p?lasmsub

Developer Reference for Intel® oneAPI Math Kernel Library for C

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ID 766684

Date 4/28/2026

Version

Public

p?lasmsub

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

Syntax

voidpslasmsub ( constfloat*a , constMKL_INT*desca , constMKL_INT*i , constMKL_INT*l , MKL_INT*k , constfloat*smlnum , float*buf , constMKL_INT*lwork );

voidpdlasmsub ( constdouble*a , constMKL_INT*desca , constMKL_INT*i , constMKL_INT*l , MKL_INT*k , constdouble*smlnum , double*buf , constMKL_INT*lwork );

voidpclasmsub ( constMKL_Complex8*a , constMKL_INT*desca , constMKL_INT*i , constMKL_INT*l , MKL_INT*k , constfloat*smlnum , MKL_Complex8*buf , constMKL_INT*lwork );

voidpzlasmsub ( constMKL_Complex16*a , constMKL_INT*desca , constMKL_INT*i , constMKL_INT*l , MKL_INT*k , constdouble*smlnum , MKL_Complex16*buf , constMKL_INT*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))lld_a * LOCc ( n_a ) .

On entry, the Hessenberg matrix whose tridiagonal part is being scanned. Unchanged on exit.

desca

(global and local)

Array of size dlen_ . The array descriptor for the distributed matrix A .

i

(global)

The global location of the bottom of the unreduced submatrix of A . Unchanged on exit.

l

(global)

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)

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)

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 C

p?lasmsub

Syntax

Include Files

Description

Input Parameters

Output Parameters

Application Notes