1*> \brief \b SLARRF finds a new relatively robust representation such that at least one of the eigenvalues is relatively isolated.
2*
3*  =========== DOCUMENTATION ===========
4*
5* Online html documentation available at
6*            http://www.netlib.org/lapack/explore-html/
7*
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15*> [TXT]</a>
16*> \endhtmlonly
17*
18*  Definition:
19*  ===========
20*
21*       SUBROUTINE SLARRF( N, D, L, LD, CLSTRT, CLEND,
22*                          W, WGAP, WERR,
23*                          SPDIAM, CLGAPL, CLGAPR, PIVMIN, SIGMA,
24*                          DPLUS, LPLUS, WORK, INFO )
25*
26*       .. Scalar Arguments ..
27*       INTEGER            CLSTRT, CLEND, INFO, N
28*       REAL               CLGAPL, CLGAPR, PIVMIN, SIGMA, SPDIAM
29*       ..
30*       .. Array Arguments ..
31*       REAL               D( * ), DPLUS( * ), L( * ), LD( * ),
32*      $          LPLUS( * ), W( * ), WGAP( * ), WERR( * ), WORK( * )
33*       ..
34*
35*
36*> \par Purpose:
37*  =============
38*>
39*> \verbatim
40*>
41*> Given the initial representation L D L^T and its cluster of close
42*> eigenvalues (in a relative measure), W( CLSTRT ), W( CLSTRT+1 ), ...
43*> W( CLEND ), SLARRF finds a new relatively robust representation
44*> L D L^T - SIGMA I = L(+) D(+) L(+)^T such that at least one of the
45*> eigenvalues of L(+) D(+) L(+)^T is relatively isolated.
46*> \endverbatim
47*
48*  Arguments:
49*  ==========
50*
51*> \param[in] N
52*> \verbatim
53*>          N is INTEGER
54*>          The order of the matrix (subblock, if the matrix splitted).
55*> \endverbatim
56*>
57*> \param[in] D
58*> \verbatim
59*>          D is REAL array, dimension (N)
60*>          The N diagonal elements of the diagonal matrix D.
61*> \endverbatim
62*>
63*> \param[in] L
64*> \verbatim
65*>          L is REAL array, dimension (N-1)
66*>          The (N-1) subdiagonal elements of the unit bidiagonal
67*>          matrix L.
68*> \endverbatim
69*>
70*> \param[in] LD
71*> \verbatim
72*>          LD is REAL array, dimension (N-1)
73*>          The (N-1) elements L(i)*D(i).
74*> \endverbatim
75*>
76*> \param[in] CLSTRT
77*> \verbatim
78*>          CLSTRT is INTEGER
79*>          The index of the first eigenvalue in the cluster.
80*> \endverbatim
81*>
82*> \param[in] CLEND
83*> \verbatim
84*>          CLEND is INTEGER
85*>          The index of the last eigenvalue in the cluster.
86*> \endverbatim
87*>
88*> \param[in] W
89*> \verbatim
90*>          W is REAL array, dimension
91*>          dimension is >=  (CLEND-CLSTRT+1)
92*>          The eigenvalue APPROXIMATIONS of L D L^T in ascending order.
93*>          W( CLSTRT ) through W( CLEND ) form the cluster of relatively
94*>          close eigenalues.
95*> \endverbatim
96*>
97*> \param[in,out] WGAP
98*> \verbatim
99*>          WGAP is REAL array, dimension
100*>          dimension is >=  (CLEND-CLSTRT+1)
101*>          The separation from the right neighbor eigenvalue in W.
102*> \endverbatim
103*>
104*> \param[in] WERR
105*> \verbatim
106*>          WERR is REAL array, dimension
107*>          dimension is >=  (CLEND-CLSTRT+1)
108*>          WERR contain the semiwidth of the uncertainty
109*>          interval of the corresponding eigenvalue APPROXIMATION in W
110*> \endverbatim
111*>
112*> \param[in] SPDIAM
113*> \verbatim
114*>          SPDIAM is REAL
115*>          estimate of the spectral diameter obtained from the
116*>          Gerschgorin intervals
117*> \endverbatim
118*>
119*> \param[in] CLGAPL
120*> \verbatim
121*>          CLGAPL is REAL
122*> \endverbatim
123*>
124*> \param[in] CLGAPR
125*> \verbatim
126*>          CLGAPR is REAL
127*>          absolute gap on each end of the cluster.
