sgbsv.f

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      SUBROUTINE SGBSV( N, KL, KU, NRHS, AB, LDAB, IPIV, B, LDB, 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 ..
      INTEGER            INFO, KL, KU, LDAB, LDB, N, NRHS
*     ..
*     .. Array Arguments ..
      INTEGER            IPIV( * )
      REAL               AB( LDAB, * ), B( LDB, * )
*     ..
*
*  Purpose
*  =======
*
*  SGBSV computes the solution to a real system of linear equations
*  A * X = B, where A is a band matrix of order N with KL subdiagonals
*  and KU superdiagonals, and X and B are N-by-NRHS matrices.
*
*  The LU decomposition with partial pivoting and row interchanges is
*  used to factor A as A = L * U, where L is a product of permutation
*  and unit lower triangular matrices with KL subdiagonals, and U is
*  upper triangular with KL+KU superdiagonals.  The factored form of A
*  is then used to solve the system of equations A * X = B.
*
*  Arguments
*  =========
*
*  N       (input) INTEGER
*          The number of linear equations, i.e., the order of the
*          matrix A.  N >= 0.
*
*  KL      (input) INTEGER
*          The number of subdiagonals within the band of A.  KL >= 0.
*
*  KU      (input) INTEGER
*          The number of superdiagonals within the band of A.  KU >= 0.
*
*  NRHS    (input) INTEGER
*          The number of right hand sides, i.e., the number of columns
*          of the matrix B.  NRHS >= 0.
*
*  AB      (input/output) REAL array, dimension (LDAB,N)
*          On entry, the matrix A in band storage, in rows KL+1 to
*          2*KL+KU+1; rows 1 to KL of the array need not be set.
*          The j-th column of A is stored in the j-th column of the
*          array AB as follows:
*          AB(KL+KU+1+i-j,j) = A(i,j) for max(1,j-KU)<=i<=min(N,j+KL)
*          On exit, details of the factorization: U is stored as an
*          upper triangular band matrix with KL+KU superdiagonals in
*          rows 1 to KL+KU+1, and the multipliers used during the
*          factorization are stored in rows KL+KU+2 to 2*KL+KU+1.
*          See below for further details.
*
*  LDAB    (input) INTEGER
*          The leading dimension of the array AB.  LDAB >= 2*KL+KU+1.
*
*  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).
*
*  B       (input/output) REAL 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).
*
*  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) is exactly zero.  The factorization
*                has been completed, but the factor U is exactly
*                singular, and the solution has not been computed.
*
*  Further Details
*  ===============
*
*  The band storage scheme is illustrated by the following example, when
*  M = N = 6, KL = 2, KU = 1:
*
*  On entry:                       On exit:
*
*      *    *    *    +    +    +       *    *    *   u14  u25  u36
*      *    *    +    +    +    +       *    *   u13  u24  u35  u46
*      *   a12  a23  a34  a45  a56      *   u12  u23  u34  u45  u56
*     a11  a22  a33  a44  a55  a66     u11  u22  u33  u44  u55  u66
*     a21  a32  a43  a54  a65   *      m21  m32  m43  m54  m65   *
*     a31  a42  a53  a64   *    *      m31  m42  m53  m64   *    *
*
*  Array elements marked * are not used by the routine; elements marked
*  + need not be set on entry, but are required by the routine to store
*  elements of U because of fill-in resulting from the row interchanges.
*
*  =====================================================================
*
*     .. External Subroutines ..
      EXTERNAL           SGBTRF, SGBTRS, XERBLA
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          MAX
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
      INFO = 0
      IF( N.LT.0 ) THEN
         INFO = -1
      ELSE IF( KL.LT.0 ) THEN
         INFO = -2
      ELSE IF( KU.LT.0 ) THEN
         INFO = -3
      ELSE IF( NRHS.LT.0 ) THEN
         INFO = -4
      ELSE IF( LDAB.LT.2*KL+KU+1 ) THEN
         INFO = -6
      ELSE IF( LDB.LT.MAX( N, 1 ) ) THEN
         INFO = -9
      END IF
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'SGBSV ', -INFO )
         RETURN
      END IF
*
*     Compute the LU factorization of the band matrix A.
*
      CALL SGBTRF( N, N, KL, KU, AB, LDAB, IPIV, INFO )
      IF( INFO.EQ.0 ) THEN
*
*        Solve the system A*X = B, overwriting B with X.
*
         CALL SGBTRS( 'No transpose', N, KL, KU, NRHS, AB, LDAB, IPIV,
     $                B, LDB, INFO )
      END IF
      RETURN
*
*     End of SGBSV
*
      END