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Annotation of rpl/lapack/lapack/dpttrs.f, revision 1.2

1.1       bertrand    1:       SUBROUTINE DPTTRS( N, NRHS, D, E, B, LDB, INFO )
                      2: *
                      3: *  -- LAPACK routine (version 3.2) --
                      4: *  -- LAPACK is a software package provided by Univ. of Tennessee,    --
                      5: *  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
                      6: *     November 2006
                      7: *
                      8: *     .. Scalar Arguments ..
                      9:       INTEGER            INFO, LDB, N, NRHS
                     10: *     ..
                     11: *     .. Array Arguments ..
                     12:       DOUBLE PRECISION   B( LDB, * ), D( * ), E( * )
                     13: *     ..
                     14: *
                     15: *  Purpose
                     16: *  =======
                     17: *
                     18: *  DPTTRS solves a tridiagonal system of the form
                     19: *     A * X = B
                     20: *  using the L*D*L' factorization of A computed by DPTTRF.  D is a
                     21: *  diagonal matrix specified in the vector D, L is a unit bidiagonal
                     22: *  matrix whose subdiagonal is specified in the vector E, and X and B
                     23: *  are N by NRHS matrices.
                     24: *
                     25: *  Arguments
                     26: *  =========
                     27: *
                     28: *  N       (input) INTEGER
                     29: *          The order of the tridiagonal matrix A.  N >= 0.
                     30: *
                     31: *  NRHS    (input) INTEGER
                     32: *          The number of right hand sides, i.e., the number of columns
                     33: *          of the matrix B.  NRHS >= 0.
                     34: *
                     35: *  D       (input) DOUBLE PRECISION array, dimension (N)
                     36: *          The n diagonal elements of the diagonal matrix D from the
                     37: *          L*D*L' factorization of A.
                     38: *
                     39: *  E       (input) DOUBLE PRECISION array, dimension (N-1)
                     40: *          The (n-1) subdiagonal elements of the unit bidiagonal factor
                     41: *          L from the L*D*L' factorization of A.  E can also be regarded
                     42: *          as the superdiagonal of the unit bidiagonal factor U from the
                     43: *          factorization A = U'*D*U.
                     44: *
                     45: *  B       (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
                     46: *          On entry, the right hand side vectors B for the system of
                     47: *          linear equations.
                     48: *          On exit, the solution vectors, X.
                     49: *
                     50: *  LDB     (input) INTEGER
                     51: *          The leading dimension of the array B.  LDB >= max(1,N).
                     52: *
                     53: *  INFO    (output) INTEGER
                     54: *          = 0: successful exit
                     55: *          < 0: if INFO = -k, the k-th argument had an illegal value
                     56: *
                     57: *  =====================================================================
                     58: *
                     59: *     .. Local Scalars ..
                     60:       INTEGER            J, JB, NB
                     61: *     ..
                     62: *     .. External Functions ..
                     63:       INTEGER            ILAENV
                     64:       EXTERNAL           ILAENV
                     65: *     ..
                     66: *     .. External Subroutines ..
                     67:       EXTERNAL           DPTTS2, XERBLA
                     68: *     ..
                     69: *     .. Intrinsic Functions ..
                     70:       INTRINSIC          MAX, MIN
                     71: *     ..
                     72: *     .. Executable Statements ..
                     73: *
                     74: *     Test the input arguments.
                     75: *
                     76:       INFO = 0
                     77:       IF( N.LT.0 ) THEN
                     78:          INFO = -1
                     79:       ELSE IF( NRHS.LT.0 ) THEN
                     80:          INFO = -2
                     81:       ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
                     82:          INFO = -6
                     83:       END IF
                     84:       IF( INFO.NE.0 ) THEN
                     85:          CALL XERBLA( 'DPTTRS', -INFO )
                     86:          RETURN
                     87:       END IF
                     88: *
                     89: *     Quick return if possible
                     90: *
                     91:       IF( N.EQ.0 .OR. NRHS.EQ.0 )
                     92:      $   RETURN
                     93: *
                     94: *     Determine the number of right-hand sides to solve at a time.
                     95: *
                     96:       IF( NRHS.EQ.1 ) THEN
                     97:          NB = 1
                     98:       ELSE
                     99:          NB = MAX( 1, ILAENV( 1, 'DPTTRS', ' ', N, NRHS, -1, -1 ) )
                    100:       END IF
                    101: *
                    102:       IF( NB.GE.NRHS ) THEN
                    103:          CALL DPTTS2( N, NRHS, D, E, B, LDB )
                    104:       ELSE
                    105:          DO 10 J = 1, NRHS, NB
                    106:             JB = MIN( NRHS-J+1, NB )
                    107:             CALL DPTTS2( N, JB, D, E, B( 1, J ), LDB )
                    108:    10    CONTINUE
                    109:       END IF
                    110: *
                    111:       RETURN
                    112: *
                    113: *     End of DPTTRS
                    114: *
                    115:       END