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csyrfs.f
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1 *> \brief \b CSYRFS
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
8 *> \htmlonly
9 *> Download CSYRFS + dependencies
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11 *> [TGZ]</a>
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13 *> [ZIP]</a>
14 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/csyrfs.f">
15 *> [TXT]</a>
16 *> \endhtmlonly
17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE CSYRFS( UPLO, N, NRHS, A, LDA, AF, LDAF, IPIV, B, LDB,
22 * X, LDX, FERR, BERR, WORK, RWORK, INFO )
23 *
24 * .. Scalar Arguments ..
25 * CHARACTER UPLO
26 * INTEGER INFO, LDA, LDAF, LDB, LDX, N, NRHS
27 * ..
28 * .. Array Arguments ..
29 * INTEGER IPIV( * )
30 * REAL BERR( * ), FERR( * ), RWORK( * )
31 * COMPLEX A( LDA, * ), AF( LDAF, * ), B( LDB, * ),
32 * $ WORK( * ), X( LDX, * )
33 * ..
34 *
35 *
36 *> \par Purpose:
37 * =============
38 *>
39 *> \verbatim
40 *>
41 *> CSYRFS improves the computed solution to a system of linear
42 *> equations when the coefficient matrix is symmetric indefinite, and
43 *> provides error bounds and backward error estimates for the solution.
44 *> \endverbatim
45 *
46 * Arguments:
47 * ==========
48 *
49 *> \param[in] UPLO
50 *> \verbatim
51 *> UPLO is CHARACTER*1
52 *> = 'U': Upper triangle of A is stored;
53 *> = 'L': Lower triangle of A is stored.
54 *> \endverbatim
55 *>
56 *> \param[in] N
57 *> \verbatim
58 *> N is INTEGER
59 *> The order of the matrix A. N >= 0.
60 *> \endverbatim
61 *>
62 *> \param[in] NRHS
63 *> \verbatim
64 *> NRHS is INTEGER
65 *> The number of right hand sides, i.e., the number of columns
66 *> of the matrices B and X. NRHS >= 0.
67 *> \endverbatim
68 *>
69 *> \param[in] A
70 *> \verbatim
71 *> A is COMPLEX array, dimension (LDA,N)
72 *> The symmetric matrix A. If UPLO = 'U', the leading N-by-N
73 *> upper triangular part of A contains the upper triangular part
74 *> of the matrix A, and the strictly lower triangular part of A
75 *> is not referenced. If UPLO = 'L', the leading N-by-N lower
76 *> triangular part of A contains the lower triangular part of
77 *> the matrix A, and the strictly upper triangular part of A is
78 *> not referenced.
79 *> \endverbatim
80 *>
81 *> \param[in] LDA
82 *> \verbatim
83 *> LDA is INTEGER
84 *> The leading dimension of the array A. LDA >= max(1,N).
85 *> \endverbatim
86 *>
87 *> \param[in] AF
88 *> \verbatim
89 *> AF is COMPLEX array, dimension (LDAF,N)
90 *> The factored form of the matrix A. AF contains the block
91 *> diagonal matrix D and the multipliers used to obtain the
92 *> factor U or L from the factorization A = U*D*U**T or
93 *> A = L*D*L**T as computed by CSYTRF.
94 *> \endverbatim
95 *>
96 *> \param[in] LDAF
97 *> \verbatim
98 *> LDAF is INTEGER
99 *> The leading dimension of the array AF. LDAF >= max(1,N).
100 *> \endverbatim
101 *>
102 *> \param[in] IPIV
103 *> \verbatim
104 *> IPIV is INTEGER array, dimension (N)
105 *> Details of the interchanges and the block structure of D
106 *> as determined by CSYTRF.
107 *> \endverbatim
108 *>
109 *> \param[in] B
110 *> \verbatim
111 *> B is COMPLEX array, dimension (LDB,NRHS)
112 *> The right hand side matrix B.
113 *> \endverbatim
114 *>
115 *> \param[in] LDB
116 *> \verbatim
117 *> LDB is INTEGER
118 *> The leading dimension of the array B. LDB >= max(1,N).
