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    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