128*>          Set by the calling routine to protect against shifts too close
129*>          to eigenvalues outside the cluster.
130*> \endverbatim
131*>
132*> \param[in] PIVMIN
133*> \verbatim
134*>          PIVMIN is REAL
135*>          The minimum pivot allowed in the Sturm sequence.
136*> \endverbatim
137*>
138*> \param[out] SIGMA
139*> \verbatim
140*>          SIGMA is REAL
141*>          The shift used to form L(+) D(+) L(+)^T.
142*> \endverbatim
143*>
144*> \param[out] DPLUS
145*> \verbatim
146*>          DPLUS is REAL array, dimension (N)
147*>          The N diagonal elements of the diagonal matrix D(+).
148*> \endverbatim
149*>
150*> \param[out] LPLUS
151*> \verbatim
152*>          LPLUS is REAL array, dimension (N-1)
153*>          The first (N-1) elements of LPLUS contain the subdiagonal
154*>          elements of the unit bidiagonal matrix L(+).
155*> \endverbatim
156*>
157*> \param[out] WORK
158*> \verbatim
159*>          WORK is REAL array, dimension (2*N)
160*>          Workspace.
161*> \endverbatim
162*>
163*> \param[out] INFO
164*> \verbatim
165*>          INFO is INTEGER
166*>          Signals processing OK (=0) or failure (=1)
167*> \endverbatim
168*
169*  Authors:
170*  ========
171*
172*> \author Univ. of Tennessee
173*> \author Univ. of California Berkeley
174*> \author Univ. of Colorado Denver
175*> \author NAG Ltd.
176*
177*> \date November 2015
178*
179*> \ingroup auxOTHERauxiliary
180*
181*> \par Contributors:
182*  ==================
183*>
184*> Beresford Parlett, University of California, Berkeley, USA \n
185*> Jim Demmel, University of California, Berkeley, USA \n
186*> Inderjit Dhillon, University of Texas, Austin, USA \n
187*> Osni Marques, LBNL/NERSC, USA \n
188*> Christof Voemel, University of California, Berkeley, USA
189*
190*  =====================================================================
191      SUBROUTINE SLARRF( N, D, L, LD, CLSTRT, CLEND,
192     $                   W, WGAP, WERR,
193     $                   SPDIAM, CLGAPL, CLGAPR, PIVMIN, SIGMA,
194     $                   DPLUS, LPLUS, WORK, INFO )
195*
196*  -- LAPACK auxiliary routine (version 3.6.0) --
197*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
198*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
199*     November 2015
200*
201*     .. Scalar Arguments ..
202      INTEGER            CLSTRT, CLEND, INFO, N
203      REAL               CLGAPL, CLGAPR, PIVMIN, SIGMA, SPDIAM
204*     ..
205*     .. Array Arguments ..
206      REAL               D( * ), DPLUS( * ), L( * ), LD( * ),
207     $          LPLUS( * ), W( * ), WGAP( * ), WERR( * ), WORK( * )
208*     ..
209*
210*  =====================================================================
211*
212*     .. Parameters ..
213      REAL               MAXGROWTH1, MAXGROWTH2, ONE, QUART, TWO
214      PARAMETER          ( ONE = 1.0E0, TWO = 2.0E0,
215     $                     QUART = 0.25E0,
216     $                     MAXGROWTH1 = 8.E0,
217     $                     MAXGROWTH2 = 8.E0 )
218*     ..
219*     .. Local Scalars ..
