SUBROUTINE DSYTRS( UPLO, N, NRHS, A, LDA, IPIV, B, LDB, INFO )
*
* -- LAPACK routine (version 3.1) --
* Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
* November 2006
*
* .. Scalar Arguments ..
CHARACTER UPLO
INTEGER INFO, LDA, LDB, N, NRHS
* ..
* .. Array Arguments ..
INTEGER IPIV( * )
DOUBLE PRECISION A( LDA, * ), B( LDB, * )
* ..
*
* Purpose
* =======
*
* DSYTRS solves a system of linear equations A*X = B with a real
* symmetric matrix A using the factorization A = U*D*U**T or
* A = L*D*L**T computed by DSYTRF.
*
* Arguments
* =========
*
* UPLO (input) CHARACTER*1
* Specifies whether the details of the factorization are stored
* as an upper or lower triangular matrix.
* = 'U': Upper triangular, form is A = U*D*U**T;
* = 'L': Lower triangular, form is A = L*D*L**T.
*
* N (input) INTEGER
* The order of the matrix A. N >= 0.
*
* NRHS (input) INTEGER
* The number of right hand sides, i.e., the number of columns
* of the matrix B. NRHS >= 0.
*
* A (input) DOUBLE PRECISION array, dimension (LDA,N)
* The block diagonal matrix D and the multipliers used to
* obtain the factor U or L as computed by DSYTRF.
*
* LDA (input) INTEGER
* The leading dimension of the array A. LDA >= max(1,N).
*
* IPIV (input) INTEGER array, dimension (N)
* Details of the interchanges and the block structure of D
* as determined by DSYTRF.
*
* B (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
* On entry, the right hand side matrix B.
* On exit, the solution matrix X.
*
* LDB (input) INTEGER
* The leading dimension of the array B. LDB >= max(1,N).
*
* INFO (output) INTEGER
* = 0: successful exit
* < 0: if INFO = -i, the i-th argument had an illegal value
*
* =====================================================================
*
* .. Parameters ..
DOUBLE PRECISION ONE
PARAMETER ( ONE = 1.0D+0 )
* ..
* .. Local Scalars ..
LOGICAL UPPER
INTEGER J, K, KP
DOUBLE PRECISION AK, AKM1, AKM1K, BK, BKM1, DENOM
* ..
* .. External Functions ..
LOGICAL LSAME
EXTERNAL LSAME
* ..
* .. External Subroutines ..
EXTERNAL DGEMV, DGER, DSCAL, DSWAP, XERBLA
* ..
* .. Intrinsic Functions ..
INTRINSIC MAX
* ..
* .. Executable Statements ..
*
INFO = 0
UPPER = LSAME( UPLO, 'U' )
IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
INFO = -1
ELSE IF( N.LT.0 ) THEN
INFO = -2
ELSE IF( NRHS.LT.0 ) THEN
INFO = -3
ELSE IF( LDA.LT.MAX( 1, N ) ) THEN
INFO = -5
ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
INFO = -8
END IF
IF( INFO.NE.0 ) THEN
CALL XERBLA( 'DSYTRS', -INFO )
RETURN
END IF
*
* Quick return if possible
*
IF( N.EQ.0 .OR. NRHS.EQ.0 )
$ RETURN
*
IF( UPPER ) THEN
*
* Solve A*X = B, where A = U*D*U'.
*
* First solve U*D*X = B, overwriting B with X.
*
* K is the main loop index, decreasing from N to 1 in steps of
* 1 or 2, depending on the size of the diagonal blocks.
*
K = N
10 CONTINUE
*
* If K < 1, exit from loop.
*
IF( K.LT.1 )
$ GO TO 30
*
IF( IPIV( K ).GT.0 ) THEN
*
* 1 x 1 diagonal block
*
* Interchange rows K and IPIV(K).
*
KP = IPIV( K )
IF( KP.NE.K )
$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB )
*
* Multiply by inv(U(K)), where U(K) is the transformation
* stored in column K of A.
