cpotrs.f

          SUBROUTINE CPOTRS( UPLO, N, NRHS, A, LDA, B, LDB, INFO )
    *
    *  -- LAPACK routine (version 3.1) --
    *     Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..
    *     November 2006
    *
    *     .. Scalar Arguments ..
          CHARACTER          UPLO
          INTEGER            INFO, LDA, LDB, N, NRHS
    *     ..
    *     .. Array Arguments ..
          COMPLEX            A( LDA, * ), B( LDB, * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  CPOTRS solves a system of linear equations A*X = B with a Hermitian
    *  positive definite matrix A using the Cholesky factorization
    *  A = U**H*U or A = L*L**H computed by CPOTRF.
    *
    *  Arguments
    *  =========
    *
    *  UPLO    (input) CHARACTER*1
    *          = 'U':  Upper triangle of A is stored;
    *          = 'L':  Lower triangle of A is stored.
    *
    *  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.
    *
    *  A       (input) COMPLEX array, dimension (LDA,N)
    *          The triangular factor U or L from the Cholesky factorization
    *          A = U**H*U or A = L*L**H, as computed by CPOTRF.
    *
    *  LDA     (input) INTEGER
    *          The leading dimension of the array A.  LDA >= max(1,N).
    *
    *  B       (input/output) COMPLEX array, dimension (LDB,NRHS)
    *          On entry, the right hand side matrix B.
    *          On exit, 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
    *
    *  =====================================================================
    *
    *     .. Parameters ..
          COMPLEX            ONE
          PARAMETER          ( ONE = ( 1.0E+0, 0.0E+0 ) )
    *     ..
    *     .. Local Scalars ..
          LOGICAL            UPPER
    *     ..
    *     .. External Functions ..
          LOGICAL            LSAME
          EXTERNAL           LSAME
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           CTRSM, XERBLA
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          MAX
    *     ..
    *     .. Executable Statements ..
    *
    *     Test the input parameters.
    *
          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 = -7
          END IF
          IF( INFO.NE.0 ) THEN
             CALL XERBLA( 'CPOTRS', -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'*U.
    *
    *        Solve U'*X = B, overwriting B with X.
    *
             CALL CTRSM( 'Left', 'Upper', 'Conjugate transpose', 'Non-unit',
         $               N, NRHS, ONE, A, LDA, B, LDB )
    *
    *        Solve U*X = B, overwriting B with X.
    *
             CALL CTRSM( 'Left', 'Upper', 'No transpose', 'Non-unit', N,
         $               NRHS, ONE, A, LDA, B, LDB )
          ELSE
    *
    *        Solve A*X = B where A = L*L'.
    *
    *        Solve L*X = B, overwriting B with X.
    *
             CALL CTRSM( 'Left', 'Lower', 'No transpose', 'Non-unit', N,
         $               NRHS, ONE, A, LDA, B, LDB )
    *
    *        Solve L'*X = B, overwriting B with X.
    *
             CALL CTRSM( 'Left', 'Lower', 'Conjugate transpose', 'Non-unit',
         $               N, NRHS, ONE, A, LDA, B, LDB )
          END IF
    *
          RETURN
    *
    *     End of CPOTRS
    *
          END