 LAPACK  3.10.1 LAPACK: Linear Algebra PACKage

## ◆ chprfs()

 subroutine chprfs ( character UPLO, integer N, integer NRHS, complex, dimension( * ) AP, complex, dimension( * ) AFP, integer, dimension( * ) IPIV, complex, dimension( ldb, * ) B, integer LDB, complex, dimension( ldx, * ) X, integer LDX, real, dimension( * ) FERR, real, dimension( * ) BERR, complex, dimension( * ) WORK, real, dimension( * ) RWORK, integer INFO )

CHPRFS

Purpose:
``` CHPRFS improves the computed solution to a system of linear
equations when the coefficient matrix is Hermitian indefinite
and packed, and provides error bounds and backward error estimates
for the solution.```
Parameters
 [in] UPLO ``` UPLO is CHARACTER*1 = 'U': Upper triangle of A is stored; = 'L': Lower triangle of A is stored.``` [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] AP ``` AP is COMPLEX array, dimension (N*(N+1)/2) The upper or lower triangle of the Hermitian matrix A, packed columnwise in a linear array. The j-th column of A is stored in the array AP as follows: if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j; if UPLO = 'L', AP(i + (j-1)*(2*n-j)/2) = A(i,j) for j<=i<=n.``` [in] AFP ``` AFP is COMPLEX array, dimension (N*(N+1)/2) The factored form of the matrix A. AFP contains the block diagonal matrix D and the multipliers used to obtain the factor U or L from the factorization A = U*D*U**H or A = L*D*L**H as computed by CHPTRF, stored as a packed triangular matrix.``` [in] IPIV ``` IPIV is INTEGER array, dimension (N) Details of the interchanges and the block structure of D as determined by CHPTRF.``` [in] B ``` B is COMPLEX 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 array, dimension (LDX,NRHS) On entry, the solution matrix X, as computed by CHPTRS. 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 REAL 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 REAL 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 array, dimension (2*N)` [out] RWORK ` RWORK is REAL 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 178 of file chprfs.f.

180 *
181 * -- LAPACK computational routine --
182 * -- LAPACK is a software package provided by Univ. of Tennessee, --
183 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
184 *
185 * .. Scalar Arguments ..
186  CHARACTER UPLO
187  INTEGER INFO, LDB, LDX, N, NRHS
188 * ..
189 * .. Array Arguments ..
190  INTEGER IPIV( * )
191  REAL BERR( * ), FERR( * ), RWORK( * )
192  COMPLEX AFP( * ), AP( * ), B( LDB, * ), WORK( * ),
193  \$ X( LDX, * )
194 * ..
195 *
196 * =====================================================================
197 *
198 * .. Parameters ..
199  INTEGER ITMAX
200  parameter( itmax = 5 )
201  REAL ZERO
202  parameter( zero = 0.0e+0 )
203  COMPLEX ONE
204  parameter( one = ( 1.0e+0, 0.0e+0 ) )
205  REAL TWO
206  parameter( two = 2.0e+0 )
207  REAL THREE
208  parameter( three = 3.0e+0 )
209 * ..
210 * .. Local Scalars ..
211  LOGICAL UPPER
212  INTEGER COUNT, I, IK, J, K, KASE, KK, NZ
213  REAL EPS, LSTRES, S, SAFE1, SAFE2, SAFMIN, XK
214  COMPLEX ZDUM
215 * ..
216 * .. Local Arrays ..
217  INTEGER ISAVE( 3 )
218 * ..
219 * .. External Subroutines ..
220  EXTERNAL caxpy, ccopy, chpmv, chptrs, clacn2, xerbla
221 * ..
222 * .. Intrinsic Functions ..
223  INTRINSIC abs, aimag, max, real
224 * ..
225 * .. External Functions ..
226  LOGICAL LSAME
227  REAL SLAMCH
228  EXTERNAL lsame, slamch
229 * ..
230 * .. Statement Functions ..
231  REAL CABS1
232 * ..
233 * .. Statement Function definitions ..
234  cabs1( zdum ) = abs( real( zdum ) ) + abs( aimag( zdum ) )
235 * ..
