          SUBROUTINE DSGESV( N, NRHS, A, LDA, IPIV, B, LDB, X, LDX, WORK,
         +                   SWORK, ITER, INFO)
    *
    *  -- LAPACK PROTOTYPE driver routine (version 3.1.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     February 2007
    *
    *     ..
    *     .. WARNING: PROTOTYPE ..
    *     This is an LAPACK PROTOTYPE routine which means that the
    *     interface of this routine is likely to be changed in the future
    *     based on community feedback.
    *
    *     ..
    *     .. Scalar Arguments ..
          INTEGER INFO,ITER,LDA,LDB,LDX,N,NRHS
    *     ..
    *     .. Array Arguments ..
          INTEGER IPIV(*)
          REAL SWORK(*)
          DOUBLE PRECISION A(LDA,*),B(LDB,*),WORK(N,*),X(LDX,*)
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  DSGESV computes the solution to a real system of linear equations
    *     A * X = B,
    *  where A is an N-by-N matrix and X and B are N-by-NRHS matrices.
    *
    *  DSGESV first attempts to factorize the matrix in SINGLE PRECISION
    *  and use this factorization within an iterative refinement procedure to
    *  produce a solution with DOUBLE PRECISION normwise backward error
    *  quality (see below). If the approach fails the method switches to a
    *  DOUBLE PRECISION factorization and solve.
    *
    *  The iterative refinement is not going to be a winning strategy if
    *  the ratio SINGLE PRECISION performance over DOUBLE PRECISION performance
    *  is too small. A reasonable strategy should take the number of right-hand
    *  sides and the size of the matrix into account. This might be done with a
    *  call to ILAENV in the future. Up to now, we always try iterative refinement.
    *
    *  The iterative refinement process is stopped if
    *      ITER > ITERMAX
    *  or for all the RHS we have:
    *      RNRM < SQRT(N)*XNRM*ANRM*EPS*BWDMAX
    *  where
    *      o ITER is the number of the current iteration in the iterative
    *        refinement process
    *      o RNRM is the infinity-norm of the residual
    *      o XNRM is the infinity-norm of the solution
    *      o ANRM is the infinity-operator-norm of the matrix A
    *      o EPS is the machine epsilon returned by DLAMCH('Epsilon')
    *  The value ITERMAX and BWDMAX are fixed to 30 and 1.0D+00 respectively.
    *
    *  Arguments
    *  =========
    *
    *  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 or input/ouptut) DOUBLE PRECISION array,
    *          dimension (LDA,N)
    *          On entry, the N-by-N coefficient matrix A.
    *          On exit, if iterative refinement has been successfully used
    *          (INFO.EQ.0 and ITER.GE.0, see description below), then A is
    *          unchanged, if double precision factorization has been used
    *          (INFO.EQ.0 and ITER.LT.0, see description below), then the
    *          array A contains the factors L and U from the factorization
    *          A = P*L*U; the unit diagonal elements of L are not stored.
    *
    *  LDA     (input) INTEGER
    *          The leading dimension of the array A.  LDA >= max(1,N).
    *
    *  IPIV    (output) INTEGER array, dimension (N)
    *          The pivot indices that define the permutation matrix P;
    *          row i of the matrix was interchanged with row IPIV(i).
    *          Corresponds either to the single precision factorization
    *          (if INFO.EQ.0 and ITER.GE.0) or the double precision
    *          factorization (if INFO.EQ.0 and ITER.LT.0).
    *
    *  B       (input) DOUBLE PRECISION array, dimension (LDB,NRHS)
    *          The N-by-NRHS matrix of right hand side matrix B.
    *
    *  LDB     (input) INTEGER
    *          The leading dimension of the array B.  LDB >= max(1,N).
    *
    *  X       (output) DOUBLE PRECISION array, dimension (LDX,NRHS)
    *          If INFO = 0, the N-by-NRHS solution matrix X.
    *
    *  LDX     (input) INTEGER
    *          The leading dimension of the array X.  LDX >= max(1,N).
    *
    *  WORK    (workspace) DOUBLE PRECISION array, dimension (N*NRHS)
    *          This array is used to hold the residual vectors.
