LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ claic1()

subroutine claic1 ( integer  JOB,
integer  J,
complex, dimension( j )  X,
real  SEST,
complex, dimension( j )  W,
complex  GAMMA,
real  SESTPR,
complex  S,
complex  C 
)

CLAIC1 applies one step of incremental condition estimation.

Download CLAIC1 + dependencies [TGZ] [ZIP] [TXT]

Purpose:
 CLAIC1 applies one step of incremental condition estimation in
 its simplest version:

 Let x, twonorm(x) = 1, be an approximate singular vector of an j-by-j
 lower triangular matrix L, such that
          twonorm(L*x) = sest
 Then CLAIC1 computes sestpr, s, c such that
 the vector
                 [ s*x ]
          xhat = [  c  ]
 is an approximate singular vector of
                 [ L      0  ]
          Lhat = [ w**H gamma ]
 in the sense that
          twonorm(Lhat*xhat) = sestpr.

 Depending on JOB, an estimate for the largest or smallest singular
 value is computed.

 Note that [s c]**H and sestpr**2 is an eigenpair of the system

     diag(sest*sest, 0) + [alpha  gamma] * [ conjg(alpha) ]
                                           [ conjg(gamma) ]

 where  alpha =  x**H*w.
Parameters
[in]JOB
          JOB is INTEGER
          = 1: an estimate for the largest singular value is computed.
          = 2: an estimate for the smallest singular value is computed.
[in]J
          J is INTEGER
          Length of X and W
[in]X
          X is COMPLEX array, dimension (J)
          The j-vector x.
[in]SEST
          SEST is REAL
          Estimated singular value of j by j matrix L
[in]W
          W is COMPLEX array, dimension (J)
          The j-vector w.
[in]GAMMA
          GAMMA is COMPLEX
          The diagonal element gamma.
[out]SESTPR
          SESTPR is REAL
          Estimated singular value of (j+1) by (j+1) matrix Lhat.
[out]S
          S is COMPLEX
          Sine needed in forming xhat.
[out]C
          C is COMPLEX
          Cosine needed in forming xhat.
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.

Definition at line 134 of file claic1.f.