220      LOGICAL   DORRR1, FORCER, NOFAIL, SAWNAN1, SAWNAN2, TRYRRR1
221      INTEGER            I, INDX, KTRY, KTRYMAX, SLEFT, SRIGHT, SHIFT
222      PARAMETER          ( KTRYMAX = 1, SLEFT = 1, SRIGHT = 2 )
223      REAL               AVGAP, BESTSHIFT, CLWDTH, EPS, FACT, FAIL,
224     $                   FAIL2, GROWTHBOUND, LDELTA, LDMAX, LSIGMA,
225     $                   MAX1, MAX2, MINGAP, OLDP, PROD, RDELTA, RDMAX,
226     $                   RRR1, RRR2, RSIGMA, S, SMLGROWTH, TMP, ZNM2
227*     ..
228*     .. External Functions ..
229      LOGICAL SISNAN
230      REAL               SLAMCH
231      EXTERNAL           SISNAN, SLAMCH
232*     ..
233*     .. External Subroutines ..
234      EXTERNAL           SCOPY
235*     ..
236*     .. Intrinsic Functions ..
237      INTRINSIC          ABS
238*     ..
239*     .. Executable Statements ..
240*
241      INFO = 0
242      FACT = REAL(2**KTRYMAX)
243      EPS = SLAMCH( 'Precision' )
244      SHIFT = 0
245      FORCER = .FALSE.
246
247
248*     Note that we cannot guarantee that for any of the shifts tried,
249*     the factorization has a small or even moderate element growth.
250*     There could be Ritz values at both ends of the cluster and despite
251*     backing off, there are examples where all factorizations tried
252*     (in IEEE mode, allowing zero pivots & infinities) have INFINITE
253*     element growth.
254*     For this reason, we should use PIVMIN in this subroutine so that at
255*     least the L D L^T factorization exists. It can be checked afterwards
256*     whether the element growth caused bad residuals/orthogonality.
257
258*     Decide whether the code should accept the best among all
259*     representations despite large element growth or signal INFO=1
260*     Setting NOFAIL to .FALSE. for quick fix for bug 113
261      NOFAIL = .FALSE.
262*
263
264*     Compute the average gap length of the cluster
265      CLWDTH = ABS(W(CLEND)-W(CLSTRT)) + WERR(CLEND) + WERR(CLSTRT)
266      AVGAP = CLWDTH / REAL(CLEND-CLSTRT)
267      MINGAP = MIN(CLGAPL, CLGAPR)
268*     Initial values for shifts to both ends of cluster
269      LSIGMA = MIN(W( CLSTRT ),W( CLEND )) - WERR( CLSTRT )
270      RSIGMA = MAX(W( CLSTRT ),W( CLEND )) + WERR( CLEND )
271
272*     Use a small fudge to make sure that we really shift to the outside
273      LSIGMA = LSIGMA - ABS(LSIGMA)* TWO * EPS
274      RSIGMA = RSIGMA + ABS(RSIGMA)* TWO * EPS
275
276*     Compute upper bounds for how much to back off the initial shifts
277      LDMAX = QUART * MINGAP + TWO * PIVMIN
278      RDMAX = QUART * MINGAP + TWO * PIVMIN
279
280      LDELTA = MAX(AVGAP,WGAP( CLSTRT ))/FACT
281      RDELTA = MAX(AVGAP,WGAP( CLEND-1 ))/FACT
282*
283*     Initialize the record of the best representation found
284*
285      S = SLAMCH( 'S' )
286      SMLGROWTH = ONE / S
287      FAIL = REAL(N-1)*MINGAP/(SPDIAM*EPS)
288      FAIL2 = REAL(N-1)*MINGAP/(SPDIAM*SQRT(EPS))
289      BESTSHIFT = LSIGMA
290*
291*     while (KTRY <= KTRYMAX)