    *
    *  SWORK   (workspace) REAL array, dimension (N*(N+NRHS))
    *          This array is used to use the single precision matrix and the
    *          right-hand sides or solutions in single precision.
    *
    *  ITER    (output) INTEGER
    *          < 0: iterative refinement has failed, double precision
    *               factorization has been performed
    *               -1 : taking into account machine parameters, N, NRHS, it
    *                    is a priori not worth working in SINGLE PRECISION
    *               -2 : overflow of an entry when moving from double to
    *                    SINGLE PRECISION
    *               -3 : failure of SGETRF
    *               -31: stop the iterative refinement after the 30th
    *                    iterations
    *          > 0: iterative refinement has been sucessfully used.
    *               Returns the number of iterations
    *
    *  INFO    (output) INTEGER
    *          = 0:  successful exit
    *          < 0:  if INFO = -i, the i-th argument had an illegal value
    *          > 0:  if INFO = i, U(i,i) computed in DOUBLE PRECISION is
    *                exactly zero.  The factorization has been completed,
    *                but the factor U is exactly singular, so the solution
    *                could not be computed.
    *
    *  =========
    *
    *     .. Parameters ..
          DOUBLE PRECISION NEGONE,ONE
          PARAMETER (NEGONE=-1.0D+0,ONE=1.0D+0)
    *
    *     .. Local Scalars ..
          LOGICAL DOITREF
          INTEGER I,IITER,ITERMAX,OK,PTSA,PTSX
          DOUBLE PRECISION ANRM,BWDMAX,CTE,EPS,RNRM,XNRM
    *
    *     .. External Subroutines ..
          EXTERNAL DAXPY,DGEMM,DLACPY,DLAG2S,SLAG2D,
         +         SGETRF,SGETRS,XERBLA
    *     ..
    *     .. External Functions ..
          INTEGER IDAMAX
          DOUBLE PRECISION DLAMCH,DLANGE
          EXTERNAL IDAMAX,DLAMCH,DLANGE
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC ABS,DBLE,MAX,SQRT
    *     ..
    *     .. Executable Statements ..
    *
          ITERMAX = 30
          BWDMAX = 1.0E+00
          DOITREF = .TRUE.
    *
          OK = 0
          INFO = 0
          ITER = 0
    *
    *     Test the input parameters.
    *
          IF (N.LT.0) THEN
              INFO = -1
          ELSE IF (NRHS.LT.0) THEN
              INFO = -2
          ELSE IF (LDA.LT.MAX(1,N)) THEN
              INFO = -4
          ELSE IF (LDB.LT.MAX(1,N)) THEN
              INFO = -7
          ELSE IF (LDX.LT.MAX(1,N)) THEN
              INFO = -9
          END IF
          IF (INFO.NE.0) THEN
              CALL XERBLA('DSGESV',-INFO)
              RETURN
          END IF
    *
    *     Quick return if (N.EQ.0).
    *
          IF (N.EQ.0) RETURN
    *
    *     Skip single precision iterative refinement if a priori slower
    *     than double precision factorization.
    *
          IF (.NOT.DOITREF) THEN
              ITER = -1
              GO TO 40
          END IF
    *
    *     Compute some constants.
    *
          ANRM = DLANGE('I',N,N,A,LDA,WORK)
          EPS = DLAMCH('Epsilon')
          CTE = ANRM*EPS*SQRT(DBLE(N))*BWDMAX
    *
    *     Set the pointers PTSA, PTSX for referencing SA and SX in SWORK.
    *
          PTSA = 1
          PTSX = PTSA + N*N
    *
    *     Convert B from double precision to single precision and store the
    *     result in SX.
    *
          CALL DLAG2S(N,NRHS,B,LDB,SWORK(PTSX),N,INFO)
    *
          IF (INFO.NE.0) THEN
              ITER = -2
              GO TO 40
          END IF
    *
    *     Convert A from double precision to single precision and store the
    *     result in SA.
    *
          CALL DLAG2S(N,N,A,LDA,SWORK(PTSA),N,INFO)
    *
          IF (INFO.NE.0) THEN
              ITER = -2
              GO TO 40
          END IF
    *
    *     Compute the LU factorization of SA.
