Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

ID 766686
Date 3/31/2023
Public

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p?laconsb

Looks for two consecutive small subdiagonal elements.

Syntax

call pslaconsb(a, desca, i, l, m, h44, h33, h43h34, buf, lwork)

call pdlaconsb(a, desca, i, l, m, h44, h33, h43h34, buf, lwork)

call pclaconsb(a, desca, i, l, m, h44, h33, h43h34, buf, lwork)

call pzlaconsb(a, desca, i, l, m, h44, h33, h43h34, buf, lwork)

Description

The p?laconsbroutine looks for two consecutive small subdiagonal elements by analyzing the effect of starting a double shift QR iteration given by h44, h33, and h43h34 to see if this process makes a subdiagonal negligible.

Input Parameters
a

(local) REAL for pslaconsb

DOUBLE PRECISION for pdlaconsb

COMPLEX for pclaconsb

DOUBLE COMPLEX for pzlaconsb

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.

h44, h33, h43h34

(global). REAL for pslaconsb

DOUBLE PRECISION for pdlaconsb

COMPLEX for pclaconsb

DOUBLE COMPLEX for pzlaconsb

These three values are for the double shift QR iteration.

lwork

(local) INTEGER.

This must be at least 7*ceil(ceil( (i-l)/mb_a )/lcm(nprow, npcol)). Here lcm is the least common multiple and nprow*npcol is the logical grid size.

Output Parameters
m

(global). On exit, this yields the starting location of the QR double shift. This will satisfy:

lmi-2.

buf

(local).

REAL for pslaconsb

DOUBLE PRECISION for pdlaconsb

COMPLEX for pclaconsb

DOUBLE COMPLEX for pzlaconsb

Array of size lwork.

See Also