292      KTRY = 0
293      GROWTHBOUND = MAXGROWTH1*SPDIAM
294
295 5    CONTINUE
296      SAWNAN1 = .FALSE.
297      SAWNAN2 = .FALSE.
298*     Ensure that we do not back off too much of the initial shifts
299      LDELTA = MIN(LDMAX,LDELTA)
300      RDELTA = MIN(RDMAX,RDELTA)
301
302*     Compute the element growth when shifting to both ends of the cluster
303*     accept the shift if there is no element growth at one of the two ends
304
305*     Left end
306      S = -LSIGMA
307      DPLUS( 1 ) = D( 1 ) + S
308      IF(ABS(DPLUS(1)).LT.PIVMIN) THEN
309         DPLUS(1) = -PIVMIN
310*        Need to set SAWNAN1 because refined RRR test should not be used
311*        in this case
312         SAWNAN1 = .TRUE.
313      ENDIF
314      MAX1 = ABS( DPLUS( 1 ) )
315      DO 6 I = 1, N - 1
316         LPLUS( I ) = LD( I ) / DPLUS( I )
317         S = S*LPLUS( I )*L( I ) - LSIGMA
318         DPLUS( I+1 ) = D( I+1 ) + S
319         IF(ABS(DPLUS(I+1)).LT.PIVMIN) THEN
320            DPLUS(I+1) = -PIVMIN
321*           Need to set SAWNAN1 because refined RRR test should not be used
322*           in this case
323            SAWNAN1 = .TRUE.
324         ENDIF
325         MAX1 = MAX( MAX1,ABS(DPLUS(I+1)) )
326 6    CONTINUE
327      SAWNAN1 = SAWNAN1 .OR.  SISNAN( MAX1 )
328
329      IF( FORCER .OR.
330     $   (MAX1.LE.GROWTHBOUND .AND. .NOT.SAWNAN1 ) ) THEN
331         SIGMA = LSIGMA
332         SHIFT = SLEFT
333         GOTO 100
334      ENDIF
335
336*     Right end
337      S = -RSIGMA
338      WORK( 1 ) = D( 1 ) + S
339      IF(ABS(WORK(1)).LT.PIVMIN) THEN
340         WORK(1) = -PIVMIN
341*        Need to set SAWNAN2 because refined RRR test should not be used
342*        in this case
343         SAWNAN2 = .TRUE.
344      ENDIF
345      MAX2 = ABS( WORK( 1 ) )
346      DO 7 I = 1, N - 1
347         WORK( N+I ) = LD( I ) / WORK( I )
348         S = S*WORK( N+I )*L( I ) - RSIGMA
349         WORK( I+1 ) = D( I+1 ) + S
350         IF(ABS(WORK(I+1)).LT.PIVMIN) THEN
351            WORK(I+1) = -PIVMIN
352*           Need to set SAWNAN2 because refined RRR test should not be used
353*           in this case
354            SAWNAN2 = .TRUE.
355         ENDIF
356         MAX2 = MAX( MAX2,ABS(WORK(I+1)) )
357 7    CONTINUE
358      SAWNAN2 = SAWNAN2 .OR.  SISNAN( MAX2 )
359
360      IF( FORCER .OR.
361     $   (MAX2.LE.GROWTHBOUND .AND. .NOT.SAWNAN2 ) ) THEN
362         SIGMA = RSIGMA
363         SHIFT = SRIGHT
364         GOTO 100
365      ENDIF
366*     If we are at this point, both shifts led to too much element growth
367
368*     Record the better of the two shifts (provided it didn't lead to NaN)
369      IF(SAWNAN1.AND.SAWNAN2) THEN
370*        both MAX1 and MAX2 are NaN
371         GOTO 50
372      ELSE
373         IF( .NOT.SAWNAN1 ) THEN
374            INDX = 1
375            IF(MAX1.LE.SMLGROWTH) THEN
376               SMLGROWTH = MAX1
377               BESTSHIFT = LSIGMA
378            ENDIF
379         ENDIF
380         IF( .NOT.SAWNAN2 ) THEN
381            IF(SAWNAN1 .OR. MAX2.LE.MAX1) INDX = 2
382            IF(MAX2.LE.SMLGROWTH) THEN
383               SMLGROWTH = MAX2
384               BESTSHIFT = RSIGMA
385            ENDIF
386         ENDIF
387      ENDIF
388
389*     If we are here, both the left and the right shift led to
390*     element growth. If the element growth is moderate, then
391*     we may still accept the representation, if it passes a
392*     refined test for RRR. This test supposes that no NaN occurred.
