 LAPACK  3.10.0 LAPACK: Linear Algebra PACKage

## ◆ zgerfs()

 subroutine zgerfs ( character TRANS, integer N, integer NRHS, complex*16, dimension( lda, * ) A, integer LDA, complex*16, dimension( ldaf, * ) AF, integer LDAF, integer, dimension( * ) IPIV, complex*16, dimension( ldb, * ) B, integer LDB, complex*16, dimension( ldx, * ) X, integer LDX, double precision, dimension( * ) FERR, double precision, dimension( * ) BERR, complex*16, dimension( * ) WORK, double precision, dimension( * ) RWORK, integer INFO )

ZGERFS

Purpose:
``` ZGERFS improves the computed solution to a system of linear
equations and provides error bounds and backward error estimates for
the solution.```
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. N >= 0.``` [in] NRHS ``` NRHS is INTEGER The number of right hand sides, i.e., the number of columns of the matrices B and X. NRHS >= 0.``` [in] A ``` A is COMPLEX*16 array, dimension (LDA,N) The original N-by-N matrix A.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,N).``` [in] AF ``` AF is COMPLEX*16 array, dimension (LDAF,N) The factors L and U from the factorization A = P*L*U as computed by ZGETRF.``` [in] LDAF ``` LDAF is INTEGER The leading dimension of the array AF. LDAF >= max(1,N).``` [in] IPIV ``` IPIV is INTEGER array, dimension (N) The pivot indices from ZGETRF; for 1<=i<=N, row i of the matrix was interchanged with row IPIV(i).``` [in] B ``` B is COMPLEX*16 array, dimension (LDB,NRHS) The right hand side matrix B.``` [in] LDB ``` LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N).``` [in,out] X ``` X is COMPLEX*16 array, dimension (LDX,NRHS) On entry, the solution matrix X, as computed by ZGETRS. On exit, the improved solution matrix X.``` [in] LDX ``` LDX is INTEGER The leading dimension of the array X. LDX >= max(1,N).``` [out] FERR ``` FERR is DOUBLE PRECISION array, dimension (NRHS) The estimated forward error bound for each solution vector X(j) (the j-th column of the solution matrix X). If XTRUE is the true solution corresponding to X(j), FERR(j) is an estimated upper bound for the magnitude of the largest element in (X(j) - XTRUE) divided by the magnitude of the largest element in X(j). The estimate is as reliable as the estimate for RCOND, and is almost always a slight overestimate of the true error.``` [out] BERR ``` BERR is DOUBLE PRECISION array, dimension (NRHS) The componentwise relative backward error of each solution vector X(j) (i.e., the smallest relative change in any element of A or B that makes X(j) an exact solution).``` [out] WORK ` WORK is COMPLEX*16 array, dimension (2*N)` [out] RWORK ` RWORK is DOUBLE PRECISION array, dimension (N)` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value```
Internal Parameters:
`  ITMAX is the maximum number of steps of iterative refinement.`

Definition at line 184 of file zgerfs.f.

186 *
187 * -- LAPACK computational routine --
188 * -- LAPACK is a software package provided by Univ. of Tennessee, --
189 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
190 *
191 * .. Scalar Arguments ..
192  CHARACTER TRANS
193  INTEGER INFO, LDA, LDAF, LDB, LDX, N, NRHS
194 * ..
195 * .. Array Arguments ..
196  INTEGER IPIV( * )
197  DOUBLE PRECISION BERR( * ), FERR( * ), RWORK( * )
198  COMPLEX*16 A( LDA, * ), AF( LDAF, * ), B( LDB, * ),
199  \$ WORK( * ), X( LDX, * )
200 * ..
201 *
202 * =====================================================================
203 *
204 * .. Parameters ..
205  INTEGER ITMAX
206  parameter( itmax = 5 )
207  DOUBLE PRECISION ZERO
208  parameter( zero = 0.0d+0 )
209  COMPLEX*16 ONE
210  parameter( one = ( 1.0d+0, 0.0d+0 ) )
211  DOUBLE PRECISION TWO
212  parameter( two = 2.0d+0 )
213  DOUBLE PRECISION THREE
214  parameter( three = 3.0d+0 )
215 * ..
216 * .. Local Scalars ..
217  LOGICAL NOTRAN
218  CHARACTER TRANSN, TRANST
219  INTEGER COUNT, I, J, K, KASE, NZ
220  DOUBLE PRECISION EPS, LSTRES, S, SAFE1, SAFE2, SAFMIN, XK
221  COMPLEX*16 ZDUM
222 * ..
223 * .. Local Arrays ..
