cpttrs.f

          SUBROUTINE CPTTRS( UPLO, N, NRHS, D, E, 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, LDB, N, NRHS
    *     ..
    *     .. Array Arguments ..
          REAL               D( * )
          COMPLEX            B( LDB, * ), E( * )
    *     ..
    *
    *  Purpose
    *  =======
    *
    *  CPTTRS solves a tridiagonal system of the form
    *     A * X = B
    *  using the factorization A = U'*D*U or A = L*D*L' computed by CPTTRF.
    *  D is a diagonal matrix specified in the vector D, U (or L) is a unit
    *  bidiagonal matrix whose superdiagonal (subdiagonal) is specified in
    *  the vector E, and X and B are N by NRHS matrices.
    *
    *  Arguments
    *  =========
    *
    *  UPLO    (input) CHARACTER*1
    *          Specifies the form of the factorization and whether the
    *          vector E is the superdiagonal of the upper bidiagonal factor
    *          U or the subdiagonal of the lower bidiagonal factor L.
    *          = 'U':  A = U'*D*U, E is the superdiagonal of U
    *          = 'L':  A = L*D*L', E is the subdiagonal of L
    *
    *  N       (input) INTEGER
    *          The order of the tridiagonal 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.
    *
    *  D       (input) REAL array, dimension (N)
    *          The n diagonal elements of the diagonal matrix D from the
    *          factorization A = U'*D*U or A = L*D*L'.
    *
    *  E       (input) COMPLEX array, dimension (N-1)
    *          If UPLO = 'U', the (n-1) superdiagonal elements of the unit
    *          bidiagonal factor U from the factorization A = U'*D*U.
    *          If UPLO = 'L', the (n-1) subdiagonal elements of the unit
    *          bidiagonal factor L from the factorization A = L*D*L'.
    *
    *  B       (input/output) REAL array, dimension (LDB,NRHS)
    *          On entry, the right hand side vectors B for the system of
    *          linear equations.
    *          On exit, the solution vectors, X.
    *
    *  LDB     (input) INTEGER
    *          The leading dimension of the array B.  LDB >= max(1,N).
    *
    *  INFO    (output) INTEGER
    *          = 0: successful exit
    *          < 0: if INFO = -k, the k-th argument had an illegal value
    *
    *  =====================================================================
    *
    *     .. Local Scalars ..
          LOGICAL            UPPER
          INTEGER            IUPLO, J, JB, NB
    *     ..
    *     .. External Functions ..
          INTEGER            ILAENV
          EXTERNAL           ILAENV
    *     ..
    *     .. External Subroutines ..
          EXTERNAL           CPTTS2, XERBLA
    *     ..
    *     .. Intrinsic Functions ..
          INTRINSIC          MAX, MIN
    *     ..
    *     .. Executable Statements ..
    *
    *     Test the input arguments.
    *
          INFO = 0
          UPPER = ( UPLO.EQ.'U' .OR. UPLO.EQ.'u' )
          IF( .NOT.UPPER .AND. .NOT.( UPLO.EQ.'L' .OR. UPLO.EQ.'l' ) ) THEN
             INFO = -1
          ELSE IF( N.LT.0 ) THEN
             INFO = -2
          ELSE IF( NRHS.LT.0 ) THEN
             INFO = -3
          ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
             INFO = -7
          END IF
          IF( INFO.NE.0 ) THEN
             CALL XERBLA( 'CPTTRS', -INFO )
             RETURN
          END IF
    *
    *     Quick return if possible
    *
          IF( N.EQ.0 .OR. NRHS.EQ.0 )
         $   RETURN
    *
    *     Determine the number of right-hand sides to solve at a time.
    *
          IF( NRHS.EQ.1 ) THEN
             NB = 1
          ELSE
             NB = MAX( 1, ILAENV( 1, 'CPTTRS', UPLO, N, NRHS, -1, -1 ) )
          END IF
    *
    *     Decode UPLO
    *
          IF( UPPER ) THEN
             IUPLO = 1
          ELSE
             IUPLO = 0
          END IF
    *
          IF( NB.GE.NRHS ) THEN
             CALL CPTTS2( IUPLO, N, NRHS, D, E, B, LDB )
          ELSE
             DO 10 J = 1, NRHS, NB
                JB = MIN( NRHS-J+1, NB )
                CALL CPTTS2( IUPLO, N, JB, D, E, B( 1, J ), LDB )
       10    CONTINUE
          END IF
    *
          RETURN
    *
    *     End of CPTTRS
    *
          END