 LAPACK  3.10.0 LAPACK: Linear Algebra PACKage

## ◆ dporfs()

 subroutine dporfs ( character UPLO, integer N, integer NRHS, double precision, dimension( lda, * ) A, integer LDA, double precision, dimension( ldaf, * ) AF, integer LDAF, double precision, dimension( ldb, * ) B, integer LDB, double precision, dimension( ldx, * ) X, integer LDX, double precision, dimension( * ) FERR, double precision, dimension( * ) BERR, double precision, dimension( * ) WORK, integer, dimension( * ) IWORK, integer INFO )

DPORFS

Purpose:
``` DPORFS improves the computed solution to a system of linear
equations when the coefficient matrix is symmetric positive definite,
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] A ``` A is DOUBLE PRECISION array, dimension (LDA,N) The symmetric matrix A. If UPLO = 'U', the leading N-by-N upper triangular part of A contains the upper triangular part of the matrix A, and the strictly lower triangular part of A is not referenced. If UPLO = 'L', the leading N-by-N lower triangular part of A contains the lower triangular part of the matrix A, and the strictly upper triangular part of A is not referenced.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,N).``` [in] AF ``` AF is DOUBLE PRECISION array, dimension (LDAF,N) The triangular factor U or L from the Cholesky factorization A = U**T*U or A = L*L**T, as computed by DPOTRF.``` [in] LDAF ``` LDAF is INTEGER The leading dimension of the array AF. LDAF >= max(1,N).``` [in] B ``` B is DOUBLE PRECISION 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 DOUBLE PRECISION array, dimension (LDX,NRHS) On entry, the solution matrix X, as computed by DPOTRS. 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 DOUBLE PRECISION array, dimension (3*N)` [out] IWORK ` IWORK is INTEGER 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 181 of file dporfs.f.

183 *
184 * -- LAPACK computational routine --
185 * -- LAPACK is a software package provided by Univ. of Tennessee, --
186 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
187 *
188 * .. Scalar Arguments ..
189  CHARACTER UPLO
190  INTEGER INFO, LDA, LDAF, LDB, LDX, N, NRHS
191 * ..
192 * .. Array Arguments ..
193  INTEGER IWORK( * )
194  DOUBLE PRECISION A( LDA, * ), AF( LDAF, * ), B( LDB, * ),
195  \$ BERR( * ), FERR( * ), WORK( * ), X( LDX, * )
196 * ..
197 *
198 * =====================================================================
199 *
200 * .. Parameters ..
201  INTEGER ITMAX
202  parameter( itmax = 5 )
203  DOUBLE PRECISION ZERO
204  parameter( zero = 0.0d+0 )
205  DOUBLE PRECISION ONE
206  parameter( one = 1.0d+0 )
207  DOUBLE PRECISION TWO
208  parameter( two = 2.0d+0 )
209  DOUBLE PRECISION THREE
210  parameter( three = 3.0d+0 )
211 * ..
212 * .. Local Scalars ..
213  LOGICAL UPPER
214  INTEGER COUNT, I, J, K, KASE, NZ
215  DOUBLE PRECISION EPS, LSTRES, S, SAFE1, SAFE2, SAFMIN, XK
216 * ..
217 * .. Local Arrays ..
218  INTEGER ISAVE( 3 )
219 * ..
220 * .. External Subroutines ..
221  EXTERNAL daxpy, dcopy, dlacn2, dpotrs, dsymv, xerbla
222 * ..
223 * .. Intrinsic Functions ..
224  INTRINSIC abs, max
225 * ..
226 * .. External Functions ..
227  LOGICAL LSAME
228  DOUBLE PRECISION DLAMCH
229  EXTERNAL lsame, dlamch
230 * ..
231 * .. Executable Statements ..
232 *
233 * Test the input parameters.
234 *
235  info = 0
236  upper = lsame( uplo, 'U' )
237  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
238  info = -1
239  ELSE IF( n.LT.0 ) THEN
240  info = -2
241  ELSE IF( nrhs.LT.0 ) THEN
242  info = -3
243  ELSE IF( lda.LT.max( 1, n ) ) THEN
244  info = -5
245  ELSE IF( ldaf.LT.max( 1, n ) ) THEN
246  info = -7
247  ELSE IF( ldb.LT.max( 1, n ) ) THEN
248  info = -9
249  ELSE IF( ldx.LT.max( 1, n ) ) THEN
250  info = -11
251  END IF
252  IF( info.NE.0 ) THEN
253  CALL xerbla( 'DPORFS', -info )
254  RETURN
255  END IF
256 *
257 * Quick return if possible
258 *
259  IF( n.EQ.0 .OR. nrhs.EQ.0 ) THEN
260  DO 10 j = 1, nrhs
261  ferr( j ) = zero
262  berr( j ) = zero
263  10 CONTINUE
264  RETURN
265  END IF
266 *
267 * NZ = maximum number of nonzero elements in each row of A, plus 1
268 *
269  nz = n + 1
270  eps = dlamch( 'Epsilon' )
271  safmin = dlamch( 'Safe minimum' )
272  safe1 = nz*safmin
273  safe2 = safe1 / eps
274 *
275 * Do for each right hand side
276 *
277  DO 140 j = 1, nrhs
278 *
279  count = 1
280  lstres = three
281  20 CONTINUE
282 *
