LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ cgttrs()

subroutine cgttrs ( character  TRANS,
integer  N,
integer  NRHS,
complex, dimension( * )  DL,
complex, dimension( * )  D,
complex, dimension( * )  DU,
complex, dimension( * )  DU2,
integer, dimension( * )  IPIV,
complex, dimension( ldb, * )  B,
integer  LDB,
integer  INFO 
)

CGTTRS

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Purpose:
 CGTTRS solves one of the systems of equations
    A * X = B,  A**T * X = B,  or  A**H * X = B,
 with a tridiagonal matrix A using the LU factorization computed
 by CGTTRF.
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**H * X = B  (Conjugate 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 COMPLEX array, dimension (N-1)
          The (n-1) multipliers that define the matrix L from the
          LU factorization of A.
[in]D
          D is COMPLEX array, dimension (N)
          The n diagonal elements of the upper triangular matrix U from
          the LU factorization of A.
[in]DU
          DU is COMPLEX array, dimension (N-1)
          The (n-1) elements of the first super-diagonal of U.
[in]DU2
          DU2 is COMPLEX 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 COMPLEX 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 = -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 136 of file cgttrs.f.

138 *
139 * -- LAPACK computational routine --
140 * -- LAPACK is a software package provided by Univ. of Tennessee, --
141 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
142 *
143 * .. Scalar Arguments ..
144  CHARACTER TRANS
145  INTEGER INFO, LDB, N, NRHS
146 * ..
147 * .. Array Arguments ..
148  INTEGER IPIV( * )
149  COMPLEX B( LDB, * ), D( * ), DL( * ), DU( * ), DU2( * )
150 * ..
151 *
152 * =====================================================================
153 *
154 * .. Local Scalars ..
155  LOGICAL NOTRAN
156  INTEGER ITRANS, J, JB, NB
157 * ..
158 * .. External Functions ..
159  INTEGER ILAENV
160  EXTERNAL ilaenv
161 * ..
162 * .. External Subroutines ..
163  EXTERNAL cgtts2, xerbla
164 * ..
165 * .. Intrinsic Functions ..
166  INTRINSIC max, min
167 * ..
168 * .. Executable Statements ..
169 *
170  info = 0
171  notran = ( trans.EQ.'N' .OR. trans.EQ.'n' )
172  IF( .NOT.notran .AND. .NOT.( trans.EQ.'T' .OR. trans.EQ.
173  $ 't' ) .AND. .NOT.( trans.EQ.'C' .OR. trans.EQ.'c' ) ) THEN
174  info = -1
175  ELSE IF( n.LT.0 ) THEN
176  info = -2
177  ELSE IF( nrhs.LT.0 ) THEN
178  info = -3
179  ELSE IF( ldb.LT.max( n, 1 ) ) THEN
180  info = -10
181  END IF
182  IF( info.NE.0 ) THEN
183  CALL xerbla( 'CGTTRS', -info )
184  RETURN
185  END IF
186 *
187 * Quick return if possible
188 *
189  IF( n.EQ.0 .OR. nrhs.EQ.0 )
190  $ RETURN
191 *
192 * Decode TRANS
193 *
194  IF( notran ) THEN
195  itrans = 0
196  ELSE IF( trans.EQ.'T' .OR. trans.EQ.'t' ) THEN
197  itrans = 1
198  ELSE
199  itrans = 2
200  END IF
201 *
202 * Determine the number of right-hand sides to solve at a time.
203 *
204  IF( nrhs.EQ.1 ) THEN
205  nb = 1
206  ELSE
207  nb = max( 1, ilaenv( 1, 'CGTTRS', trans, n, nrhs, -1, -1 ) )
208  END IF
209 *
210  IF( nb.GE.nrhs ) THEN
211  CALL cgtts2( itrans, n, nrhs, dl, d, du, du2, ipiv, b, ldb )
212  ELSE
213  DO 10 j = 1, nrhs, nb
214  jb = min( nrhs-j+1, nb )
215  CALL cgtts2( itrans, n, jb, dl, d, du, du2, ipiv, b( 1, j ),
216  $ ldb )
217  10 CONTINUE
218  END IF
219 *
220 * End of CGTTRS
221 *
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 cgtts2(ITRANS, N, NRHS, DL, D, DU, DU2, IPIV, B, LDB)
CGTTS2 solves a system of linear equations with a tridiagonal matrix using the LU factorization compu...
Definition: cgtts2.f:128
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