1SLAEXC(1)           LAPACK auxiliary routine (version 3.2)           SLAEXC(1)
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NAME

6       SLAEXC  - swaps adjacent diagonal blocks T11 and T22 of order 1 or 2 in
7       an upper quasi-triangular matrix T by an orthogonal  similarity  trans‐
8       formation
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SYNOPSIS

11       SUBROUTINE SLAEXC( WANTQ, N, T, LDT, Q, LDQ, J1, N1, N2, WORK, INFO )
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13           LOGICAL        WANTQ
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15           INTEGER        INFO, J1, LDQ, LDT, N, N1, N2
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17           REAL           Q( LDQ, * ), T( LDT, * ), WORK( * )
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PURPOSE

20       SLAEXC swaps adjacent diagonal blocks T11 and T22 of order 1 or 2 in an
21       upper quasi-triangular matrix T by an orthogonal similarity transforma‐
22       tion.  T must be in Schur canonical form, that is, block upper triangu‐
23       lar with 1-by-1 and 2-by-2 diagonal blocks; each 2-by-2 diagonal  block
24       has  its  diagonal elemnts equal and its off-diagonal elements of oppo‐
25       site sign.
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ARGUMENTS

28       WANTQ   (input) LOGICAL
29               = .TRUE. : accumulate the transformation in the matrix Q;
30               = .FALSE.: do not accumulate the transformation.
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32       N       (input) INTEGER
33               The order of the matrix T. N >= 0.
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35       T       (input/output) REAL array, dimension (LDT,N)
36               On entry, the upper quasi-triangular matrix T, in Schur canoni‐
37               cal form.  On exit, the updated matrix T, again in Schur canon‐
38               ical form.
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40       LDT     (input)  INTEGER
41               The leading dimension of the array T. LDT >= max(1,N).
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43       Q       (input/output) REAL array, dimension (LDQ,N)
44               On entry, if WANTQ is .TRUE.,  the  orthogonal  matrix  Q.   On
45               exit,  if  WANTQ  is .TRUE., the updated matrix Q.  If WANTQ is
46               .FALSE., Q is not referenced.
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48       LDQ     (input) INTEGER
49               The leading dimension of the array Q.  LDQ >= 1; and  if  WANTQ
50               is .TRUE., LDQ >= N.
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52       J1      (input) INTEGER
53               The index of the first row of the first block T11.
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55       N1      (input) INTEGER
56               The order of the first block T11. N1 = 0, 1 or 2.
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58       N2      (input) INTEGER
59               The order of the second block T22. N2 = 0, 1 or 2.
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61       WORK    (workspace) REAL array, dimension (N)
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63       INFO    (output) INTEGER
64               = 0: successful exit
65               = 1: the transformed matrix T would be too far from Schur form;
66               the blocks are not swapped and T and Q are unchanged.
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70 LAPACK auxiliary routine (versionNo3v.e2m)ber 2008                       SLAEXC(1)
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