119 *> \endverbatim
120 *>
121 *> \param[in,out] X
122 *> \verbatim
123 *> X is COMPLEX array, dimension (LDX,NRHS)
124 *> On entry, the solution matrix X, as computed by CSYTRS.
125 *> On exit, the improved solution matrix X.
126 *> \endverbatim
127 *>
128 *> \param[in] LDX
129 *> \verbatim
130 *> LDX is INTEGER
131 *> The leading dimension of the array X. LDX >= max(1,N).
132 *> \endverbatim
133 *>
134 *> \param[out] FERR
135 *> \verbatim
136 *> FERR is REAL array, dimension (NRHS)
137 *> The estimated forward error bound for each solution vector
138 *> X(j) (the j-th column of the solution matrix X).
139 *> If XTRUE is the true solution corresponding to X(j), FERR(j)
140 *> is an estimated upper bound for the magnitude of the largest
141 *> element in (X(j) - XTRUE) divided by the magnitude of the
142 *> largest element in X(j). The estimate is as reliable as
143 *> the estimate for RCOND, and is almost always a slight
144 *> overestimate of the true error.
145 *> \endverbatim
146 *>
147 *> \param[out] BERR
148 *> \verbatim
149 *> BERR is REAL array, dimension (NRHS)
150 *> The componentwise relative backward error of each solution
151 *> vector X(j) (i.e., the smallest relative change in
152 *> any element of A or B that makes X(j) an exact solution).
153 *> \endverbatim
154 *>
155 *> \param[out] WORK
156 *> \verbatim
157 *> WORK is COMPLEX array, dimension (2*N)
158 *> \endverbatim
159 *>
160 *> \param[out] RWORK
161 *> \verbatim
162 *> RWORK is REAL array, dimension (N)
163 *> \endverbatim
164 *>
165 *> \param[out] INFO
166 *> \verbatim
167 *> INFO is INTEGER
168 *> = 0: successful exit
169 *> < 0: if INFO = -i, the i-th argument had an illegal value
170 *> \endverbatim
171 *
172 *> \par Internal Parameters:
173 * =========================
174 *>
175 *> \verbatim
176 *> ITMAX is the maximum number of steps of iterative refinement.
177 *> \endverbatim
178 *
179 * Authors:
180 * ========
181 *
182 *> \author Univ. of Tennessee
183 *> \author Univ. of California Berkeley
184 *> \author Univ. of Colorado Denver
185 *> \author NAG Ltd.
186 *
187 *> \date November 2011
188 *
189 *> \ingroup complexSYcomputational
190 *
191 * =====================================================================
192  SUBROUTINE csyrfs( UPLO, N, NRHS, A, LDA, AF, LDAF, IPIV, B, LDB,
193  $ x, ldx, ferr, berr, work, rwork, info )
194 *
195 * -- LAPACK computational routine (version 3.4.0) --
196 * -- LAPACK is a software package provided by Univ. of Tennessee, --
197 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
198 * November 2011
199 *
200 * .. Scalar Arguments ..
201  CHARACTER uplo
202  INTEGER info, lda, ldaf, ldb, ldx, n, nrhs
203 * ..
204 * .. Array Arguments ..
205  INTEGER ipiv( * )
206  REAL berr( * ), ferr( * ), rwork( * )
207  COMPLEX a( lda, * ), af( ldaf, * ), b( ldb, * ),
208  $ work( * ), x( ldx, * )
209 * ..
210 *
211 * =====================================================================
212 *
213 * .. Parameters ..
214  INTEGER itmax
215  parameter( itmax = 5 )
216  REAL zero
217  parameter( zero = 0.0e+0 )
218  COMPLEX one
219  parameter( one = ( 1.0e+0, 0.0e+0 ) )
220  REAL two
221  parameter( two = 2.0e+0 )
222  REAL three
223  parameter( three = 3.0e+0 )
224 * ..
225 * .. Local Scalars ..
226  LOGICAL upper
227  INTEGER count, i, j, k, kase, nz
228  REAL eps, lstres, s, safe1, safe2, safmin, xk
229  COMPLEX zdum
230 * ..
231 * .. Local Arrays ..
