p?lasmsub

Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

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ID 766686

Date 10/15/2026

Version

Public

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)

Include Files

mkl_scalapack.h

Description

pslasmsub pdlasmsub pclasmsub pzlasmsub The p?lasmsub routine function looks for a small subdiagonal element from the bottom of the matrix that it can safely set to zero. This routine function 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: 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.

NOTE:

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Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

p?lasmsub

Syntax

Include Files

Description

Input Parameters

Output Parameters

Application Notes