dtptrs.f

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      SUBROUTINE DTPTRS( UPLO, TRANS, DIAG, N, NRHS, AP, B, LDB, INFO )
*
*  -- LAPACK 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          DIAG, TRANS, UPLO
      INTEGER            INFO, LDB, N, NRHS
*     ..
*     .. Array Arguments ..
      DOUBLE PRECISION   AP( * ), B( LDB, * )
*     ..
*
*  Purpose
*  =======
*
*  DTPTRS solves a triangular system of the form
*
*     A * X = B  or  A**T * X = B,
*
*  where A is a triangular matrix of order N stored in packed format,
*  and B is an N-by-NRHS matrix.  A check is made to verify that A is
*  nonsingular.
*
*  Arguments
*  =========
*
*  UPLO    (input) CHARACTER*1
*          = 'U':  A is upper triangular;
*          = 'L':  A is lower triangular.
*
*  TRANS   (input) CHARACTER*1
*          Specifies the form of the system of equations:
*          = 'N':  A * X = B  (No transpose)
*          = 'T':  A**T * X = B  (Transpose)
*          = 'C':  A**H * X = B  (Conjugate transpose = Transpose)
*
*  DIAG    (input) CHARACTER*1
*          = 'N':  A is non-unit triangular;
*          = 'U':  A is unit triangular.
*
*  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.
*
*  AP      (input) DOUBLE PRECISION array, dimension (N*(N+1)/2)
*          The upper or lower triangular matrix A, packed columnwise in
*          a linear array.  The j-th column of A is stored in the array
*          AP as follows:
*          if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j;
*          if UPLO = 'L', AP(i + (j-1)*(2*n-j)/2) = A(i,j) for j<=i<=n.
*
*  B       (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
*          On entry, the right hand side matrix B.
*          On exit, if INFO = 0, 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
*          > 0:  if INFO = i, the i-th diagonal element of A is zero,
*                indicating that the matrix is singular and the
*                solutions X have not been computed.
*
*  =====================================================================
*
*     .. Parameters ..
      DOUBLE PRECISION   ZERO
      PARAMETER          ( ZERO = 0.0D+0 )
*     ..
*     .. Local Scalars ..
      LOGICAL            NOUNIT, UPPER
      INTEGER            J, JC
*     ..
*     .. External Functions ..
      LOGICAL            LSAME
      EXTERNAL           LSAME
*     ..
*     .. External Subroutines ..
      EXTERNAL           DTPSV, XERBLA
*     ..
*     .. Intrinsic Functions ..
      INTRINSIC          MAX
*     ..
*     .. Executable Statements ..
*
*     Test the input parameters.
*
      INFO = 0
      UPPER = LSAME( UPLO, 'U' )
      NOUNIT = LSAME( DIAG, 'N' )
      IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
         INFO = -1
      ELSE IF( .NOT.LSAME( TRANS, 'N' ) .AND. .NOT.
     $         LSAME( TRANS, 'T' ) .AND. .NOT.LSAME( TRANS, 'C' ) ) THEN
         INFO = -2
      ELSE IF( .NOT.NOUNIT .AND. .NOT.LSAME( DIAG, 'U' ) ) THEN
         INFO = -3
      ELSE IF( N.LT.0 ) THEN
         INFO = -4
      ELSE IF( NRHS.LT.0 ) THEN
         INFO = -5
      ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
         INFO = -8
      END IF
      IF( INFO.NE.0 ) THEN
         CALL XERBLA( 'DTPTRS', -INFO )
         RETURN
      END IF
*
*     Quick return if possible
*
      IF( N.EQ.0 )
     $   RETURN
*
*     Check for singularity.
*
      IF( NOUNIT ) THEN
         IF( UPPER ) THEN
            JC = 1
            DO 10 INFO = 1, N
               IF( AP( JC+INFO-1 ).EQ.ZERO )
     $            RETURN
               JC = JC + INFO
   10       CONTINUE
         ELSE
            JC = 1
            DO 20 INFO = 1, N
               IF( AP( JC ).EQ.ZERO )
     $            RETURN
               JC = JC + N - INFO + 1
   20       CONTINUE
         END IF
      END IF
      INFO = 0
*
*     Solve A * x = b  or  A' * x = b.
*
      DO 30 J = 1, NRHS
         CALL DTPSV( UPLO, TRANS, DIAG, N, AP, B( 1, J ), 1 )
   30 CONTINUE
*
      RETURN
*
*     End of DTPTRS
*
      END