232  INTEGER isave( 3 )
233 * ..
234 * .. External Subroutines ..
235  EXTERNAL caxpy, ccopy, clacn2, csymv, csytrs, xerbla
236 * ..
237 * .. Intrinsic Functions ..
238  INTRINSIC abs, aimag, max, real
239 * ..
240 * .. External Functions ..
241  LOGICAL lsame
242  REAL slamch
243  EXTERNAL lsame, slamch
244 * ..
245 * .. Statement Functions ..
246  REAL cabs1
247 * ..
248 * .. Statement Function definitions ..
249  cabs1( zdum ) = abs( REAL( ZDUM ) ) + abs( aimag( zdum ) )
250 * ..
251 * .. Executable Statements ..
252 *
253 * Test the input parameters.
254 *
255  info = 0
256  upper = lsame( uplo, 'U' )
257  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
258  info = -1
259  ELSE IF( n.LT.0 ) THEN
260  info = -2
261  ELSE IF( nrhs.LT.0 ) THEN
262  info = -3
263  ELSE IF( lda.LT.max( 1, n ) ) THEN
264  info = -5
265  ELSE IF( ldaf.LT.max( 1, n ) ) THEN
266  info = -7
267  ELSE IF( ldb.LT.max( 1, n ) ) THEN
268  info = -10
269  ELSE IF( ldx.LT.max( 1, n ) ) THEN
270  info = -12
271  END IF
272  IF( info.NE.0 ) THEN
273  CALL xerbla( 'CSYRFS', -info )
274  RETURN
275  END IF
276 *
277 * Quick return if possible
278 *
279  IF( n.EQ.0 .OR. nrhs.EQ.0 ) THEN
280  DO 10 j = 1, nrhs
281  ferr( j ) = zero
282  berr( j ) = zero
283  10 CONTINUE
284  RETURN
285  END IF
286 *
287 * NZ = maximum number of nonzero elements in each row of A, plus 1
288 *
289  nz = n + 1
290  eps = slamch( 'Epsilon' )
291  safmin = slamch( 'Safe minimum' )
292  safe1 = nz*safmin
293  safe2 = safe1 / eps
294 *
295 * Do for each right hand side
296 *
297  DO 140 j = 1, nrhs
298 *
299  count = 1
300  lstres = three
301  20 CONTINUE
302 *
303 * Loop until stopping criterion is satisfied.
304 *
305 * Compute residual R = B - A * X
306 *
307  CALL ccopy( n, b( 1, j ), 1, work, 1 )
308  CALL csymv( uplo, n, -one, a, lda, x( 1, j ), 1, one, work, 1 )
309 *
310 * Compute componentwise relative backward error from formula
311 *
312 * max(i) ( abs(R(i)) / ( abs(A)*abs(X) + abs(B) )(i) )
313 *
314 * where abs(Z) is the componentwise absolute value of the matrix
315 * or vector Z. If the i-th component of the denominator is less
316 * than SAFE2, then SAFE1 is added to the i-th components of the
317 * numerator and denominator before dividing.
318 *
319  DO 30 i = 1, n
320  rwork( i ) = cabs1( b( i, j ) )
321  30 CONTINUE
322 *
323 * Compute abs(A)*abs(X) + abs(B).
