# 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)

## Description

This routine 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

  (global). REAL for pslaconsb

  DOUBLE PRECISION for pdlaconsb

  Array, DIMENSION (desca (lld_),*). On entry, the Hessenberg matrix whose tridiagonal part is being scanned. Unchanged on exit.

desca

  (global and local) INTEGER.

  Array of DIMENSION (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

  These three values are for the double shift QR iteration.

lwork

  (global) INTEGER.

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

## Output Parameters

m

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

  l ≤ m ≤ i-2.``

buf

  (local).

  REAL for pslaconsb

  DOUBLE PRECISION for pdlaconsb

  Array of size lwork.

lwork

  (global). On exit, lwork is the size of the work buffer.

Parent topic: Auxiliary Routines