283 * Loop until stopping criterion is satisfied.
284 *
285 * Compute residual R = B - A * X
286 *
287  CALL dcopy( n, b( 1, j ), 1, work( n+1 ), 1 )
288  CALL dsymv( uplo, n, -one, a, lda, x( 1, j ), 1, one,
289  \$ work( n+1 ), 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  work( i ) = abs( b( i, j ) )
302  30 CONTINUE
303 *
304 * Compute abs(A)*abs(X) + abs(B).
305 *
306  IF( upper ) THEN
307  DO 50 k = 1, n
308  s = zero
309  xk = abs( x( k, j ) )
310  DO 40 i = 1, k - 1
311  work( i ) = work( i ) + abs( a( i, k ) )*xk
312  s = s + abs( a( i, k ) )*abs( x( i, j ) )
313  40 CONTINUE
314  work( k ) = work( k ) + abs( a( k, k ) )*xk + s
315  50 CONTINUE
316  ELSE
317  DO 70 k = 1, n
318  s = zero
319  xk = abs( x( k, j ) )
320  work( k ) = work( k ) + abs( a( k, k ) )*xk
321  DO 60 i = k + 1, n
322  work( i ) = work( i ) + abs( a( i, k ) )*xk
323  s = s + abs( a( i, k ) )*abs( x( i, j ) )
324  60 CONTINUE
325  work( k ) = work( k ) + s
326  70 CONTINUE
327  END IF
328  s = zero
329  DO 80 i = 1, n
330  IF( work( i ).GT.safe2 ) THEN
331  s = max( s, abs( work( n+i ) ) / work( i ) )
332  ELSE
333  s = max( s, ( abs( work( n+i ) )+safe1 ) /
334  \$ ( work( i )+safe1 ) )
335  END IF
336  80 CONTINUE
337  berr( j ) = s
338 *
339 * Test stopping criterion. Continue iterating if
340 * 1) The residual BERR(J) is larger than machine epsilon, and
341 * 2) BERR(J) decreased by at least a factor of 2 during the
342 * last iteration, and
343 * 3) At most ITMAX iterations tried.
344 *
345  IF( berr( j ).GT.eps .AND. two*berr( j ).LE.lstres .AND.
346  \$ count.LE.itmax ) THEN
347 *
348 * Update solution and try again.
349 *
350  CALL dpotrs( uplo, n, 1, af, ldaf, work( n+1 ), n, info )
351  CALL daxpy( n, one, work( n+1 ), 1, x( 1, j ), 1 )
352  lstres = berr( j )
353  count = count + 1
354  GO TO 20
355  END IF
356 *
357 * Bound error from formula
358 *
359 * norm(X - XTRUE) / norm(X) .le. FERR =
360 * norm( abs(inv(A))*
361 * ( abs(R) + NZ*EPS*( abs(A)*abs(X)+abs(B) ))) / norm(X)
362 *
363 * where
364 * norm(Z) is the magnitude of the largest component of Z
365 * inv(A) is the inverse of A
366 * abs(Z) is the componentwise absolute value of the matrix or
367 * vector Z
368 * NZ is the maximum number of nonzeros in any row of A, plus 1
369 * EPS is machine epsilon
370 *
371 * The i-th component of abs(R)+NZ*EPS*(abs(A)*abs(X)+abs(B))
372 * is incremented by SAFE1 if the i-th component of
373 * abs(A)*abs(X) + abs(B) is less than SAFE2.
374 *
375 * Use DLACN2 to estimate the infinity-norm of the matrix
376 * inv(A) * diag(W),
377 * where W = abs(R) + NZ*EPS*( abs(A)*abs(X)+abs(B) )))
378 *
379  DO 90 i = 1, n
380  IF( work( i ).GT.safe2 ) THEN
381  work( i ) = abs( work( n+i ) ) + nz*eps*work( i )
382  ELSE
383  work( i ) = abs( work( n+i ) ) + nz*eps*work( i ) + safe1
384  END IF
385  90 CONTINUE
386 *
387  kase = 0
388  100 CONTINUE
389  CALL dlacn2( n, work( 2*n+1 ), work( n+1 ), iwork, ferr( j ),
390  \$ kase, isave )
391  IF( kase.NE.0 ) THEN
392  IF( kase.EQ.1 ) THEN
393 *
394 * Multiply by diag(W)*inv(A**T).
395 *
396  CALL dpotrs( uplo, n, 1, af, ldaf, work( n+1 ), n, info )
397  DO 110 i = 1, n
398  work( n+i ) = work( i )*work( n+i )
399  110 CONTINUE
400  ELSE IF( kase.EQ.2 ) THEN
401 *
402 * Multiply by inv(A)*diag(W).
403 *
404  DO 120 i = 1, n
405  work( n+i ) = work( i )*work( n+i )
406  120 CONTINUE
407  CALL dpotrs( uplo, n, 1, af, ldaf, work( n+1 ), n, info )
408  END IF
409  GO TO 100
410  END IF
411 *
412 * Normalize error.
413 *
414  lstres = zero
415  DO 130 i = 1, n
416  lstres = max( lstres, abs( x( i, j ) ) )
417  130 CONTINUE
418  IF( lstres.NE.zero )
419  \$ ferr( j ) = ferr( j ) / lstres
420 *
421  140 CONTINUE
422 *
423  RETURN
424 *
425 * End of DPORFS
426 *
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 dcopy(N, DX, INCX, DY, INCY)
DCOPY
Definition: dcopy.f:82
subroutine daxpy(N, DA, DX, INCX, DY, INCY)
DAXPY
Definition: daxpy.f:89
subroutine dsymv(UPLO, N, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
DSYMV
Definition: dsymv.f:152
subroutine dlacn2(N, V, X, ISGN, EST, KASE, ISAVE)
DLACN2 estimates the 1-norm of a square matrix, using reverse communication for evaluating matrix-vec...
Definition: dlacn2.f:136
subroutine dpotrs(UPLO, N, NRHS, A, LDA, B, LDB, INFO)
DPOTRS
Definition: dpotrs.f:110
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