236 * .. Executable Statements ..
237 *
238 * Test the input parameters.
239 *
240  info = 0
241  upper = lsame( uplo, 'U' )
242  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
243  info = -1
244  ELSE IF( n.LT.0 ) THEN
245  info = -2
246  ELSE IF( nrhs.LT.0 ) THEN
247  info = -3
248  ELSE IF( ldb.LT.max( 1, n ) ) THEN
249  info = -8
250  ELSE IF( ldx.LT.max( 1, n ) ) THEN
251  info = -10
252  END IF
253  IF( info.NE.0 ) THEN
254  CALL xerbla( 'CHPRFS', -info )
255  RETURN
256  END IF
257 *
258 * Quick return if possible
259 *
260  IF( n.EQ.0 .OR. nrhs.EQ.0 ) THEN
261  DO 10 j = 1, nrhs
262  ferr( j ) = zero
263  berr( j ) = zero
264  10 CONTINUE
265  RETURN
266  END IF
267 *
268 * NZ = maximum number of nonzero elements in each row of A, plus 1
269 *
270  nz = n + 1
271  eps = slamch( 'Epsilon' )
272  safmin = slamch( 'Safe minimum' )
273  safe1 = nz*safmin
274  safe2 = safe1 / eps
275 *
276 * Do for each right hand side
277 *
278  DO 140 j = 1, nrhs
279 *
280  count = 1
281  lstres = three
282  20 CONTINUE
283 *
284 * Loop until stopping criterion is satisfied.
285 *
286 * Compute residual R = B - A * X
287 *
288  CALL ccopy( n, b( 1, j ), 1, work, 1 )
289  CALL chpmv( uplo, n, -one, ap, x( 1, j ), 1, one, work, 1 )
290 *
291 * Compute componentwise relative backward error from formula
292 *
293 * max(i) ( abs(R(i)) / ( abs(A)*abs(X) + abs(B) )(i) )
294 *
295 * where abs(Z) is the componentwise absolute value of the matrix
296 * or vector Z. If the i-th component of the denominator is less
297 * than SAFE2, then SAFE1 is added to the i-th components of the
298 * numerator and denominator before dividing.
299 *
300  DO 30 i = 1, n
301  rwork( i ) = cabs1( b( i, j ) )
302  30 CONTINUE
303 *
304 * Compute abs(A)*abs(X) + abs(B).
305 *
306  kk = 1
307  IF( upper ) THEN
308  DO 50 k = 1, n
309  s = zero
310  xk = cabs1( x( k, j ) )
311  ik = kk
312  DO 40 i = 1, k - 1
313  rwork( i ) = rwork( i ) + cabs1( ap( ik ) )*xk
314  s = s + cabs1( ap( ik ) )*cabs1( x( i, j ) )
315  ik = ik + 1
316  40 CONTINUE
317  rwork( k ) = rwork( k ) + abs( real( ap( kk+k-1 ) ) )*
318  \$ xk + s
319  kk = kk + k
320  50 CONTINUE
321  ELSE
322  DO 70 k = 1, n
323  s = zero
324  xk = cabs1( x( k, j ) )
325  rwork( k ) = rwork( k ) + abs( real( ap( kk ) ) )*xk
326  ik = kk + 1
327  DO 60 i = k + 1, n
328  rwork( i ) = rwork( i ) + cabs1( ap( ik ) )*xk
329  s = s + cabs1( ap( ik ) )*cabs1( x( i, j ) )
330  ik = ik + 1
331  60 CONTINUE
332  rwork( k ) = rwork( k ) + s
333  kk = kk + ( n-k+1 )
334  70 CONTINUE
335  END IF
336  s = zero
337  DO 80 i = 1, n
338  IF( rwork( i ).GT.safe2 ) THEN
339  s = max( s, cabs1( work( i ) ) / rwork( i ) )
340  ELSE
341  s = max( s, ( cabs1( work( i ) )+safe1 ) /
342  \$ ( rwork( i )+safe1 ) )
343  END IF
344  80 CONTINUE
345  berr( j ) = s
346 *
347 * Test stopping criterion. Continue iterating if
348 * 1) The residual BERR(J) is larger than machine epsilon, and
349 * 2) BERR(J) decreased by at least a factor of 2 during the
350 * last iteration, and
351 * 3) At most ITMAX iterations tried.
