Developer Reference

  • 2020.2
  • 07/15/2020
  • Public Content
Contents

p?lasmsub

Looks for a small subdiagonal element from the bottom of the matrix that it can safely set to zero.

Syntax

call pslasmsub
(
a
,
desca
,
i
,
l
,
k
,
smlnum
,
buf
,
lwork
)
call pdlasmsub
(
a
,
desca
,
i
,
l
,
k
,
smlnum
,
buf
,
lwork
)
call pclasmsub
(
a
,
desca
,
i
,
l
,
k
,
smlnum
,
buf
,
lwork
)
call pzlasmsub
(
a
,
desca
,
i
,
l
,
k
,
smlnum
,
buf
,
lwork
)
Description
The
p?lasmsub
routine
looks for a small subdiagonal element from the bottom of the matrix that it can safely set to zero. This
routine
performs a global maximum and must be called by all processes.
Input Parameters
a
(local)
REAL
for
pslasmsub
DOUBLE PRECISION
for
pdlasmsub
COMPLEX
for
pclasmsub
DOUBLE COMPLEX
for
pzlasmsub
Array of size
(
lld_a
,
LOCc
(
n_a
))
.
On entry, the Hessenberg matrix whose tridiagonal part is being scanned. Unchanged on exit.
desca
(global and local)
INTEGER
.
Array of size
dlen_
. The array descriptor for the distributed matrix
A
.
i
(global)
INTEGER
.
The global location of the bottom of the unreduced submatrix of
A
. Unchanged on exit.
l
(global)
INTEGER
.
The global location of the top of the unreduced submatrix of
A
.
Unchanged on exit.
smlnum
(global)
REAL
for
pslasmsub
DOUBLE PRECISION
for
pdlasmsub
REAL
for
pclasmsub
DOUBLE PRECISION
for
pzlasmsub
On entry, a "small number" for the given matrix. Unchanged on exit. The machine-dependent constants for the stopping criterion.
lwork
(local)
INTEGER
.
This must be at least
2*ceil(ceil((
i
-l)
/mb_a
)/ lcm(
nprow
,
npcol
))
. Here
lcm
is least common multiple and
nprow
x
npcol
is the logical grid size.
Output Parameters
k
(global)
INTEGER
.
On exit, this yields the bottom portion of the unreduced submatrix. This will satisfy:
l
k
i
-1
.
buf
(local).
REAL
for
pslasmsub
DOUBLE PRECISION
for
pdlasmsub
COMPLEX
for
pclasmsub
DOUBLE COMPLEX
for
pzlasmsub
Array of size
lwork
.
Application Notes
This routine parallelizes the code from
?lahqr
that looks for a single small subdiagonal element.