Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

ID 766686
Date 3/22/2024
Public

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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?lasmsubroutine 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 nprowxnpcol is the logical grid size.

Output Parameters

k

(global) INTEGER.

On exit, this yields the bottom portion of the unreduced submatrix. This will satisfy: lki-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.

See Also