LAPACK  3.10.1
LAPACK: Linear Algebra PACKage

◆ dpttrs()

subroutine dpttrs ( integer  N,
integer  NRHS,
double precision, dimension( * )  D,
double precision, dimension( * )  E,
double precision, dimension( ldb, * )  B,
integer  LDB,
integer  INFO 
)

DPTTRS

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Purpose:
 DPTTRS solves a tridiagonal system of the form
    A * X = B
 using the L*D*L**T factorization of A computed by DPTTRF.  D is a
 diagonal matrix specified in the vector D, L is a unit bidiagonal
 matrix whose subdiagonal is specified in the vector E, and X and B
 are N by NRHS matrices.
Parameters
[in]N
          N is INTEGER
          The order of the tridiagonal matrix A.  N >= 0.
[in]NRHS
          NRHS is INTEGER
          The number of right hand sides, i.e., the number of columns
          of the matrix B.  NRHS >= 0.
[in]D
          D is DOUBLE PRECISION array, dimension (N)
          The n diagonal elements of the diagonal matrix D from the
          L*D*L**T factorization of A.
[in]E
          E is DOUBLE PRECISION array, dimension (N-1)
          The (n-1) subdiagonal elements of the unit bidiagonal factor
          L from the L*D*L**T factorization of A.  E can also be regarded
          as the superdiagonal of the unit bidiagonal factor U from the
          factorization A = U**T*D*U.
[in,out]B
          B is DOUBLE PRECISION array, dimension (LDB,NRHS)
          On entry, the right hand side vectors B for the system of
          linear equations.
          On exit, the solution vectors, X.
[in]LDB
          LDB is INTEGER
          The leading dimension of the array B.  LDB >= max(1,N).
[out]INFO
          INFO is INTEGER
          = 0: successful exit
          < 0: if INFO = -k, the k-th argument had an illegal value
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.

Definition at line 108 of file dpttrs.f.

109 *
110 * -- LAPACK computational routine --
111 * -- LAPACK is a software package provided by Univ. of Tennessee, --
112 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
113 *
114 * .. Scalar Arguments ..
115  INTEGER INFO, LDB, N, NRHS
116 * ..
117 * .. Array Arguments ..
118  DOUBLE PRECISION B( LDB, * ), D( * ), E( * )
119 * ..
120 *
121 * =====================================================================
122 *
123 * .. Local Scalars ..
124  INTEGER J, JB, NB
125 * ..
126 * .. External Functions ..
127  INTEGER ILAENV
128  EXTERNAL ilaenv
129 * ..
130 * .. External Subroutines ..
131  EXTERNAL dptts2, xerbla
132 * ..
133 * .. Intrinsic Functions ..
134  INTRINSIC max, min
135 * ..
136 * .. Executable Statements ..
137 *
138 * Test the input arguments.
139 *
140  info = 0
141  IF( n.LT.0 ) THEN
142  info = -1
143  ELSE IF( nrhs.LT.0 ) THEN
144  info = -2
145  ELSE IF( ldb.LT.max( 1, n ) ) THEN
146  info = -6
147  END IF
148  IF( info.NE.0 ) THEN
149  CALL xerbla( 'DPTTRS', -info )
150  RETURN
151  END IF
152 *
153 * Quick return if possible
154 *
155  IF( n.EQ.0 .OR. nrhs.EQ.0 )
156  $ RETURN
157 *
158 * Determine the number of right-hand sides to solve at a time.
159 *
160  IF( nrhs.EQ.1 ) THEN
161  nb = 1
162  ELSE
163  nb = max( 1, ilaenv( 1, 'DPTTRS', ' ', n, nrhs, -1, -1 ) )
164  END IF
165 *
166  IF( nb.GE.nrhs ) THEN
167  CALL dptts2( n, nrhs, d, e, b, ldb )
168  ELSE
169  DO 10 j = 1, nrhs, nb
170  jb = min( nrhs-j+1, nb )
171  CALL dptts2( n, jb, d, e, b( 1, j ), ldb )
172  10 CONTINUE
173  END IF
174 *
175  RETURN
176 *
177 * End of DPTTRS
178 *
integer function ilaenv(ISPEC, NAME, OPTS, N1, N2, N3, N4)
ILAENV
Definition: ilaenv.f:162
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
subroutine dptts2(N, NRHS, D, E, B, LDB)
DPTTS2 solves a tridiagonal system of the form AX=B using the L D LH factorization computed by spttrf...
Definition: dptts2.f:102
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