224  INTEGER ISAVE( 3 )
225 * ..
226 * .. External Functions ..
227  LOGICAL LSAME
228  DOUBLE PRECISION DLAMCH
229  EXTERNAL lsame, dlamch
230 * ..
231 * .. External Subroutines ..
232  EXTERNAL xerbla, zaxpy, zcopy, zgemv, zgetrs, zlacn2
233 * ..
234 * .. Intrinsic Functions ..
235  INTRINSIC abs, dble, dimag, max
236 * ..
237 * .. Statement Functions ..
238  DOUBLE PRECISION CABS1
239 * ..
240 * .. Statement Function definitions ..
241  cabs1( zdum ) = abs( dble( zdum ) ) + abs( dimag( zdum ) )
242 * ..
243 * .. Executable Statements ..
244 *
245 * Test the input parameters.
246 *
247  info = 0
248  notran = lsame( trans, 'N' )
249  IF( .NOT.notran .AND. .NOT.lsame( trans, 'T' ) .AND. .NOT.
250  \$ lsame( trans, 'C' ) ) THEN
251  info = -1
252  ELSE IF( n.LT.0 ) THEN
253  info = -2
254  ELSE IF( nrhs.LT.0 ) THEN
255  info = -3
256  ELSE IF( lda.LT.max( 1, n ) ) THEN
257  info = -5
258  ELSE IF( ldaf.LT.max( 1, n ) ) THEN
259  info = -7
260  ELSE IF( ldb.LT.max( 1, n ) ) THEN
261  info = -10
262  ELSE IF( ldx.LT.max( 1, n ) ) THEN
263  info = -12
264  END IF
265  IF( info.NE.0 ) THEN
266  CALL xerbla( 'ZGERFS', -info )
267  RETURN
268  END IF
269 *
270 * Quick return if possible
271 *
272  IF( n.EQ.0 .OR. nrhs.EQ.0 ) THEN
273  DO 10 j = 1, nrhs
274  ferr( j ) = zero
275  berr( j ) = zero
276  10 CONTINUE
277  RETURN
278  END IF
279 *
280  IF( notran ) THEN
281  transn = 'N'
282  transt = 'C'
283  ELSE
284  transn = 'C'
285  transt = 'N'
286  END IF
287 *
288 * NZ = maximum number of nonzero elements in each row of A, plus 1
289 *
290  nz = n + 1
291  eps = dlamch( 'Epsilon' )
292  safmin = dlamch( 'Safe minimum' )
293  safe1 = nz*safmin
294  safe2 = safe1 / eps
295 *
296 * Do for each right hand side
297 *
298  DO 140 j = 1, nrhs
299 *
300  count = 1
301  lstres = three
302  20 CONTINUE
303 *
304 * Loop until stopping criterion is satisfied.
305 *
306 * Compute residual R = B - op(A) * X,
307 * where op(A) = A, A**T, or A**H, depending on TRANS.
308 *
309  CALL zcopy( n, b( 1, j ), 1, work, 1 )
310  CALL zgemv( trans, n, n, -one, a, lda, x( 1, j ), 1, one, work,
311  \$ 1 )
312 *
313 * Compute componentwise relative backward error from formula
314 *
315 * max(i) ( abs(R(i)) / ( abs(op(A))*abs(X) + abs(B) )(i) )
316 *
317 * where abs(Z) is the componentwise absolute value of the matrix
318 * or vector Z. If the i-th component of the denominator is less
319 * than SAFE2, then SAFE1 is added to the i-th components of the
320 * numerator and denominator before dividing.
321 *
322  DO 30 i = 1, n
323  rwork( i ) = cabs1( b( i, j ) )
324  30 CONTINUE
325 *
326 * Compute abs(op(A))*abs(X) + abs(B).
327 *
328  IF( notran ) THEN
329  DO 50 k = 1, n
330  xk = cabs1( x( k, j ) )
331  DO 40 i = 1, n
332  rwork( i ) = rwork( i ) + cabs1( a( i, k ) )*xk
333  40 CONTINUE
334  50 CONTINUE
335  ELSE
336  DO 70 k = 1, n
337  s = zero
338  DO 60 i = 1, n
339  s = s + cabs1( a( i, k ) )*cabs1( x( i, j ) )
340  60 CONTINUE
341  rwork( k ) = rwork( k ) + s
342  70 CONTINUE
343  END IF
344  s = zero
345  DO 80 i = 1, n
346  IF( rwork( i ).GT.safe2 ) THEN
347  s = max( s, cabs1( work( i ) ) / rwork( i ) )
348  ELSE
349  s = max( s, ( cabs1( work( i ) )+safe1 ) /
350  \$ ( rwork( i )+safe1 ) )
351  END IF
352  80 CONTINUE
353  berr( j ) = s
354 *
355 * Test stopping criterion. Continue iterating if
356 * 1) The residual BERR(J) is larger than machine epsilon, and
357 * 2) BERR(J) decreased by at least a factor of 2 during the
358 * last iteration, and
359 * 3) At most ITMAX iterations tried.
