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Annotation of rpl/lapack/lapack/dpptrs.f, revision 1.1.1.1

1.1       bertrand    1:       SUBROUTINE DPPTRS( UPLO, N, NRHS, AP, 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:       CHARACTER          UPLO
                     10:       INTEGER            INFO, LDB, N, NRHS
                     11: *     ..
                     12: *     .. Array Arguments ..
                     13:       DOUBLE PRECISION   AP( * ), B( LDB, * )
                     14: *     ..
                     15: *
                     16: *  Purpose
                     17: *  =======
                     18: *
                     19: *  DPPTRS solves a system of linear equations A*X = B with a symmetric
                     20: *  positive definite matrix A in packed storage using the Cholesky
                     21: *  factorization A = U**T*U or A = L*L**T computed by DPPTRF.
                     22: *
                     23: *  Arguments
                     24: *  =========
                     25: *
                     26: *  UPLO    (input) CHARACTER*1
                     27: *          = 'U':  Upper triangle of A is stored;
                     28: *          = 'L':  Lower triangle of A is stored.
                     29: *
                     30: *  N       (input) INTEGER
                     31: *          The order of the matrix A.  N >= 0.
                     32: *
                     33: *  NRHS    (input) INTEGER
                     34: *          The number of right hand sides, i.e., the number of columns
                     35: *          of the matrix B.  NRHS >= 0.
                     36: *
                     37: *  AP      (input) DOUBLE PRECISION array, dimension (N*(N+1)/2)
                     38: *          The triangular factor U or L from the Cholesky factorization
                     39: *          A = U**T*U or A = L*L**T, packed columnwise in a linear
                     40: *          array.  The j-th column of U or L is stored in the array AP
                     41: *          as follows:
                     42: *          if UPLO = 'U', AP(i + (j-1)*j/2) = U(i,j) for 1<=i<=j;
                     43: *          if UPLO = 'L', AP(i + (j-1)*(2n-j)/2) = L(i,j) for j<=i<=n.
                     44: *
                     45: *  B       (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
                     46: *          On entry, the right hand side matrix B.
                     47: *          On exit, the solution matrix X.
                     48: *
                     49: *  LDB     (input) INTEGER
                     50: *          The leading dimension of the array B.  LDB >= max(1,N).
                     51: *
                     52: *  INFO    (output) INTEGER
                     53: *          = 0:  successful exit
                     54: *          < 0:  if INFO = -i, the i-th argument had an illegal value
                     55: *
                     56: *  =====================================================================
                     57: *
                     58: *     .. Local Scalars ..
                     59:       LOGICAL            UPPER
                     60:       INTEGER            I
                     61: *     ..
                     62: *     .. External Functions ..
                     63:       LOGICAL            LSAME
                     64:       EXTERNAL           LSAME
                     65: *     ..
                     66: *     .. External Subroutines ..
                     67:       EXTERNAL           DTPSV, XERBLA
                     68: *     ..
                     69: *     .. Intrinsic Functions ..
                     70:       INTRINSIC          MAX
                     71: *     ..
                     72: *     .. Executable Statements ..
                     73: *
                     74: *     Test the input parameters.
                     75: *
                     76:       INFO = 0
                     77:       UPPER = LSAME( UPLO, 'U' )
                     78:       IF( .NOT.UPPER .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
                     79:          INFO = -1
                     80:       ELSE IF( N.LT.0 ) THEN
                     81:          INFO = -2
                     82:       ELSE IF( NRHS.LT.0 ) THEN
                     83:          INFO = -3
                     84:       ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
                     85:          INFO = -6
                     86:       END IF
                     87:       IF( INFO.NE.0 ) THEN
                     88:          CALL XERBLA( 'DPPTRS', -INFO )
                     89:          RETURN
                     90:       END IF
                     91: *
                     92: *     Quick return if possible
                     93: *
                     94:       IF( N.EQ.0 .OR. NRHS.EQ.0 )
                     95:      $   RETURN
                     96: *
                     97:       IF( UPPER ) THEN
                     98: *
                     99: *        Solve A*X = B where A = U'*U.
                    100: *
                    101:          DO 10 I = 1, NRHS
                    102: *
                    103: *           Solve U'*X = B, overwriting B with X.
                    104: *
                    105:             CALL DTPSV( 'Upper', 'Transpose', 'Non-unit', N, AP,
                    106:      $                  B( 1, I ), 1 )
                    107: *
                    108: *           Solve U*X = B, overwriting B with X.
                    109: *
                    110:             CALL DTPSV( 'Upper', 'No transpose', 'Non-unit', N, AP,
                    111:      $                  B( 1, I ), 1 )
                    112:    10    CONTINUE
                    113:       ELSE
                    114: *
                    115: *        Solve A*X = B where A = L*L'.
                    116: *
                    117:          DO 20 I = 1, NRHS
                    118: *
                    119: *           Solve L*Y = B, overwriting B with X.
                    120: *
                    121:             CALL DTPSV( 'Lower', 'No transpose', 'Non-unit', N, AP,
                    122:      $                  B( 1, I ), 1 )
                    123: *
                    124: *           Solve L'*X = Y, overwriting B with X.
                    125: *
                    126:             CALL DTPSV( 'Lower', 'Transpose', 'Non-unit', N, AP,
                    127:      $                  B( 1, I ), 1 )
                    128:    20    CONTINUE
                    129:       END IF
                    130: *
                    131:       RETURN
                    132: *
                    133: *     End of DPPTRS
                    134: *
                    135:       END