 LAPACK  3.10.1 LAPACK: Linear Algebra PACKage

## ◆ spbrfs()

 subroutine spbrfs ( character UPLO, integer N, integer KD, integer NRHS, real, dimension( ldab, * ) AB, integer LDAB, real, dimension( ldafb, * ) AFB, integer LDAFB, real, dimension( ldb, * ) B, integer LDB, real, dimension( ldx, * ) X, integer LDX, real, dimension( * ) FERR, real, dimension( * ) BERR, real, dimension( * ) WORK, integer, dimension( * ) IWORK, integer INFO )

SPBRFS

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Purpose:
``` SPBRFS improves the computed solution to a system of linear
equations when the coefficient matrix is symmetric positive definite
and banded, 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] KD ``` KD is INTEGER The number of superdiagonals of the matrix A if UPLO = 'U', or the number of subdiagonals if UPLO = 'L'. KD >= 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] AB ``` AB is REAL array, dimension (LDAB,N) The upper or lower triangle of the symmetric band matrix A, stored in the first KD+1 rows of the array. The j-th column of A is stored in the j-th column of the array AB as follows: if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j; if UPLO = 'L', AB(1+i-j,j) = A(i,j) for j<=i<=min(n,j+kd).``` [in] LDAB ``` LDAB is INTEGER The leading dimension of the array AB. LDAB >= KD+1.``` [in] AFB ``` AFB is REAL array, dimension (LDAFB,N) The triangular factor U or L from the Cholesky factorization A = U**T*U or A = L*L**T of the band matrix A as computed by SPBTRF, in the same storage format as A (see AB).``` [in] LDAFB ``` LDAFB is INTEGER The leading dimension of the array AFB. LDAFB >= KD+1.``` [in] B ``` B is REAL 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 REAL array, dimension (LDX,NRHS) On entry, the solution matrix X, as computed by SPBTRS. 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 REAL 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 187 of file spbrfs.f.

189 *
190 * -- LAPACK computational routine --
191 * -- LAPACK is a software package provided by Univ. of Tennessee, --
192 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
193 *
194 * .. Scalar Arguments ..
195  CHARACTER UPLO
196  INTEGER INFO, KD, LDAB, LDAFB, LDB, LDX, N, NRHS
197 * ..
198 * .. Array Arguments ..
199  INTEGER IWORK( * )
200  REAL AB( LDAB, * ), AFB( LDAFB, * ), B( LDB, * ),
201  \$ BERR( * ), FERR( * ), WORK( * ), X( LDX, * )
202 * ..
203 *
204 * =====================================================================
205 *
206 * .. Parameters ..
207  INTEGER ITMAX
208  parameter( itmax = 5 )
209  REAL ZERO
210  parameter( zero = 0.0e+0 )
211  REAL ONE
212  parameter( one = 1.0e+0 )
213  REAL TWO
214  parameter( two = 2.0e+0 )
215  REAL THREE
216  parameter( three = 3.0e+0 )
217 * ..
218 * .. Local Scalars ..
219  LOGICAL UPPER
220  INTEGER COUNT, I, J, K, KASE, L, NZ
221  REAL EPS, LSTRES, S, SAFE1, SAFE2, SAFMIN, XK
222 * ..
223 * .. Local Arrays ..
224  INTEGER ISAVE( 3 )
225 * ..
226 * .. External Subroutines ..
227  EXTERNAL saxpy, scopy, slacn2, spbtrs, ssbmv, xerbla
228 * ..
229 * .. Intrinsic Functions ..
230  INTRINSIC abs, max, min
231 * ..
232 * .. External Functions ..
233  LOGICAL LSAME
234  REAL SLAMCH
235  EXTERNAL lsame, slamch
236 * ..
237 * .. Executable Statements ..
238 *
239 * Test the input parameters.