393*     Moreover, we use the refined RRR test only for isolated clusters.
394      IF((CLWDTH.LT.MINGAP/REAL(128)) .AND.
395     $   (MIN(MAX1,MAX2).LT.FAIL2)
396     $  .AND.(.NOT.SAWNAN1).AND.(.NOT.SAWNAN2)) THEN
397         DORRR1 = .TRUE.
398      ELSE
399         DORRR1 = .FALSE.
400      ENDIF
401      TRYRRR1 = .TRUE.
402      IF( TRYRRR1 .AND. DORRR1 ) THEN
403      IF(INDX.EQ.1) THEN
404         TMP = ABS( DPLUS( N ) )
405         ZNM2 = ONE
406         PROD = ONE
407         OLDP = ONE
408         DO 15 I = N-1, 1, -1
409            IF( PROD .LE. EPS ) THEN
410               PROD =
411     $         ((DPLUS(I+1)*WORK(N+I+1))/(DPLUS(I)*WORK(N+I)))*OLDP
412            ELSE
413               PROD = PROD*ABS(WORK(N+I))
414            END IF
415            OLDP = PROD
416            ZNM2 = ZNM2 + PROD**2
417            TMP = MAX( TMP, ABS( DPLUS( I ) * PROD ))
418 15      CONTINUE
419         RRR1 = TMP/( SPDIAM * SQRT( ZNM2 ) )
420         IF (RRR1.LE.MAXGROWTH2) THEN
421            SIGMA = LSIGMA
422            SHIFT = SLEFT
423            GOTO 100
424         ENDIF
425      ELSE IF(INDX.EQ.2) THEN
426         TMP = ABS( WORK( N ) )
427         ZNM2 = ONE
428         PROD = ONE
429         OLDP = ONE
430         DO 16 I = N-1, 1, -1
431            IF( PROD .LE. EPS ) THEN
432               PROD = ((WORK(I+1)*LPLUS(I+1))/(WORK(I)*LPLUS(I)))*OLDP
433            ELSE
434               PROD = PROD*ABS(LPLUS(I))
435            END IF
436            OLDP = PROD
437            ZNM2 = ZNM2 + PROD**2
438            TMP = MAX( TMP, ABS( WORK( I ) * PROD ))
439 16      CONTINUE
440         RRR2 = TMP/( SPDIAM * SQRT( ZNM2 ) )
441         IF (RRR2.LE.MAXGROWTH2) THEN
442            SIGMA = RSIGMA
443            SHIFT = SRIGHT
444            GOTO 100
445         ENDIF
446      END IF
447      ENDIF
448
449 50   CONTINUE
450
451      IF (KTRY.LT.KTRYMAX) THEN
452*        If we are here, both shifts failed also the RRR test.
453*        Back off to the outside
454         LSIGMA = MAX( LSIGMA - LDELTA,
455     $     LSIGMA - LDMAX)
456         RSIGMA = MIN( RSIGMA + RDELTA,
457     $     RSIGMA + RDMAX )
458         LDELTA = TWO * LDELTA
459         RDELTA = TWO * RDELTA
460         KTRY = KTRY + 1
461         GOTO 5
462      ELSE
463*        None of the representations investigated satisfied our
464*        criteria. Take the best one we found.
465         IF((SMLGROWTH.LT.FAIL).OR.NOFAIL) THEN
466            LSIGMA = BESTSHIFT
467            RSIGMA = BESTSHIFT
468            FORCER = .TRUE.
469            GOTO 5
470         ELSE
471            INFO = 1
472            RETURN
473         ENDIF
474      END IF
475
476 100  CONTINUE
477      IF (SHIFT.EQ.SLEFT) THEN
478      ELSEIF (SHIFT.EQ.SRIGHT) THEN
479*        store new L and D back into DPLUS, LPLUS
480         CALL SCOPY( N, WORK, 1, DPLUS, 1 )
481         CALL SCOPY( N-1, WORK(N+1), 1, LPLUS, 1 )
482      ENDIF
483
484      RETURN
485*
486*     End of SLARRF
487*
488      END
489