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SUBROUTINE SGEBAK( JOB, SIDE, N, ILO, IHI, SCALE, M, V, LDV,
$ INFO )
*
* -- LAPACK routine (version 3.2) --
* -- LAPACK is a software package provided by Univ. of Tennessee, --
* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
* November 2006
*
* .. Scalar Arguments ..
CHARACTER JOB, SIDE
INTEGER IHI, ILO, INFO, LDV, M, N
* ..
* .. Array Arguments ..
REAL V( LDV, * ), SCALE( * )
* ..
*
* Purpose
* =======
*
* SGEBAK forms the right or left eigenvectors of a real general matrix
* by backward transformation on the computed eigenvectors of the
* balanced matrix output by SGEBAL.
*
* Arguments
* =========
*
* JOB (input) CHARACTER*1
* Specifies the type of backward transformation required:
* = 'N', do nothing, return immediately;
* = 'P', do backward transformation for permutation only;
* = 'S', do backward transformation for scaling only;
* = 'B', do backward transformations for both permutation and
* scaling.
* JOB must be the same as the argument JOB supplied to SGEBAL.
*
* SIDE (input) CHARACTER*1
* = 'R': V contains right eigenvectors;
* = 'L': V contains left eigenvectors.
*
* N (input) INTEGER
* The number of rows of the matrix V. N >= 0.
*
* ILO (input) INTEGER
* IHI (input) INTEGER
* The integers ILO and IHI determined by SGEBAL.
* 1 <= ILO <= IHI <= N, if N > 0; ILO=1 and IHI=0, if N=0.
*
* SCALE (input) REAL array, dimension (N)
* Details of the permutation and scaling factors, as returned
* by SGEBAL.
*
* M (input) INTEGER
* The number of columns of the matrix V. M >= 0.
*
* V (input/output) REAL array, dimension (LDV,M)
* On entry, the matrix of right or left eigenvectors to be
* transformed, as returned by SHSEIN or STREVC.
* On exit, V is overwritten by the transformed eigenvectors.
*
* LDV (input) INTEGER
* The leading dimension of the array V. LDV >= max(1,N).
*
* INFO (output) INTEGER
* = 0: successful exit
* < 0: if INFO = -i, the i-th argument had an illegal value.
*
* =====================================================================
*
* .. Parameters ..
REAL ONE
PARAMETER ( ONE = 1.0E+0 )
* ..
* .. Local Scalars ..
LOGICAL LEFTV, RIGHTV
INTEGER I, II, K
REAL S
* ..
* .. External Functions ..
LOGICAL LSAME
EXTERNAL LSAME
* ..
* .. External Subroutines ..
EXTERNAL SSCAL, SSWAP, XERBLA
* ..
* .. Intrinsic Functions ..
INTRINSIC MAX, MIN
* ..
* .. Executable Statements ..
*
* Decode and Test the input parameters
*
RIGHTV = LSAME( SIDE, 'R' )
LEFTV = LSAME( SIDE, 'L' )
*
INFO = 0
IF( .NOT.LSAME( JOB, 'N' ) .AND. .NOT.LSAME( JOB, 'P' ) .AND.
$ .NOT.LSAME( JOB, 'S' ) .AND. .NOT.LSAME( JOB, 'B' ) ) THEN
INFO = -1
ELSE IF( .NOT.RIGHTV .AND. .NOT.LEFTV ) THEN
INFO = -2
ELSE IF( N.LT.0 ) THEN
INFO = -3
ELSE IF( ILO.LT.1 .OR. ILO.GT.MAX( 1, N ) ) THEN
INFO = -4
ELSE IF( IHI.LT.MIN( ILO, N ) .OR. IHI.GT.N ) THEN
INFO = -5
ELSE IF( M.LT.0 ) THEN
INFO = -7
ELSE IF( LDV.LT.MAX( 1, N ) ) THEN
INFO = -9
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'SGEBAK', -INFO )
RETURN
END IF
*
* Quick return if possible
*
IF( N.EQ.0 )
$ RETURN
IF( M.EQ.0 )
$ RETURN
IF( LSAME( JOB, 'N' ) )
$ RETURN
*
IF( ILO.EQ.IHI )
$ GO TO 30
*
* Backward balance
*
IF( LSAME( JOB, 'S' ) .OR. LSAME( JOB, 'B' ) ) THEN
*
IF( RIGHTV ) THEN
DO 10 I = ILO, IHI
S = SCALE( I )
CALL SSCAL( M, S, V( I, 1 ), LDV )
10 CONTINUE
END IF
*
IF( LEFTV ) THEN
DO 20 I = ILO, IHI
S = ONE / SCALE( I )
CALL SSCAL( M, S, V( I, 1 ), LDV )
20 CONTINUE
END IF
*
END IF
*
* Backward permutation
*
* For I = ILO-1 step -1 until 1,
* IHI+1 step 1 until N do --
*
30 CONTINUE
IF( LSAME( JOB, 'P' ) .OR. LSAME( JOB, 'B' ) ) THEN
IF( RIGHTV ) THEN
DO 40 II = 1, N
I = II
IF( I.GE.ILO .AND. I.LE.IHI )
$ GO TO 40
IF( I.LT.ILO )
$ I = ILO - II
K = SCALE( I )
IF( K.EQ.I )
$ GO TO 40
CALL SSWAP( M, V( I, 1 ), LDV, V( K, 1 ), LDV )
40 CONTINUE
END IF
*
IF( LEFTV ) THEN
DO 50 II = 1, N
I = II
IF( I.GE.ILO .AND. I.LE.IHI )
$ GO TO 50
IF( I.LT.ILO )
$ I = ILO - II
K = SCALE( I )
IF( K.EQ.I )
$ GO TO 50
CALL SSWAP( M, V( I, 1 ), LDV, V( K, 1 ), LDV )
50 CONTINUE
END IF
END IF
*
RETURN
*
* End of SGEBAK
*
END
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