324 *
325  IF( upper ) THEN
326  DO 50 k = 1, n
327  s = zero
328  xk = cabs1( x( k, j ) )
329  DO 40 i = 1, k - 1
330  rwork( i ) = rwork( i ) + cabs1( a( i, k ) )*xk
331  s = s + cabs1( a( i, k ) )*cabs1( x( i, j ) )
332  40 CONTINUE
333  rwork( k ) = rwork( k ) + cabs1( a( k, k ) )*xk + s
334  50 CONTINUE
335  ELSE
336  DO 70 k = 1, n
337  s = zero
338  xk = cabs1( x( k, j ) )
339  rwork( k ) = rwork( k ) + cabs1( a( k, k ) )*xk
340  DO 60 i = k + 1, n
341  rwork( i ) = rwork( i ) + cabs1( a( i, k ) )*xk
342  s = s + cabs1( a( i, k ) )*cabs1( x( i, j ) )
343  60 CONTINUE
344  rwork( k ) = rwork( k ) + s
345  70 CONTINUE
346  END IF
347  s = zero
348  DO 80 i = 1, n
349  IF( rwork( i ).GT.safe2 ) THEN
350  s = max( s, cabs1( work( i ) ) / rwork( i ) )
351  ELSE
352  s = max( s, ( cabs1( work( i ) )+safe1 ) /
353  $ ( rwork( i )+safe1 ) )
354  END IF
355  80 CONTINUE
356  berr( j ) = s
357 *
358 * Test stopping criterion. Continue iterating if
359 * 1) The residual BERR(J) is larger than machine epsilon, and
360 * 2) BERR(J) decreased by at least a factor of 2 during the
361 * last iteration, and
362 * 3) At most ITMAX iterations tried.
363 *
364  IF( berr( j ).GT.eps .AND. two*berr( j ).LE.lstres .AND.
365  $ count.LE.itmax ) THEN
366 *
367 * Update solution and try again.
368 *
369  CALL csytrs( uplo, n, 1, af, ldaf, ipiv, work, n, info )
370  CALL caxpy( n, one, work, 1, x( 1, j ), 1 )
371  lstres = berr( j )
372  count = count + 1
373  go to 20
374  END IF
375 *
376 * Bound error from formula
377 *
378 * norm(X - XTRUE) / norm(X) .le. FERR =
379 * norm( abs(inv(A))*
380 * ( abs(R) + NZ*EPS*( abs(A)*abs(X)+abs(B) ))) / norm(X)
381 *
382 * where
383 * norm(Z) is the magnitude of the largest component of Z
384 * inv(A) is the inverse of A
385 * abs(Z) is the componentwise absolute value of the matrix or
386 * vector Z
387 * NZ is the maximum number of nonzeros in any row of A, plus 1
388 * EPS is machine epsilon
389 *
390 * The i-th component of abs(R)+NZ*EPS*(abs(A)*abs(X)+abs(B))
391 * is incremented by SAFE1 if the i-th component of
392 * abs(A)*abs(X) + abs(B) is less than SAFE2.
393 *
394 * Use CLACN2 to estimate the infinity-norm of the matrix
395 * inv(A) * diag(W),
396 * where W = abs(R) + NZ*EPS*( abs(A)*abs(X)+abs(B) )))
397 *
398  DO 90 i = 1, n
399  IF( rwork( i ).GT.safe2 ) THEN
400  rwork( i ) = cabs1( work( i ) ) + nz*eps*rwork( i )
401  ELSE
402  rwork( i ) = cabs1( work( i ) ) + nz*eps*rwork( i ) +
403  $ safe1
404  END IF
405  90 CONTINUE
406 *
407  kase = 0
408  100 CONTINUE
409  CALL clacn2( n, work( n+1 ), work, ferr( j ), kase, isave )
410  IF( kase.NE.0 ) THEN
411  IF( kase.EQ.1 ) THEN
412 *
413 * Multiply by diag(W)*inv(A**T).
414 *
415  CALL csytrs( uplo, n, 1, af, ldaf, ipiv, work, n, info )
416  DO 110 i = 1, n
417  work( i ) = rwork( i )*work( i )
418  110 CONTINUE
419  ELSE IF( kase.EQ.2 ) THEN
420 *
421 * Multiply by inv(A)*diag(W).
422 *
423  DO 120 i = 1, n
424  work( i ) = rwork( i )*work( i )
425  120 CONTINUE
426  CALL csytrs( uplo, n, 1, af, ldaf, ipiv, work, n, info )
427  END IF
428  go to 100
429  END IF
430 *
431 * Normalize error.
432 *
433  lstres = zero
434  DO 130 i = 1, n
435  lstres = max( lstres, cabs1( x( i, j ) ) )
436  130 CONTINUE
437  IF( lstres.NE.zero )
438  $ ferr( j ) = ferr( j ) / lstres
439 *
440  140 CONTINUE
441 *
442  RETURN
443 *
444 * End of CSYRFS
445 *
446  END