ssytrs_rook()

subroutine ssytrs_rook	(	character	uplo,
		integer	n,
		integer	nrhs,
		real, dimension( lda, * )	a,
		integer	lda,
		integer, dimension( * )	ipiv,
		real, dimension( ldb, * )	b,
		integer	ldb,
		integer	info
	)

SSYTRS_ROOK

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Purpose:

 SSYTRS_ROOK 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 SSYTRF_ROOK.

Parameters

[in]	UPLO	UPLO is 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.
[in]	N	N is INTEGER The order of the matrix A. N >= 0.
[in]	NRHS	NRHS is INTEGER The number of right hand sides, i.e., the number of columns of the matrix B. NRHS >= 0.
[in]	A	A is REAL array, dimension (LDA,N) The block diagonal matrix D and the multipliers used to obtain the factor U or L as computed by SSYTRF_ROOK.
[in]	LDA	LDA is INTEGER The leading dimension of the array A. LDA >= max(1,N).
[in]	IPIV	IPIV is INTEGER array, dimension (N) Details of the interchanges and the block structure of D as determined by SSYTRF_ROOK.
[in,out]	B	B is REAL array, dimension (LDB,NRHS) On entry, the right hand side matrix B. On exit, the solution matrix X.
[in]	LDB	LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N).
[out]	INFO	INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value

Author

Univ. of Tennessee

Univ. of California Berkeley

Univ. of Colorado Denver

NAG Ltd.

Contributors:

   April 2012, Igor Kozachenko,
                  Computer Science Division,
                  University of California, Berkeley

  September 2007, Sven Hammarling, Nicholas J. Higham, Craig Lucas,
                  School of Mathematics,
                  University of Manchester

Definition at line 134 of file ssytrs_rook.f.

*
*  -- LAPACK computational routine --
*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
*
*     .. Scalar Arguments ..
      CHARACTER          UPLO
      INTEGER            INFO, LDA, LDB, N, NRHS
*     ..
*     .. Array Arguments ..
      INTEGER            IPIV( * )
      REAL               A( LDA, * ), B( LDB, * )
*     ..
*
*  =====================================================================
*
*     .. Parameters ..
      REAL               ONE
      parameter( one = 1.0e+0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            UPPER
      INTEGER            J, K, KP
      REAL               AK, AKM1, AKM1K, BK, BKM1, DENOM
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      EXTERNAL           lsame
*     ..
*     .. External Subroutines ..
      EXTERNAL           sgemv, sger, sscal, sswap, 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( 'SSYTRS_ROOK', -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**T.
*
*        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 sswap( 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 sger( 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 sscal( nrhs, one / a( k, k ), b( k, 1 ), ldb )
            k = k - 1
         ELSE
*
*           2 x 2 diagonal block
*
*           Interchange rows K and -IPIV(K) THEN K-1 and -IPIV(K-1)
*
            kp = -ipiv( k )
            IF( kp.NE.k )
     $         CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
*
            kp = -ipiv( k-1 )
            IF( kp.NE.k-1 )
     $         CALL sswap( 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.
*
            IF( k.GT.2 ) THEN
               CALL sger( k-2, nrhs, -one, a( 1, k ), 1, b( k, 1 ),
     $                    ldb, b( 1, 1 ), ldb )
               CALL sger( k-2, nrhs, -one, a( 1, k-1 ), 1, b( k-1, 1 ),
     $                    ldb, b( 1, 1 ), ldb )
            END IF
*
*           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**T *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**T(K)), where U(K) is the transformation
*           stored in column K of A.
*
            IF( k.GT.1 )
     $         CALL sgemv( '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 sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
            k = k + 1
         ELSE
*
*           2 x 2 diagonal block
*
*           Multiply by inv(U**T(K+1)), where U(K+1) is the transformation
*           stored in columns K and K+1 of A.
*
            IF( k.GT.1 ) THEN
               CALL sgemv( 'Transpose', k-1, nrhs, -one, b,
     $                     ldb, a( 1, k ), 1, one, b( k, 1 ), ldb )
               CALL sgemv( 'Transpose', k-1, nrhs, -one, b,
     $                     ldb, a( 1, k+1 ), 1, one, b( k+1, 1 ), ldb )
            END IF
*
*           Interchange rows K and -IPIV(K) THEN K+1 and -IPIV(K+1).
*
            kp = -ipiv( k )
            IF( kp.NE.k )
     $         CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
*
            kp = -ipiv( k+1 )
            IF( kp.NE.k+1 )
     $         CALL sswap( nrhs, b( k+1, 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**T.
*
*        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 sswap( 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 sger( 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 sscal( nrhs, one / a( k, k ), b( k, 1 ), ldb )
            k = k + 1
         ELSE
*
*           2 x 2 diagonal block
*
*           Interchange rows K and -IPIV(K) THEN K+1 and -IPIV(K+1)
*
            kp = -ipiv( k )
            IF( kp.NE.k )
     $         CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
*
            kp = -ipiv( k+1 )
            IF( kp.NE.k+1 )
     $         CALL sswap( 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 sger( n-k-1, nrhs, -one, a( k+2, k ), 1, b( k, 1 ),
     $                    ldb, b( k+2, 1 ), ldb )
               CALL sger( 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**T *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**T(K)), where L(K) is the transformation
*           stored in column K of A.
*
            IF( k.LT.n )
     $         CALL sgemv( '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 sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
            k = k - 1
         ELSE
*
*           2 x 2 diagonal block
*
*           Multiply by inv(L**T(K-1)), where L(K-1) is the transformation
*           stored in columns K-1 and K of A.
*
            IF( k.LT.n ) THEN
               CALL sgemv( 'Transpose', n-k, nrhs, -one, b( k+1, 1 ),
     $                     ldb, a( k+1, k ), 1, one, b( k, 1 ), ldb )
               CALL sgemv( '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) THEN K-1 and -IPIV(K-1)
*
            kp = -ipiv( k )
            IF( kp.NE.k )
     $         CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
*
            kp = -ipiv( k-1 )
            IF( kp.NE.k-1 )
     $         CALL sswap( nrhs, b( k-1, 1 ), ldb, b( kp, 1 ), ldb )
*
            k = k - 2
         END IF
*
         GO TO 90
  100    CONTINUE
      END IF
*
      RETURN
*
*     End of SSYTRS_ROOK
*

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