240 *
241  info = 0
242  upper = lsame( uplo, 'U' )
243  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
244  info = -1
245  ELSE IF( n.LT.0 ) THEN
246  info = -2
247  ELSE IF( kd.LT.0 ) THEN
248  info = -3
249  ELSE IF( nrhs.LT.0 ) THEN
250  info = -4
251  ELSE IF( ldab.LT.kd+1 ) THEN
252  info = -6
253  ELSE IF( ldafb.LT.kd+1 ) THEN
254  info = -8
255  ELSE IF( ldb.LT.max( 1, n ) ) THEN
256  info = -10
257  ELSE IF( ldx.LT.max( 1, n ) ) THEN
258  info = -12
259  END IF
260  IF( info.NE.0 ) THEN
261  CALL xerbla( 'SPBRFS', -info )
262  RETURN
263  END IF
264 *
265 * Quick return if possible
266 *
267  IF( n.EQ.0 .OR. nrhs.EQ.0 ) THEN
268  DO 10 j = 1, nrhs
269  ferr( j ) = zero
270  berr( j ) = zero
271  10 CONTINUE
272  RETURN
273  END IF
274 *
275 * NZ = maximum number of nonzero elements in each row of A, plus 1
276 *
277  nz = min( n+1, 2*kd+2 )
278  eps = slamch( 'Epsilon' )
279  safmin = slamch( 'Safe minimum' )
280  safe1 = nz*safmin
281  safe2 = safe1 / eps
282 *
283 * Do for each right hand side
284 *
285  DO 140 j = 1, nrhs
286 *
287  count = 1
288  lstres = three
289  20 CONTINUE
290 *
291 * Loop until stopping criterion is satisfied.
292 *
293 * Compute residual R = B - A * X
294 *
295  CALL scopy( n, b( 1, j ), 1, work( n+1 ), 1 )
296  CALL ssbmv( uplo, n, kd, -one, ab, ldab, x( 1, j ), 1, one,
297  \$ work( n+1 ), 1 )
298 *
299 * Compute componentwise relative backward error from formula
300 *
301 * max(i) ( abs(R(i)) / ( abs(A)*abs(X) + abs(B) )(i) )
302 *
303 * where abs(Z) is the componentwise absolute value of the matrix
304 * or vector Z. If the i-th component of the denominator is less
305 * than SAFE2, then SAFE1 is added to the i-th components of the
306 * numerator and denominator before dividing.
307 *
308  DO 30 i = 1, n
309  work( i ) = abs( b( i, j ) )
310  30 CONTINUE
311 *
312 * Compute abs(A)*abs(X) + abs(B).
313 *
314  IF( upper ) THEN
315  DO 50 k = 1, n
316  s = zero
317  xk = abs( x( k, j ) )
318  l = kd + 1 - k
319  DO 40 i = max( 1, k-kd ), k - 1
320  work( i ) = work( i ) + abs( ab( l+i, k ) )*xk
321  s = s + abs( ab( l+i, k ) )*abs( x( i, j ) )
322  40 CONTINUE
323  work( k ) = work( k ) + abs( ab( kd+1, k ) )*xk + s
324  50 CONTINUE
325  ELSE
326  DO 70 k = 1, n
327  s = zero
328  xk = abs( x( k, j ) )
329  work( k ) = work( k ) + abs( ab( 1, k ) )*xk
330  l = 1 - k
331  DO 60 i = k + 1, min( n, k+kd )
332  work( i ) = work( i ) + abs( ab( l+i, k ) )*xk
333  s = s + abs( ab( l+i, k ) )*abs( x( i, j ) )
334  60 CONTINUE
335  work( k ) = work( k ) + s
336  70 CONTINUE
337  END IF
338  s = zero
339  DO 80 i = 1, n
340  IF( work( i ).GT.safe2 ) THEN
341  s = max( s, abs( work( n+i ) ) / work( i ) )
342  ELSE
343  s = max( s, ( abs( work( n+i ) )+safe1 ) /
344  \$ ( work( i )+safe1 ) )
345  END IF
346  80 CONTINUE
347  berr( j ) = s
348 *
349 * Test stopping criterion. Continue iterating if
350 * 1) The residual BERR(J) is larger than machine epsilon, and
351 * 2) BERR(J) decreased by at least a factor of 2 during the
352 * last iteration, and
353 * 3) At most ITMAX iterations tried.
