slaexc(3S)

NAME
     SLAEXC - swap adjacent diagonal blocks T11 and T22 of order 1 or 2 in an
     upper quasi-triangular matrix T by an orthogonal similarity
     transformation

SYNOPSIS
     SUBROUTINE SLAEXC( WANTQ, N, T, LDT, Q, LDQ, J1, N1, N2, WORK, INFO )

         LOGICAL        WANTQ

         INTEGER        INFO, J1, LDQ, LDT, N, N1, N2

         REAL           Q( LDQ, * ), T( LDT, * ), WORK( * )

IMPLEMENTATION
     These routines are part of the SCSL Scientific Library and can be loaded
     using either the -lscs or the -lscs_mp option.  The -lscs_mp option
     directs the linker to use the multi-processor version of the library.

     When linking to SCSL with -lscs or -lscs_mp, the default integer size is
     4 bytes (32 bits). Another version of SCSL is available in which integers
     are 8 bytes (64 bits).  This version allows the user access to larger
     memory sizes and helps when porting legacy Cray codes.  It can be loaded
     by using the -lscs_i8 option or the -lscs_i8_mp option. A program may use
     only one of the two versions; 4-byte integer and 8-byte integer library
     calls cannot be mixed.

PURPOSE
     SLAEXC 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 elemnts equal and its off-diagonal elements of
     opposite sign.


ARGUMENTS
     WANTQ   (input) LOGICAL
             = .TRUE. : accumulate the transformation in the matrix Q;
             = .FALSE.: do not accumulate the transformation.

     N       (input) INTEGER
             The order of the matrix T. N >= 0.

     T       (input/output) REAL 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     (input)  INTEGER
             The leading dimension of the array T. LDT >= max(1,N).
     Q       (input/output) REAL 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     (input) INTEGER
             The leading dimension of the array Q.  LDQ >= 1; and if WANTQ is
             .TRUE., LDQ >= N.

     J1      (input) INTEGER
             The index of the first row of the first block T11.

     N1      (input) INTEGER
             The order of the first block T11. N1 = 0, 1 or 2.

     N2      (input) INTEGER
             The order of the second block T22. N2 = 0, 1 or 2.

     WORK    (workspace) REAL array, dimension (N)

     INFO    (output) 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.

SEE ALSO
     INTRO_LAPACK(3S), INTRO_SCSL(3S)

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