, JBLK, H, LDH, N, ULP )
INTEGER LDS, NBULGE, JBLK, LDH, N
DOUBLE PRECISION ULP
DOUBLE PRECISION S(LDS,*), H(LDH,*)
PURPOSE
DLAMSH sends multiple shifts through a small (single node) matrix to
see how consecutive small subdiagonal elements are modified by subse‐
quent shifts in an effort to maximize the number of bulges that can be
sent through. DLAMSH should only be called when there are multiple
shifts/bulges
(NBULGE > 1) and the first shift is starting in the middle of an
unreduced Hessenberg matrix because of two or more consecutive small
subdiagonal elements.
ARGUMENTS
S (local input/output) DOUBLE PRECISION array, (LDS,*)
On entry, the matrix of shifts. Only the 2x2 diagonal of S is
referenced. It is assumed that S has JBLK double shifts (size
2). On exit, the data is rearranged in the best order for
applying.
LDS (local input) INTEGER
On entry, the leading dimension of S. Unchanged on exit. 1 <
NBULGE <= JBLK <= LDS/2
NBULGE (local input/output) INTEGER
On entry, the number of bulges to send through H ( >1 ).
NBULGE should be less than the maximum determined (JBLK). 1 <
NBULGE <= JBLK <= LDS/2 On exit, the maximum number of bulges
that can be sent through.
JBLK (local input) INTEGER
On entry, the number of shifts determined for S. Unchanged on
exit.
H (local input/output) DOUBLE PRECISION array (LDH,N)
On entry, the local matrix to apply the shifts on. H should be
aligned so that the starting row is 2. On exit, the data is
destroyed.
LDS (local input) INTEGER
On entry, the leading dimension of S. Unchanged on exit.
N (local input) INTEGER
On entry, the size of H. If all the bulges are expected to go
through, N should be at least 4*NBULGE+2. Otherwise, NBULGE
may be reduced by this routine.
ULP (local input) DOUBLE PRECISION
On entry, machine precision Unchanged on exit.
Implemented by: G. Henry, May 1, 1997
ScaLAPACK version 1.7 13 August 2001 DLAMSH(l)