354 *
355  IF( berr( j ).GT.eps .AND. two*berr( j ).LE.lstres .AND.
356  \$ count.LE.itmax ) THEN
357 *
358 * Update solution and try again.
359 *
360  CALL spbtrs( uplo, n, kd, 1, afb, ldafb, work( n+1 ), n,
361  \$ info )
362  CALL saxpy( n, one, work( n+1 ), 1, x( 1, j ), 1 )
363  lstres = berr( j )
364  count = count + 1
365  GO TO 20
366  END IF
367 *
368 * Bound error from formula
369 *
370 * norm(X - XTRUE) / norm(X) .le. FERR =
371 * norm( abs(inv(A))*
372 * ( abs(R) + NZ*EPS*( abs(A)*abs(X)+abs(B) ))) / norm(X)
373 *
374 * where
375 * norm(Z) is the magnitude of the largest component of Z
376 * inv(A) is the inverse of A
377 * abs(Z) is the componentwise absolute value of the matrix or
378 * vector Z
379 * NZ is the maximum number of nonzeros in any row of A, plus 1
380 * EPS is machine epsilon
381 *
382 * The i-th component of abs(R)+NZ*EPS*(abs(A)*abs(X)+abs(B))
383 * is incremented by SAFE1 if the i-th component of
384 * abs(A)*abs(X) + abs(B) is less than SAFE2.
385 *
386 * Use SLACN2 to estimate the infinity-norm of the matrix
387 * inv(A) * diag(W),
388 * where W = abs(R) + NZ*EPS*( abs(A)*abs(X)+abs(B) )))
389 *
390  DO 90 i = 1, n
391  IF( work( i ).GT.safe2 ) THEN
392  work( i ) = abs( work( n+i ) ) + nz*eps*work( i )
393  ELSE
394  work( i ) = abs( work( n+i ) ) + nz*eps*work( i ) + safe1
395  END IF
396  90 CONTINUE
397 *
398  kase = 0
399  100 CONTINUE
400  CALL slacn2( n, work( 2*n+1 ), work( n+1 ), iwork, ferr( j ),
401  \$ kase, isave )
402  IF( kase.NE.0 ) THEN
403  IF( kase.EQ.1 ) THEN
404 *
405 * Multiply by diag(W)*inv(A**T).
406 *
407  CALL spbtrs( uplo, n, kd, 1, afb, ldafb, work( n+1 ), n,
408  \$ info )
409  DO 110 i = 1, n
410  work( n+i ) = work( n+i )*work( i )
411  110 CONTINUE
412  ELSE IF( kase.EQ.2 ) THEN
413 *
414 * Multiply by inv(A)*diag(W).
415 *
416  DO 120 i = 1, n
417  work( n+i ) = work( n+i )*work( i )
418  120 CONTINUE
419  CALL spbtrs( uplo, n, kd, 1, afb, ldafb, work( n+1 ), n,
420  \$ info )
421  END IF
422  GO TO 100
423  END IF
424 *
425 * Normalize error.
426 *
427  lstres = zero
428  DO 130 i = 1, n
429  lstres = max( lstres, abs( x( i, j ) ) )
430  130 CONTINUE
431  IF( lstres.NE.zero )
432  \$ ferr( j ) = ferr( j ) / lstres
433 *
434  140 CONTINUE
435 *
436  RETURN
437 *
438 * End of SPBRFS
439 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine slacn2(N, V, X, ISGN, EST, KASE, ISAVE)
SLACN2 estimates the 1-norm of a square matrix, using reverse communication for evaluating matrix-vec...
Definition: slacn2.f:136
subroutine spbtrs(UPLO, N, KD, NRHS, AB, LDAB, B, LDB, INFO)
SPBTRS
Definition: spbtrs.f:121
subroutine scopy(N, SX, INCX, SY, INCY)
SCOPY
Definition: scopy.f:82
subroutine saxpy(N, SA, SX, INCX, SY, INCY)
SAXPY
Definition: saxpy.f:89
subroutine ssbmv(UPLO, N, K, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
SSBMV
Definition: ssbmv.f:184
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
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