*
CALL DGER( K-1, NRHS, -ONE, A( 1, K ), 1, B( K, 1 ), LDB,
$ B( 1, 1 ), LDB )
*
* Multiply by the inverse of the diagonal block.
*
CALL DSCAL( NRHS, ONE / A( K, K ), B( K, 1 ), LDB )
K = K - 1
ELSE
*
* 2 x 2 diagonal block
*
* Interchange rows K-1 and -IPIV(K).
*
KP = -IPIV( K )
IF( KP.NE.K-1 )
$ CALL DSWAP( NRHS, B( K-1, 1 ), LDB, B( KP, 1 ), LDB )
*
* Multiply by inv(U(K)), where U(K) is the transformation
* stored in columns K-1 and K of A.
*
CALL DGER( K-2, NRHS, -ONE, A( 1, K ), 1, B( K, 1 ), LDB,
$ B( 1, 1 ), LDB )
CALL DGER( K-2, NRHS, -ONE, A( 1, K-1 ), 1, B( K-1, 1 ),
$ LDB, B( 1, 1 ), LDB )
*
* Multiply by the inverse of the diagonal block.
*
AKM1K = A( K-1, K )
AKM1 = A( K-1, K-1 ) / AKM1K
AK = A( K, K ) / AKM1K
DENOM = AKM1*AK - ONE
DO 20 J = 1, NRHS
BKM1 = B( K-1, J ) / AKM1K
BK = B( K, J ) / AKM1K
B( K-1, J ) = ( AK*BKM1-BK ) / DENOM
B( K, J ) = ( AKM1*BK-BKM1 ) / DENOM
20 CONTINUE
K = K - 2
END IF
*
GO TO 10
30 CONTINUE
*
* Next solve U'*X = B, overwriting B with X.
*
* K is the main loop index, increasing from 1 to N in steps of
* 1 or 2, depending on the size of the diagonal blocks.
*
K = 1
40 CONTINUE
*
* If K > N, exit from loop.
*
IF( K.GT.N )
$ GO TO 50
*
IF( IPIV( K ).GT.0 ) THEN
*
* 1 x 1 diagonal block
*
* Multiply by inv(U'(K)), where U(K) is the transformation
* stored in column K of A.
*
CALL DGEMV( 'Transpose', K-1, NRHS, -ONE, B, LDB, A( 1, K ),
$ 1, ONE, B( K, 1 ), LDB )
*
* Interchange rows K and IPIV(K).
*
KP = IPIV( K )
IF( KP.NE.K )
$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB )
K = K + 1
ELSE
*
* 2 x 2 diagonal block
*
* Multiply by inv(U'(K+1)), where U(K+1) is the transformation
* stored in columns K and K+1 of A.
*
CALL DGEMV( 'Transpose', K-1, NRHS, -ONE, B, LDB, A( 1, K ),
$ 1, ONE, B( K, 1 ), LDB )
CALL DGEMV( 'Transpose', K-1, NRHS, -ONE, B, LDB,
$ A( 1, K+1 ), 1, ONE, B( K+1, 1 ), LDB )
*
* Interchange rows K and -IPIV(K).
*
KP = -IPIV( K )
IF( KP.NE.K )
$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB )
K = K + 2
END IF
*
GO TO 40
50 CONTINUE
*
ELSE
*
* Solve A*X = B, where A = L*D*L'.
*
* First solve L*D*X = B, overwriting B with X.
*
* K is the main loop index, increasing from 1 to N in steps of
* 1 or 2, depending on the size of the diagonal blocks.
*
K = 1
60 CONTINUE
*
* If K > N, exit from loop.
*
IF( K.GT.N )
$ GO TO 80
*
IF( IPIV( K ).GT.0 ) THEN
*
* 1 x 1 diagonal block
*
* Interchange rows K and IPIV(K).
