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    1:       SUBROUTINE ZPTTRS( UPLO, 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:       CHARACTER          UPLO
   10:       INTEGER            INFO, LDB, N, NRHS
   11: *     ..
   12: *     .. Array Arguments ..
   13:       DOUBLE PRECISION   D( * )
   14:       COMPLEX*16         B( LDB, * ), E( * )
   15: *     ..
   16: *
   17: *  Purpose
   18: *  =======
   19: *
   20: *  ZPTTRS solves a tridiagonal system of the form
   21: *     A * X = B
   22: *  using the factorization A = U'*D*U or A = L*D*L' computed by ZPTTRF.
   23: *  D is a diagonal matrix specified in the vector D, U (or L) is a unit
   24: *  bidiagonal matrix whose superdiagonal (subdiagonal) is specified in
   25: *  the vector E, and X and B are N by NRHS matrices.
   26: *
   27: *  Arguments
   28: *  =========
   29: *
   30: *  UPLO    (input) CHARACTER*1
   31: *          Specifies the form of the factorization and whether the
   32: *          vector E is the superdiagonal of the upper bidiagonal factor
   33: *          U or the subdiagonal of the lower bidiagonal factor L.
   34: *          = 'U':  A = U'*D*U, E is the superdiagonal of U
   35: *          = 'L':  A = L*D*L', E is the subdiagonal of L
   36: *
   37: *  N       (input) INTEGER
   38: *          The order of the tridiagonal matrix A.  N >= 0.
   39: *
   40: *  NRHS    (input) INTEGER
   41: *          The number of right hand sides, i.e., the number of columns
   42: *          of the matrix B.  NRHS >= 0.
   43: *
   44: *  D       (input) DOUBLE PRECISION array, dimension (N)
   45: *          The n diagonal elements of the diagonal matrix D from the
   46: *          factorization A = U'*D*U or A = L*D*L'.
   47: *
   48: *  E       (input) COMPLEX*16 array, dimension (N-1)
   49: *          If UPLO = 'U', the (n-1) superdiagonal elements of the unit
   50: *          bidiagonal factor U from the factorization A = U'*D*U.
   51: *          If UPLO = 'L', the (n-1) subdiagonal elements of the unit
   52: *          bidiagonal factor L from the factorization A = L*D*L'.
   53: *
   54: *  B       (input/output) DOUBLE PRECISION array, dimension (LDB,NRHS)
   55: *          On entry, the right hand side vectors B for the system of
   56: *          linear equations.
   57: *          On exit, the solution vectors, X.
   58: *
   59: *  LDB     (input) INTEGER
   60: *          The leading dimension of the array B.  LDB >= max(1,N).
   61: *
   62: *  INFO    (output) INTEGER
   63: *          = 0: successful exit
   64: *          < 0: if INFO = -k, the k-th argument had an illegal value
   65: *
   66: *  =====================================================================
   67: *
   68: *     .. Local Scalars ..
   69:       LOGICAL            UPPER
   70:       INTEGER            IUPLO, J, JB, NB
   71: *     ..
   72: *     .. External Functions ..
   73:       INTEGER            ILAENV
   74:       EXTERNAL           ILAENV
   75: *     ..
   76: *     .. External Subroutines ..
   77:       EXTERNAL           XERBLA, ZPTTS2
   78: *     ..
   79: *     .. Intrinsic Functions ..
   80:       INTRINSIC          MAX, MIN
   81: *     ..
   82: *     .. Executable Statements ..
   83: *
   84: *     Test the input arguments.
   85: *
   86:       INFO = 0
   87:       UPPER = ( UPLO.EQ.'U' .OR. UPLO.EQ.'u' )
   88:       IF( .NOT.UPPER .AND. .NOT.( UPLO.EQ.'L' .OR. UPLO.EQ.'l' ) ) THEN
   89:          INFO = -1
   90:       ELSE IF( N.LT.0 ) THEN
   91:          INFO = -2
   92:       ELSE IF( NRHS.LT.0 ) THEN
   93:          INFO = -3
   94:       ELSE IF( LDB.LT.MAX( 1, N ) ) THEN
   95:          INFO = -7
   96:       END IF
   97:       IF( INFO.NE.0 ) THEN
   98:          CALL XERBLA( 'ZPTTRS', -INFO )
   99:          RETURN
  100:       END IF
  101: *
  102: *     Quick return if possible
  103: *
  104:       IF( N.EQ.0 .OR. NRHS.EQ.0 )
  105:      $   RETURN
  106: *
  107: *     Determine the number of right-hand sides to solve at a time.
  108: *
  109:       IF( NRHS.EQ.1 ) THEN
  110:          NB = 1
  111:       ELSE
  112:          NB = MAX( 1, ILAENV( 1, 'ZPTTRS', UPLO, N, NRHS, -1, -1 ) )
  113:       END IF
  114: *
  115: *     Decode UPLO
  116: *
  117:       IF( UPPER ) THEN
  118:          IUPLO = 1
  119:       ELSE
  120:          IUPLO = 0
  121:       END IF
  122: *
  123:       IF( NB.GE.NRHS ) THEN
  124:          CALL ZPTTS2( IUPLO, N, NRHS, D, E, B, LDB )
  125:       ELSE
  126:          DO 10 J = 1, NRHS, NB
  127:             JB = MIN( NRHS-J+1, NB )
  128:             CALL ZPTTS2( IUPLO, N, JB, D, E, B( 1, J ), LDB )
  129:    10    CONTINUE
  130:       END IF
  131: *
  132:       RETURN
  133: *
  134: *     End of ZPTTRS
  135: *
  136:       END