LAPACK  3.6.1 LAPACK: Linear Algebra PACKage
 subroutine dgttrs ( character TRANS, integer N, integer NRHS, double precision, dimension( * ) DL, double precision, dimension( * ) D, double precision, dimension( * ) DU, double precision, dimension( * ) DU2, integer, dimension( * ) IPIV, double precision, dimension( ldb, * ) B, integer LDB, integer INFO )

DGTTRS

Purpose:
``` DGTTRS solves one of the systems of equations
A*X = B  or  A**T*X = B,
with a tridiagonal matrix A using the LU factorization computed
by DGTTRF.```
Parameters
 [in] TRANS ``` TRANS is CHARACTER*1 Specifies the form of the system of equations. = 'N': A * X = B (No transpose) = 'T': A**T* X = B (Transpose) = 'C': A**T* X = B (Conjugate transpose = Transpose)``` [in] N ``` N is INTEGER The order of the matrix A.``` [in] NRHS ``` NRHS is INTEGER The number of right hand sides, i.e., the number of columns of the matrix B. NRHS >= 0.``` [in] DL ``` DL is DOUBLE PRECISION array, dimension (N-1) The (n-1) multipliers that define the matrix L from the LU factorization of A.``` [in] D ``` D is DOUBLE PRECISION array, dimension (N) The n diagonal elements of the upper triangular matrix U from the LU factorization of A.``` [in] DU ``` DU is DOUBLE PRECISION array, dimension (N-1) The (n-1) elements of the first super-diagonal of U.``` [in] DU2 ``` DU2 is DOUBLE PRECISION array, dimension (N-2) The (n-2) elements of the second super-diagonal of U.``` [in] IPIV ``` IPIV is INTEGER array, dimension (N) The pivot indices; for 1 <= i <= n, row i of the matrix was interchanged with row IPIV(i). IPIV(i) will always be either i or i+1; IPIV(i) = i indicates a row interchange was not required.``` [in,out] B ``` B is DOUBLE PRECISION array, dimension (LDB,NRHS) On entry, the matrix of right hand side vectors B. On exit, B is overwritten by 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 = -i, the i-th argument had an illegal value```
Date
September 2012

Definition at line 140 of file dgttrs.f.

140 *
141 * -- LAPACK computational routine (version 3.4.2) --
142 * -- LAPACK is a software package provided by Univ. of Tennessee, --
143 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
144 * September 2012
145 *
146 * .. Scalar Arguments ..
147  CHARACTER trans
148  INTEGER info, ldb, n, nrhs
149 * ..
150 * .. Array Arguments ..
151  INTEGER ipiv( * )
152  DOUBLE PRECISION b( ldb, * ), d( * ), dl( * ), du( * ), du2( * )
153 * ..
154 *
155 * =====================================================================
156 *
157 * .. Local Scalars ..
158  LOGICAL notran
159  INTEGER itrans, j, jb, nb
160 * ..
161 * .. External Functions ..
162  INTEGER ilaenv
163  EXTERNAL ilaenv
164 * ..
165 * .. External Subroutines ..
166  EXTERNAL dgtts2, xerbla
167 * ..
168 * .. Intrinsic Functions ..
169  INTRINSIC max, min
170 * ..
171 * .. Executable Statements ..
172 *
173  info = 0
174  notran = ( trans.EQ.'N' .OR. trans.EQ.'n' )
175  IF( .NOT.notran .AND. .NOT.( trans.EQ.'T' .OR. trans.EQ.
176  \$ 't' ) .AND. .NOT.( trans.EQ.'C' .OR. trans.EQ.'c' ) ) THEN
177  info = -1
178  ELSE IF( n.LT.0 ) THEN
179  info = -2
180  ELSE IF( nrhs.LT.0 ) THEN
181  info = -3
182  ELSE IF( ldb.LT.max( n, 1 ) ) THEN
183  info = -10
184  END IF
185  IF( info.NE.0 ) THEN
186  CALL xerbla( 'DGTTRS', -info )
187  RETURN
188  END IF
189 *
190 * Quick return if possible
191 *
192  IF( n.EQ.0 .OR. nrhs.EQ.0 )
193  \$ RETURN
194 *
195 * Decode TRANS
196 *
197  IF( notran ) THEN
198  itrans = 0
199  ELSE
200  itrans = 1
201  END IF
202 *
203 * Determine the number of right-hand sides to solve at a time.
204 *
205  IF( nrhs.EQ.1 ) THEN
206  nb = 1
207  ELSE
208  nb = max( 1, ilaenv( 1, 'DGTTRS', trans, n, nrhs, -1, -1 ) )
209  END IF
210 *
211  IF( nb.GE.nrhs ) THEN
212  CALL dgtts2( itrans, n, nrhs, dl, d, du, du2, ipiv, b, ldb )
213  ELSE
214  DO 10 j = 1, nrhs, nb
215  jb = min( nrhs-j+1, nb )
216  CALL dgtts2( itrans, n, jb, dl, d, du, du2, ipiv, b( 1, j ),
217  \$ ldb )
218  10 CONTINUE
219  END IF
220 *
221 * End of DGTTRS
222 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:62
subroutine dgtts2(ITRANS, N, NRHS, DL, D, DU, DU2, IPIV, B, LDB)
DGTTS2 solves a system of linear equations with a tridiagonal matrix using the LU factorization compu...
Definition: dgtts2.f:130
integer function ilaenv(ISPEC, NAME, OPTS, N1, N2, N3, N4)
Definition: tstiee.f:83

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