360 *
361  IF( berr( j ).GT.eps .AND. two*berr( j ).LE.lstres .AND.
362  \$ count.LE.itmax ) THEN
363 *
364 * Update solution and try again.
365 *
366  CALL zgetrs( trans, n, 1, af, ldaf, ipiv, work, n, info )
367  CALL zaxpy( n, one, work, 1, x( 1, j ), 1 )
368  lstres = berr( j )
369  count = count + 1
370  GO TO 20
371  END IF
372 *
373 * Bound error from formula
374 *
375 * norm(X - XTRUE) / norm(X) .le. FERR =
376 * norm( abs(inv(op(A)))*
377 * ( abs(R) + NZ*EPS*( abs(op(A))*abs(X)+abs(B) ))) / norm(X)
378 *
379 * where
380 * norm(Z) is the magnitude of the largest component of Z
381 * inv(op(A)) is the inverse of op(A)
382 * abs(Z) is the componentwise absolute value of the matrix or
383 * vector Z
384 * NZ is the maximum number of nonzeros in any row of A, plus 1
385 * EPS is machine epsilon
386 *
387 * The i-th component of abs(R)+NZ*EPS*(abs(op(A))*abs(X)+abs(B))
388 * is incremented by SAFE1 if the i-th component of
389 * abs(op(A))*abs(X) + abs(B) is less than SAFE2.
390 *
391 * Use ZLACN2 to estimate the infinity-norm of the matrix
392 * inv(op(A)) * diag(W),
393 * where W = abs(R) + NZ*EPS*( abs(op(A))*abs(X)+abs(B) )))
394 *
395  DO 90 i = 1, n
396  IF( rwork( i ).GT.safe2 ) THEN
397  rwork( i ) = cabs1( work( i ) ) + nz*eps*rwork( i )
398  ELSE
399  rwork( i ) = cabs1( work( i ) ) + nz*eps*rwork( i ) +
400  \$ safe1
401  END IF
402  90 CONTINUE
403 *
404  kase = 0
405  100 CONTINUE
406  CALL zlacn2( n, work( n+1 ), work, ferr( j ), kase, isave )
407  IF( kase.NE.0 ) THEN
408  IF( kase.EQ.1 ) THEN
409 *
410 * Multiply by diag(W)*inv(op(A)**H).
411 *
412  CALL zgetrs( transt, n, 1, af, ldaf, ipiv, work, n,
413  \$ info )
414  DO 110 i = 1, n
415  work( i ) = rwork( i )*work( i )
416  110 CONTINUE
417  ELSE
418 *
419 * Multiply by inv(op(A))*diag(W).
420 *
421  DO 120 i = 1, n
422  work( i ) = rwork( i )*work( i )
423  120 CONTINUE
424  CALL zgetrs( transn, n, 1, af, ldaf, ipiv, work, n,
425  \$ info )
426  END IF
427  GO TO 100
428  END IF
429 *
430 * Normalize error.
431 *
432  lstres = zero
433  DO 130 i = 1, n
434  lstres = max( lstres, cabs1( x( i, j ) ) )
435  130 CONTINUE
436  IF( lstres.NE.zero )
437  \$ ferr( j ) = ferr( j ) / lstres
438 *
439  140 CONTINUE
440 *
441  RETURN
442 *
443 * End of ZGERFS
444 *
double precision function dlamch(CMACH)
DLAMCH
Definition: dlamch.f:69
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine zaxpy(N, ZA, ZX, INCX, ZY, INCY)
ZAXPY
Definition: zaxpy.f:88
subroutine zcopy(N, ZX, INCX, ZY, INCY)
ZCOPY
Definition: zcopy.f:81
subroutine zgemv(TRANS, M, N, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
ZGEMV
Definition: zgemv.f:158
subroutine zgetrs(TRANS, N, NRHS, A, LDA, IPIV, B, LDB, INFO)
ZGETRS
Definition: zgetrs.f:121
subroutine zlacn2(N, V, X, EST, KASE, ISAVE)
ZLACN2 estimates the 1-norm of a square matrix, using reverse communication for evaluating matrix-vec...
Definition: zlacn2.f:133
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