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.