 LAPACK  3.10.0 LAPACK: Linear Algebra PACKage

## ◆ cgbt02()

 subroutine cgbt02 ( character TRANS, integer M, integer N, integer KL, integer KU, integer NRHS, complex, dimension( lda, * ) A, integer LDA, complex, dimension( ldx, * ) X, integer LDX, complex, dimension( ldb, * ) B, integer LDB, real, dimension( * ) RWORK, real RESID )

CGBT02

Purpose:
``` CGBT02 computes the residual for a solution of a banded system of
equations op(A)*X = B:
RESID = norm(B - op(A)*X) / ( norm(op(A)) * norm(X) * EPS ),
where op(A) = A, A**T, or A**H, depending on TRANS, and EPS is the
machine epsilon.```
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] M ``` M is INTEGER The number of rows of the matrix A. M >= 0.``` [in] N ``` N is INTEGER The number of columns 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 columns of B. NRHS >= 0.``` [in] A ``` A is COMPLEX array, dimension (LDA,N) The original matrix A in band storage, stored in rows 1 to KL+KU+1.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,KL+KU+1).``` [in] X ``` X is COMPLEX array, dimension (LDX,NRHS) The computed solution vectors for the system of linear equations.``` [in] LDX ``` LDX is INTEGER The leading dimension of the array X. If TRANS = 'N', LDX >= max(1,N); if TRANS = 'T' or 'C', LDX >= max(1,M).``` [in,out] B ``` B is COMPLEX array, dimension (LDB,NRHS) On entry, the right hand side vectors for the system of linear equations. On exit, B is overwritten with the difference B - A*X.``` [in] LDB ``` LDB is INTEGER The leading dimension of the array B. IF TRANS = 'N', LDB >= max(1,M); if TRANS = 'T' or 'C', LDB >= max(1,N).``` [out] RWORK ``` RWORK is REAL array, dimension (MAX(1,LRWORK)), where LRWORK >= M when TRANS = 'T' or 'C'; otherwise, RWORK is not referenced.``` [out] RESID ``` RESID is REAL The maximum over the number of right hand sides of norm(B - op(A)*X) / ( norm(op(A)) * norm(X) * EPS ).```

Definition at line 146 of file cgbt02.f.

148 *
149 * -- LAPACK test routine --
150 * -- LAPACK is a software package provided by Univ. of Tennessee, --
151 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
152 *
153 * .. Scalar Arguments ..
154  CHARACTER TRANS
155  INTEGER KL, KU, LDA, LDB, LDX, M, N, NRHS
156  REAL RESID
157 * ..
158 * .. Array Arguments ..
159  REAL RWORK( * )
160  COMPLEX A( LDA, * ), B( LDB, * ), X( LDX, * )
161 * ..
162 *
163 * =====================================================================
164 *
165 * .. Parameters ..
166  REAL ZERO, ONE
167  parameter( zero = 0.0e+0, one = 1.0e+0 )
168  COMPLEX CONE
169  parameter( cone = ( 1.0e+0, 0.0e+0 ) )
170 * ..
171 * .. Local Scalars ..
172  INTEGER I1, I2, J, KD, N1
173  REAL ANORM, BNORM, EPS, TEMP, XNORM
174  COMPLEX CDUM
175 * ..
176 * .. External Functions ..
177  LOGICAL LSAME, SISNAN
178  REAL SCASUM, SLAMCH
179  EXTERNAL lsame, scasum, sisnan, slamch
180 * ..
181 * .. External Subroutines ..
182  EXTERNAL cgbmv
183 * ..
184 * .. Statement Functions ..
185  REAL CABS1
186 * ..
187 * .. Intrinsic Functions ..
188  INTRINSIC abs, aimag, max, min, real
189 * ..
190 * .. Statement Function definitions ..
191  cabs1( cdum ) = abs( real( cdum ) ) + abs( aimag( cdum ) )
192 * ..
193 * .. Executable Statements ..
194 *
195 * Quick return if N = 0 pr NRHS = 0
196 *
197  IF( m.LE.0 .OR. n.LE.0 .OR. nrhs.LE.0 ) THEN
198  resid = zero
199  RETURN
200  END IF
201 *
202 * Exit with RESID = 1/EPS if ANORM = 0.
203 *
204  eps = slamch( 'Epsilon' )
205  anorm = zero
206  IF( lsame( trans, 'N' ) ) THEN
207 *
208 * Find norm1(A).
209 *
210  kd = ku + 1
211  DO 10 j = 1, n
212  i1 = max( kd+1-j, 1 )
213  i2 = min( kd+m-j, kl+kd )
214  IF( i2.GE.i1 ) THEN
215  temp = scasum( i2-i1+1, a( i1, j ), 1 )
216  IF( anorm.LT.temp .OR. sisnan( temp ) ) anorm = temp
217  END IF
218  10 CONTINUE
219  ELSE
220 *
221 * Find normI(A).
222 *
223  DO 12 i1 = 1, m
224  rwork( i1 ) = zero
225  12 CONTINUE
226  DO 16 j = 1, n
227  kd = ku + 1 - j
228  DO 14 i1 = max( 1, j-ku ), min( m, j+kl )
229  rwork( i1 ) = rwork( i1 ) + cabs1( a( kd+i1, j ) )
230  14 CONTINUE
231  16 CONTINUE
232  DO 18 i1 = 1, m
233  temp = rwork( i1 )
234  IF( anorm.LT.temp .OR. sisnan( temp ) ) anorm = temp
235  18 CONTINUE
236  END IF
237  IF( anorm.LE.zero ) THEN
238  resid = one / eps
239  RETURN
240  END IF
241 *
242  IF( lsame( trans, 'T' ) .OR. lsame( trans, 'C' ) ) THEN
243  n1 = n
244  ELSE
245  n1 = m
246  END IF
247 *
248 * Compute B - op(A)*X
249 *
250  DO 20 j = 1, nrhs
251  CALL cgbmv( trans, m, n, kl, ku, -cone, a, lda, x( 1, j ), 1,
252  \$ cone, b( 1, j ), 1 )
253  20 CONTINUE
254 *
255 * Compute the maximum over the number of right hand sides of
256 * norm(B - op(A)*X) / ( norm(op(A)) * norm(X) * EPS ).
257 *
258  resid = zero
259  DO 30 j = 1, nrhs
260  bnorm = scasum( n1, b( 1, j ), 1 )
261  xnorm = scasum( n1, x( 1, j ), 1 )
262  IF( xnorm.LE.zero ) THEN
263  resid = one / eps
264  ELSE
265  resid = max( resid, ( ( bnorm/anorm )/xnorm )/eps )
266  END IF
267  30 CONTINUE
268 *
269  RETURN
270 *
271 * End of CGBT02
272 *
logical function sisnan(SIN)
SISNAN tests input for NaN.
Definition: sisnan.f:59
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine cgbmv(TRANS, M, N, KL, KU, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
CGBMV
Definition: cgbmv.f:187
real function scasum(N, CX, INCX)
SCASUM
Definition: scasum.f:72
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
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