    *
          CALL SGETRF(N,N,SWORK(PTSA),N,IPIV,INFO)
    *
          IF (INFO.NE.0) THEN
              ITER = -3
              GO TO 40
          END IF
    *
    *     Solve the system SA*SX = SB.
    *
          CALL SGETRS('No transpose',N,NRHS,SWORK(PTSA),N,IPIV,
         +            SWORK(PTSX),N,INFO)
    *
    *     Convert SX back to double precision
    *
          CALL SLAG2D(N,NRHS,SWORK(PTSX),N,X,LDX,INFO)
    *
    *     Compute R = B - AX (R is WORK).
    *
          CALL DLACPY('All',N,NRHS,B,LDB,WORK,N)
    *
          CALL DGEMM('No Transpose','No Transpose',N,NRHS,N,NEGONE,A,LDA,X,
         +           LDX,ONE,WORK,N)
    *
    *     Check whether the NRHS normwised backward errors satisfy the
    *     stopping criterion. If yes, set ITER=0 and return.
    *
          DO I = 1,NRHS
              XNRM = ABS(X(IDAMAX(N,X(1,I),1),I))
              RNRM = ABS(WORK(IDAMAX(N,WORK(1,I),1),I))
              IF (RNRM.GT.XNRM*CTE) GOTO 10
          END DO
    *
    *     If we are here, the NRHS normwised backward errors satisfy the
    *     stopping criterion. We are good to exit.
    *
          ITER = 0
          RETURN
    *
     10   CONTINUE
    *
          DO 30 IITER = 1,ITERMAX
    *
    *         Convert R (in WORK) from double precision to single precision
    *         and store the result in SX.
    *
              CALL DLAG2S(N,NRHS,WORK,N,SWORK(PTSX),N,INFO)
    *
              IF (INFO.NE.0) THEN
                  ITER = -2
                  GO TO 40
              END IF
    *
    *         Solve the system SA*SX = SR.
    *
              CALL SGETRS('No transpose',N,NRHS,SWORK(PTSA),N,IPIV,
         +                SWORK(PTSX),N,INFO)
    *
    *         Convert SX back to double precision and update the current
    *         iterate.
    *
              CALL SLAG2D(N,NRHS,SWORK(PTSX),N,WORK,N,INFO)
    *
              CALL DAXPY(N*NRHS,ONE,WORK,1,X,1)
    *
    *         Compute R = B - AX (R is WORK).
    *
              CALL DLACPY('All',N,NRHS,B,LDB,WORK,N)
    *
              CALL DGEMM('No Transpose','No Transpose',N,NRHS,N,NEGONE,A,
         +               LDA,X,LDX,ONE,WORK,N)
    *
    *         Check whether the NRHS normwised backward errors satisfy the
    *         stopping criterion. If yes, set ITER=IITER>0 and return.
    *
              DO I = 1,NRHS
                  XNRM = ABS(X(IDAMAX(N,X(1,I),1),I))
                  RNRM = ABS(WORK(IDAMAX(N,WORK(1,I),1),I))
                  IF (RNRM.GT.XNRM*CTE) GOTO 20
              END DO
    *
    *         If we are here, the NRHS normwised backward errors satisfy the
    *         stopping criterion, we are good to exit.
    *
              ITER = IITER
    *
              RETURN
    *
       20     CONTINUE
    *
       30 CONTINUE
    *
    *     If we are at this place of the code, this is because we have
    *     performed ITER=ITERMAX iterations and never satisified the stopping
    *     criterion, set up the ITER flag accordingly and follow up on double
    *     precision routine.
    *
          ITER = -ITERMAX - 1
    *
       40 CONTINUE
    *
    *     Single-precision iterative refinement failed to converge to a
    *     satisfactory solution, so we resort to double precision.
    *
          CALL DGETRF(N,N,A,LDA,IPIV,INFO)
    *
          CALL DLACPY('All',N,NRHS,B,LDB,X,LDX)
    *
          IF (INFO.EQ.0) THEN
              CALL DGETRS('No transpose',N,NRHS,A,LDA,IPIV,X,LDX,INFO)
          END IF
    *
          RETURN
    *
    *     End of DSGESV.
    *
          END