Library Functions Manual

NAME¶

laexc - laexc: reorder Schur form

SYNOPSIS¶

Functions¶

subroutine DLAEXC (wantq, n, t, ldt, q, ldq, j1, n1, n2, work, info)
DLAEXC swaps adjacent diagonal blocks of a real upper quasi-triangular matrix in Schur canonical form, by an orthogonal similarity transformation. subroutine SLAEXC (wantq, n, t, ldt, q, ldq, j1, n1, n2, work, info)
SLAEXC swaps adjacent diagonal blocks of a real upper quasi-triangular matrix in Schur canonical form, by an orthogonal similarity transformation.

Detailed Description¶

Function Documentation¶

subroutine DLAEXC (logical wantq, integer n, double precision, dimension( ldt, * ) t, integer ldt, double precision, dimension( ldq, * ) q, integer ldq, integer j1, integer n1, integer n2, double precision, dimension( * ) work, integer info)¶

DLAEXC swaps adjacent diagonal blocks of a real upper quasi-triangular matrix in Schur canonical form, by an orthogonal similarity transformation.

Purpose:

!>
!> DLAEXC swaps adjacent diagonal blocks T11 and T22 of order 1 or 2 in
!> an upper quasi-triangular matrix T by an orthogonal similarity
!> transformation.
!>
!> T must be in Schur canonical form, that is, block upper triangular
!> with 1-by-1 and 2-by-2 diagonal blocks; each 2-by-2 diagonal block
!> has its diagonal elements equal and its off-diagonal elements of
!> opposite sign.
!>

Parameters

WANTQ

!>          WANTQ is LOGICAL
!>          = .TRUE. : accumulate the transformation in the matrix Q;
!>          = .FALSE.: do not accumulate the transformation.
!>

!>          N is INTEGER
!>          The order of the matrix T. N >= 0.
!>

!>          T is DOUBLE PRECISION array, dimension (LDT,N)
!>          On entry, the upper quasi-triangular matrix T, in Schur
!>          canonical form.
!>          On exit, the updated matrix T, again in Schur canonical form.
!>

LDT

!>          LDT is INTEGER
!>          The leading dimension of the array T. LDT >= max(1,N).
!>

!>          Q is DOUBLE PRECISION array, dimension (LDQ,N)
!>          On entry, if WANTQ is .TRUE., the orthogonal matrix Q.
!>          On exit, if WANTQ is .TRUE., the updated matrix Q.
!>          If WANTQ is .FALSE., Q is not referenced.
!>

LDQ

!>          LDQ is INTEGER
!>          The leading dimension of the array Q.
!>          LDQ >= 1; and if WANTQ is .TRUE., LDQ >= N.
!>

!>          J1 is INTEGER
!>          The index of the first row of the first block T11.
!>

!>          N1 is INTEGER
!>          The order of the first block T11. N1 = 0, 1 or 2.
!>

!>          N2 is INTEGER
!>          The order of the second block T22. N2 = 0, 1 or 2.
!>

WORK

!>          WORK is DOUBLE PRECISION array, dimension (N)
!>

INFO

!>          INFO is INTEGER
!>          = 0: successful exit
!>          = 1: the transformed matrix T would be too far from Schur
!>               form; the blocks are not swapped and T and Q are
!>               unchanged.
!>

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Definition at line 136 of file dlaexc.f.

subroutine SLAEXC (logical wantq, integer n, real, dimension( ldt, * ) t, integer ldt, real, dimension( ldq, * ) q, integer ldq, integer j1, integer n1, integer n2, real, dimension( * ) work, integer info)¶

SLAEXC swaps adjacent diagonal blocks of a real upper quasi-triangular matrix in Schur canonical form, by an orthogonal similarity transformation.