*
KP = IPIV( K )
IF( KP.NE.K )
$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB )
*
* Multiply by inv(L(K)), where L(K) is the transformation
* stored in column K of A.
*
IF( K.LT.N )
$ CALL DGER( N-K, NRHS, -ONE, A( K+1, K ), 1, B( K, 1 ),
$ LDB, B( K+1, 1 ), LDB )
*
* Multiply by the inverse of the diagonal block.
*
CALL DSCAL( NRHS, ONE / A( K, K ), B( K, 1 ), LDB )
K = K + 1
ELSE
*
* 2 x 2 diagonal block
*
* Interchange rows K+1 and -IPIV(K).
*
KP = -IPIV( K )
IF( KP.NE.K+1 )
$ CALL DSWAP( NRHS, B( K+1, 1 ), LDB, B( KP, 1 ), LDB )
*
* Multiply by inv(L(K)), where L(K) is the transformation
* stored in columns K and K+1 of A.
*
IF( K.LT.N-1 ) THEN
CALL DGER( N-K-1, NRHS, -ONE, A( K+2, K ), 1, B( K, 1 ),
$ LDB, B( K+2, 1 ), LDB )
CALL DGER( N-K-1, NRHS, -ONE, A( K+2, K+1 ), 1,
$ B( K+1, 1 ), LDB, B( K+2, 1 ), LDB )
END IF
*
* Multiply by the inverse of the diagonal block.
*
AKM1K = A( K+1, K )
AKM1 = A( K, K ) / AKM1K
AK = A( K+1, K+1 ) / AKM1K
DENOM = AKM1*AK - ONE
DO 70 J = 1, NRHS
BKM1 = B( K, J ) / AKM1K
BK = B( K+1, J ) / AKM1K
B( K, J ) = ( AK*BKM1-BK ) / DENOM
B( K+1, J ) = ( AKM1*BK-BKM1 ) / DENOM
70 CONTINUE
K = K + 2
END IF
*
GO TO 60
80 CONTINUE
*
* Next solve L'*X = B, overwriting B with X.
*
* K is the main loop index, decreasing from N to 1 in steps of
* 1 or 2, depending on the size of the diagonal blocks.
*
K = N
90 CONTINUE
*
* If K < 1, exit from loop.
*
IF( K.LT.1 )
$ GO TO 100
*
IF( IPIV( K ).GT.0 ) THEN
*
* 1 x 1 diagonal block
*
* Multiply by inv(L'(K)), where L(K) is the transformation
* stored in column K of A.
*
IF( K.LT.N )
$ CALL DGEMV( 'Transpose', N-K, NRHS, -ONE, B( K+1, 1 ),
$ LDB, A( K+1, K ), 1, ONE, B( K, 1 ), LDB )
*
* Interchange rows K and IPIV(K).
*
KP = IPIV( K )
IF( KP.NE.K )
$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB )
K = K - 1
ELSE
*
* 2 x 2 diagonal block
*
* Multiply by inv(L'(K-1)), where L(K-1) is the transformation
* stored in columns K-1 and K of A.
*
IF( K.LT.N ) THEN
CALL DGEMV( 'Transpose', N-K, NRHS, -ONE, B( K+1, 1 ),
$ LDB, A( K+1, K ), 1, ONE, B( K, 1 ), LDB )
CALL DGEMV( 'Transpose', N-K, NRHS, -ONE, B( K+1, 1 ),
$ LDB, A( K+1, K-1 ), 1, ONE, B( K-1, 1 ),
$ LDB )
END IF
*
* Interchange rows K and -IPIV(K).
*
KP = -IPIV( K )
IF( KP.NE.K )
$ CALL DSWAP( NRHS, B( K, 1 ), LDB, B( KP, 1 ), LDB )
K = K - 2
END IF
*
GO TO 90
100 CONTINUE
END IF
*
RETURN
*
* End of DSYTRS
*
END