      SUBROUTINE DSYSV( UPLO, N, NRHS, A, LDA, IPIV, B, LDB, WORK,
     $                  LWORK, INFO )
*
*  -- LAPACK driver 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          UPLO
      INTEGER            INFO, LDA, LDB, LWORK, N, NRHS
*     ..
*     .. Array Arguments ..
      INTEGER            IPIV( * )
      DOUBLE PRECISION   A( LDA, * ), B( LDB, * ), WORK( * )
*     ..
*
*  Purpose
*  =======
*
*  DSYSV computes the solution to a real system of linear equations
*     A * X = B,
*  where A is an N-by-N symmetric matrix and X and B are N-by-NRHS
*  matrices.
*
*  The diagonal pivoting method is used to factor A as
*     A = U * D * U**T,  if UPLO = 'U', or
*     A = L * D * L**T,  if UPLO = 'L',
*  where U (or L) is a product of permutation and unit upper (lower)
*  triangular matrices, and D is symmetric and block diagonal with
*  1-by-1 and 2-by-2 diagonal blocks.  The factored form of A is then
*  used to solve the system of equations A * X = B.
*
*  Arguments
*  =========
*
*  UPLO    (input) CHARACTER*1
*          = 'U':  Upper triangle of A is stored;
*          = 'L':  Lower triangle of A is stored.
*
*  N       (input) INTEGER
*          The number of linear equations, i.e., 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/output) DOUBLE PRECISION array, dimension (LDA,N)
*          On entry, the symmetric matrix A.  If UPLO = 'U', the leading
*          N-by-N upper triangular part of A contains the upper
*          triangular part of the matrix A, and the strictly lower
*          triangular part of A is not referenced.  If UPLO = 'L', the
*          leading N-by-N lower triangular part of A contains the lower
*          triangular part of the matrix A, and the strictly upper
*          triangular part of A is not referenced.
*
*          On exit, if INFO = 0, the block diagonal matrix D and the
*          multipliers used to obtain the factor U or L from the
*          factorization A = U*D*U**T or A = L*D*L**T as computed by
*          DSYTRF.
*
*  LDA     (input) INTEGER
*          The leading dimension of the array A.  LDA >= max(1,N).
*
*  IPIV    (output) INTEGER array, dimension (N)
*          Details of the interchanges and the block structure of D, as
*          determined by DSYTRF.  If IPIV(k) > 0, then rows and columns
*          k and IPIV(k) were interchanged, and D(k,k) is a 1-by-1
*          diagonal block.  If UPLO = 'U' and IPIV(k) = IPIV(k-1) < 0,
*          then rows and columns k-1 and -IPIV(k) were interchanged and
*          D(k-1:k,k-1:k) is a 2-by-2 diagonal block.  If UPLO = 'L' and
*          IPIV(k) = IPIV(k+1) < 0, then rows and columns k+1 and
*          -IPIV(k) were interchanged and D(k:k+1,k:k+1) is a 2-by-2
*          diagonal block.
*
*  B       (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
*          On entry, the N-by-NRHS right hand side matrix B.
*          On exit, if INFO = 0, the N-by-NRHS solution matrix X.
*
*  LDB     (input) INTEGER
*          The leading dimension of the array B.  LDB >= max(1,N).
*
*  WORK    (workspace/output) DOUBLE PRECISION array, dimension (MAX(1,LWORK))
*          On exit, if INFO = 0, WORK(1) returns the optimal LWORK.
*
*  LWORK   (input) INTEGER
*          The length of WORK.  LWORK >= 1, and for best performance
*          LWORK >= max(1,N*NB), where NB is the optimal blocksize for
*          DSYTRF.
*
*          If LWORK = -1, then a workspace query is assumed; the routine
*          only calculates the optimal size of the WORK array, returns
*          this value as the first entry of the WORK array, and no error
*          message related to LWORK is issued by XERBLA.
*
*  INFO    (output) INTEGER
*          = 0: successful exit
*          < 0: if INFO = -i, the i-th argument had an illegal value
*          > 0: if INFO = i, D(i,i) is exactly zero.  The factorization
*               has been completed, but the block diagonal matrix D is
*               exactly singular, so the solution could not be computed.
*
*  =====================================================================
*
*     .. Local Scalars ..
      LOGICAL            LQUERY
      INTEGER            LWKOPT, NB
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      INTEGER            ILAENV
      EXTERNAL           LSAME, ILAENV
*     ..
*     .. External Subroutines ..
      EXTERNAL           DSYTRF, DSYTRS, XERBLA
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          MAX
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
      INFO = 0
      LQUERY = ( LWORK.EQ.-1 )
      IF( .NOT.LSAME( UPLO, 'U' ) .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
      ELSE IF( LWORK.LT.1 .AND. .NOT.LQUERY ) THEN
         INFO = -10
      END IF
*
      IF( INFO.EQ.0 ) THEN
         IF( N.EQ.0 ) THEN
            LWKOPT = 1
         ELSE
            NB = ILAENV( 1, 'DSYTRF', UPLO, N, -1, -1, -1 )
            LWKOPT = N*NB
         END IF
         WORK( 1 ) = LWKOPT
      END IF
*
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'DSYSV ', -INFO )
         RETURN
      ELSE IF( LQUERY ) THEN
         RETURN
      END IF
*
*     Compute the factorization A = U*D*U' or A = L*D*L'.
*
      CALL DSYTRF( UPLO, N, A, LDA, IPIV, WORK, LWORK, INFO )
      IF( INFO.EQ.0 ) THEN
*
*        Solve the system A*X = B, overwriting B with X.
*
         CALL DSYTRS( UPLO, N, NRHS, A, LDA, IPIV, B, LDB, INFO )
*
      END IF
*
      WORK( 1 ) = LWKOPT
*
      RETURN
*
*     End of DSYSV
*
      END
