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

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

     LOGICAL WANTQ

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

     DOUBLE PRECISION Q( LDQ, * ), T( LDT, * ), WORK( * )



     #include <sunperf.h>

     void dlaexc(int wantq, int n, double *t, int ldt, double *q,
               int ldq, int j1, int n1, int n2, int *info) ;

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 tri-
     angular  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) 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       (input)  INTEGER
               The leading dimension  of  the  array  T.  LDT  >=
               max(1,N).

     Q         (input/output) 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       (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) DOUBLE PRECISION 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.

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