LAPACK  3.9.0 LAPACK: Linear Algebra PACKage

## ◆ dgbtrs()

 subroutine dgbtrs ( character TRANS, integer N, integer KL, integer KU, integer NRHS, double precision, dimension( ldab, * ) AB, integer LDAB, integer, dimension( * ) IPIV, double precision, dimension( ldb, * ) B, integer LDB, integer INFO )

DGBTRS

Download DGBTRS + dependencies [TGZ] [ZIP] [TXT]

Purpose:
``` DGBTRS solves a system of linear equations
A * X = B  or  A**T * X = B
with a general band matrix A using the LU factorization computed
by DGBTRF.```
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. N >= 0.``` [in] KL ``` KL is INTEGER The number of subdiagonals within the band of A. KL >= 0.``` [in] KU ``` KU is INTEGER The number of superdiagonals within the band of A. KU >= 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] AB ``` AB is DOUBLE PRECISION array, dimension (LDAB,N) Details of the LU factorization of the band matrix A, as computed by DGBTRF. U is stored as an upper triangular band matrix with KL+KU superdiagonals in rows 1 to KL+KU+1, and the multipliers used during the factorization are stored in rows KL+KU+2 to 2*KL+KU+1.``` [in] LDAB ``` LDAB is INTEGER The leading dimension of the array AB. LDAB >= 2*KL+KU+1.``` [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).``` [in,out] B ``` B is DOUBLE PRECISION array, dimension (LDB,NRHS) On entry, the right hand side matrix B. On exit, the solution matrix 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
December 2016

Definition at line 140 of file dgbtrs.f.

140 *
141 * -- LAPACK computational routine (version 3.7.0) --
142 * -- LAPACK is a software package provided by Univ. of Tennessee, --
143 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
144 * December 2016
145 *
146 * .. Scalar Arguments ..
147  CHARACTER TRANS
148  INTEGER INFO, KL, KU, LDAB, LDB, N, NRHS
149 * ..
150 * .. Array Arguments ..
151  INTEGER IPIV( * )
152  DOUBLE PRECISION AB( LDAB, * ), B( LDB, * )
153 * ..
154 *
155 * =====================================================================
156 *
157 * .. Parameters ..
158  DOUBLE PRECISION ONE
159  parameter( one = 1.0d+0 )
160 * ..
161 * .. Local Scalars ..
162  LOGICAL LNOTI, NOTRAN
163  INTEGER I, J, KD, L, LM
164 * ..
165 * .. External Functions ..
166  LOGICAL LSAME
167  EXTERNAL lsame
168 * ..
169 * .. External Subroutines ..
170  EXTERNAL dgemv, dger, dswap, dtbsv, xerbla
171 * ..
172 * .. Intrinsic Functions ..
173  INTRINSIC max, min
174 * ..
175 * .. Executable Statements ..
176 *
177 * Test the input parameters.
178 *
179  info = 0
180  notran = lsame( trans, 'N' )
181  IF( .NOT.notran .AND. .NOT.lsame( trans, 'T' ) .AND. .NOT.
182  \$ lsame( trans, 'C' ) ) THEN
183  info = -1
184  ELSE IF( n.LT.0 ) THEN
185  info = -2
186  ELSE IF( kl.LT.0 ) THEN
187  info = -3
188  ELSE IF( ku.LT.0 ) THEN
189  info = -4
190  ELSE IF( nrhs.LT.0 ) THEN
191  info = -5
192  ELSE IF( ldab.LT.( 2*kl+ku+1 ) ) THEN
193  info = -7
194  ELSE IF( ldb.LT.max( 1, n ) ) THEN
195  info = -10
196  END IF
197  IF( info.NE.0 ) THEN
198  CALL xerbla( 'DGBTRS', -info )
199  RETURN
200  END IF
201 *
202 * Quick return if possible
203 *
204  IF( n.EQ.0 .OR. nrhs.EQ.0 )
205  \$ RETURN
206 *
207  kd = ku + kl + 1
208  lnoti = kl.GT.0
209 *
210  IF( notran ) THEN
211 *
212 * Solve A*X = B.
213 *
214 * Solve L*X = B, overwriting B with X.
215 *
216 * L is represented as a product of permutations and unit lower
217 * triangular matrices L = P(1) * L(1) * ... * P(n-1) * L(n-1),
218 * where each transformation L(i) is a rank-one modification of
219 * the identity matrix.
220 *
221  IF( lnoti ) THEN
222  DO 10 j = 1, n - 1
223  lm = min( kl, n-j )
224  l = ipiv( j )
225  IF( l.NE.j )
226  \$ CALL dswap( nrhs, b( l, 1 ), ldb, b( j, 1 ), ldb )
227  CALL dger( lm, nrhs, -one, ab( kd+1, j ), 1, b( j, 1 ),
228  \$ ldb, b( j+1, 1 ), ldb )
229  10 CONTINUE
230  END IF
231 *
232  DO 20 i = 1, nrhs
233 *
234 * Solve U*X = B, overwriting B with X.
235 *
236  CALL dtbsv( 'Upper', 'No transpose', 'Non-unit', n, kl+ku,
237  \$ ab, ldab, b( 1, i ), 1 )
238  20 CONTINUE
239 *
240  ELSE
241 *
242 * Solve A**T*X = B.
243 *
244  DO 30 i = 1, nrhs
245 *
246 * Solve U**T*X = B, overwriting B with X.
247 *
248  CALL dtbsv( 'Upper', 'Transpose', 'Non-unit', n, kl+ku, ab,
249  \$ ldab, b( 1, i ), 1 )
250  30 CONTINUE
251 *
252 * Solve L**T*X = B, overwriting B with X.
253 *
254  IF( lnoti ) THEN
255  DO 40 j = n - 1, 1, -1
256  lm = min( kl, n-j )
257  CALL dgemv( 'Transpose', lm, nrhs, -one, b( j+1, 1 ),
258  \$ ldb, ab( kd+1, j ), 1, one, b( j, 1 ), ldb )
259  l = ipiv( j )
260  IF( l.NE.j )
261  \$ CALL dswap( nrhs, b( l, 1 ), ldb, b( j, 1 ), ldb )
262  40 CONTINUE
263  END IF
264  END IF
265  RETURN
266 *
267 * End of DGBTRS
268 *
Here is the call graph for this function:
Here is the caller graph for this function:
dger
subroutine dger(M, N, ALPHA, X, INCX, Y, INCY, A, LDA)
DGER
Definition: dger.f:132
dtbsv
subroutine dtbsv(UPLO, TRANS, DIAG, N, K, A, LDA, X, INCX)
DTBSV
Definition: dtbsv.f:191
xerbla
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:62
lsame
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:55
dgemv
subroutine dgemv(TRANS, M, N, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
DGEMV
Definition: dgemv.f:158
dswap
subroutine dswap(N, DX, INCX, DY, INCY)
DSWAP
Definition: dswap.f:84