352 *
353  IF( berr( j ).GT.eps .AND. two*berr( j ).LE.lstres .AND.
354  \$ count.LE.itmax ) THEN
355 *
356 * Update solution and try again.
357 *
358  CALL chptrs( uplo, n, 1, afp, ipiv, work, n, info )
359  CALL caxpy( n, one, work, 1, x( 1, j ), 1 )
360  lstres = berr( j )
361  count = count + 1
362  GO TO 20
363  END IF
364 *
365 * Bound error from formula
366 *
367 * norm(X - XTRUE) / norm(X) .le. FERR =
368 * norm( abs(inv(A))*
369 * ( abs(R) + NZ*EPS*( abs(A)*abs(X)+abs(B) ))) / norm(X)
370 *
371 * where
372 * norm(Z) is the magnitude of the largest component of Z
373 * inv(A) is the inverse of A
374 * abs(Z) is the componentwise absolute value of the matrix or
375 * vector Z
376 * NZ is the maximum number of nonzeros in any row of A, plus 1
377 * EPS is machine epsilon
378 *
379 * The i-th component of abs(R)+NZ*EPS*(abs(A)*abs(X)+abs(B))
380 * is incremented by SAFE1 if the i-th component of
381 * abs(A)*abs(X) + abs(B) is less than SAFE2.
382 *
383 * Use CLACN2 to estimate the infinity-norm of the matrix
384 * inv(A) * diag(W),
385 * where W = abs(R) + NZ*EPS*( abs(A)*abs(X)+abs(B) )))
386 *
387  DO 90 i = 1, n
388  IF( rwork( i ).GT.safe2 ) THEN
389  rwork( i ) = cabs1( work( i ) ) + nz*eps*rwork( i )
390  ELSE
391  rwork( i ) = cabs1( work( i ) ) + nz*eps*rwork( i ) +
392  \$ safe1
393  END IF
394  90 CONTINUE
395 *
396  kase = 0
397  100 CONTINUE
398  CALL clacn2( n, work( n+1 ), work, ferr( j ), kase, isave )
399  IF( kase.NE.0 ) THEN
400  IF( kase.EQ.1 ) THEN
401 *
402 * Multiply by diag(W)*inv(A**H).
403 *
404  CALL chptrs( uplo, n, 1, afp, ipiv, work, n, info )
405  DO 110 i = 1, n
406  work( i ) = rwork( i )*work( i )
407  110 CONTINUE
408  ELSE IF( kase.EQ.2 ) THEN
409 *
410 * Multiply by inv(A)*diag(W).
411 *
412  DO 120 i = 1, n
413  work( i ) = rwork( i )*work( i )
414  120 CONTINUE
415  CALL chptrs( uplo, n, 1, afp, ipiv, work, n, info )
416  END IF
417  GO TO 100
418  END IF
419 *
420 * Normalize error.
421 *
422  lstres = zero
423  DO 130 i = 1, n
424  lstres = max( lstres, cabs1( x( i, j ) ) )
425  130 CONTINUE
426  IF( lstres.NE.zero )
427  \$ ferr( j ) = ferr( j ) / lstres
428 *
429  140 CONTINUE
430 *
431  RETURN
432 *
433 * End of CHPRFS
434 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine ccopy(N, CX, INCX, CY, INCY)
CCOPY
Definition: ccopy.f:81
subroutine caxpy(N, CA, CX, INCX, CY, INCY)
CAXPY
Definition: caxpy.f:88
subroutine chpmv(UPLO, N, ALPHA, AP, X, INCX, BETA, Y, INCY)
CHPMV
Definition: chpmv.f:149
subroutine clacn2(N, V, X, EST, KASE, ISAVE)
CLACN2 estimates the 1-norm of a square matrix, using reverse communication for evaluating matrix-vec...
Definition: clacn2.f:133
subroutine chptrs(UPLO, N, NRHS, AP, IPIV, B, LDB, INFO)
CHPTRS
Definition: chptrs.f:115
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
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