135 *
136 * -- LAPACK auxiliary routine --
137 * -- LAPACK is a software package provided by Univ. of Tennessee, --
138 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
139 *
140 * .. Scalar Arguments ..
141  INTEGER J, JOB
142  REAL SEST, SESTPR
143  COMPLEX C, GAMMA, S
144 * ..
145 * .. Array Arguments ..
146  COMPLEX W( J ), X( J )
147 * ..
148 *
149 * =====================================================================
150 *
151 * .. Parameters ..
152  REAL ZERO, ONE, TWO
153  parameter( zero = 0.0e0, one = 1.0e0, two = 2.0e0 )
154  REAL HALF, FOUR
155  parameter( half = 0.5e0, four = 4.0e0 )
156 * ..
157 * .. Local Scalars ..
158  REAL ABSALP, ABSEST, ABSGAM, B, EPS, NORMA, S1, S2,
159  $ SCL, T, TEST, TMP, ZETA1, ZETA2
160  COMPLEX ALPHA, COSINE, SINE
161 * ..
162 * .. Intrinsic Functions ..
163  INTRINSIC abs, conjg, max, sqrt
164 * ..
165 * .. External Functions ..
166  REAL SLAMCH
167  COMPLEX CDOTC
168  EXTERNAL slamch, cdotc
169 * ..
170 * .. Executable Statements ..
171 *
172  eps = slamch( 'Epsilon' )
173  alpha = cdotc( j, x, 1, w, 1 )
174 *
175  absalp = abs( alpha )
176  absgam = abs( gamma )
177  absest = abs( sest )
178 *
179  IF( job.EQ.1 ) THEN
180 *
181 * Estimating largest singular value
182 *
183 * special cases
184 *
185  IF( sest.EQ.zero ) THEN
186  s1 = max( absgam, absalp )
187  IF( s1.EQ.zero ) THEN
188  s = zero
189  c = one
190  sestpr = zero
191  ELSE
192  s = alpha / s1
193  c = gamma / s1
194  tmp = real( sqrt( s*conjg( s )+c*conjg( c ) ) )
195  s = s / tmp
196  c = c / tmp
197  sestpr = s1*tmp
198  END IF
199  RETURN
200  ELSE IF( absgam.LE.eps*absest ) THEN
201  s = one
202  c = zero
203  tmp = max( absest, absalp )
204  s1 = absest / tmp
205  s2 = absalp / tmp
206  sestpr = tmp*sqrt( s1*s1+s2*s2 )
207  RETURN
208  ELSE IF( absalp.LE.eps*absest ) THEN
209  s1 = absgam
210  s2 = absest
211  IF( s1.LE.s2 ) THEN
212  s = one
213  c = zero
214  sestpr = s2
215  ELSE
216  s = zero
217  c = one
218  sestpr = s1
219  END IF
220  RETURN
221  ELSE IF( absest.LE.eps*absalp .OR. absest.LE.eps*absgam ) THEN
222  s1 = absgam
223  s2 = absalp
224  IF( s1.LE.s2 ) THEN
225  tmp = s1 / s2
226  scl = sqrt( one+tmp*tmp )
227  sestpr = s2*scl
228  s = ( alpha / s2 ) / scl
229  c = ( gamma / s2 ) / scl
230  ELSE
231  tmp = s2 / s1
232  scl = sqrt( one+tmp*tmp )
233  sestpr = s1*scl
234  s = ( alpha / s1 ) / scl
235  c = ( gamma / s1 ) / scl
236  END IF
237  RETURN
238  ELSE
239 *
240 * normal case
241 *
242  zeta1 = absalp / absest
243  zeta2 = absgam / absest
244 *
245  b = ( one-zeta1*zeta1-zeta2*zeta2 )*half
246  c = zeta1*zeta1
247  IF( b.GT.zero ) THEN
248  t = real( c / ( b+sqrt( b*b+c ) ) )
249  ELSE
250  t = real( sqrt( b*b+c ) - b )
251  END IF
252 *
253  sine = -( alpha / absest ) / t
254  cosine = -( gamma / absest ) / ( one+t )
255  tmp = real( sqrt( sine * conjg( sine )
256  $ + cosine * conjg( cosine ) ) )
257  s = sine / tmp
258  c = cosine / tmp
259  sestpr = sqrt( t+one )*absest
260  RETURN
261  END IF
262 *
263  ELSE IF( job.EQ.2 ) THEN
264 *
265 * Estimating smallest singular value
266 *
267 * special cases
268 *
269  IF( sest.EQ.zero ) THEN
270  sestpr = zero
271  IF( max( absgam, absalp ).EQ.zero ) THEN
272  sine = one
273  cosine = zero
274  ELSE
275  sine = -conjg( gamma )
276  cosine = conjg( alpha )
277  END IF
278  s1 = max( abs( sine ), abs( cosine ) )
279  s = sine / s1
280  c = cosine / s1
281  tmp = real( sqrt( s*conjg( s )+c*conjg( c ) ) )
282  s = s / tmp
283  c = c / tmp
284  RETURN
285  ELSE IF( absgam.LE.eps*absest ) THEN
286  s = zero
287  c = one
288  sestpr = absgam
289  RETURN
290  ELSE IF( absalp.LE.eps*absest ) THEN
291  s1 = absgam
292  s2 = absest
293  IF( s1.LE.s2 ) THEN
294  s = zero
295  c = one
296  sestpr = s1
297  ELSE
298  s = one
299  c = zero
300  sestpr = s2
301  END IF
302  RETURN
303  ELSE IF( absest.LE.eps*absalp .OR. absest.LE.eps*absgam ) THEN
304  s1 = absgam
305  s2 = absalp
306  IF( s1.LE.s2 ) THEN
307  tmp = s1 / s2
308  scl = sqrt( one+tmp*tmp )
309  sestpr = absest*( tmp / scl )
310  s = -( conjg( gamma ) / s2 ) / scl
311  c = ( conjg( alpha ) / s2 ) / scl
312  ELSE
313  tmp = s2 / s1
314  scl = sqrt( one+tmp*tmp )
315  sestpr = absest / scl
316  s = -( conjg( gamma ) / s1 ) / scl
317  c = ( conjg( alpha ) / s1 ) / scl
318  END IF
319  RETURN
320  ELSE
321 *
322 * normal case
323 *
324  zeta1 = absalp / absest
325  zeta2 = absgam / absest
326 *
327  norma = max( one+zeta1*zeta1+zeta1*zeta2,
328  $ zeta1*zeta2+zeta2*zeta2 )
329 *
330 * See if root is closer to zero or to ONE
331 *
332  test = one + two*( zeta1-zeta2 )*( zeta1+zeta2 )
333  IF( test.GE.zero ) THEN
334 *
335 * root is close to zero, compute directly
336 *
337  b = ( zeta1*zeta1+zeta2*zeta2+one )*half
338  c = zeta2*zeta2
339  t = real( c / ( b+sqrt( abs( b*b-c ) ) ) )
340  sine = ( alpha / absest ) / ( one-t )
341  cosine = -( gamma / absest ) / t
342  sestpr = sqrt( t+four*eps*eps*norma )*absest
343  ELSE
344 *
345 * root is closer to ONE, shift by that amount
346 *
347  b = ( zeta2*zeta2+zeta1*zeta1-one )*half
348  c = zeta1*zeta1
349  IF( b.GE.zero ) THEN
350  t = real( -c / ( b+sqrt( b*b+c ) ) )
351  ELSE
352  t = real( b - sqrt( b*b+c ) )
353  END IF
354  sine = -( alpha / absest ) / t
355  cosine = -( gamma / absest ) / ( one+t )
356  sestpr = sqrt( one+t+four*eps*eps*norma )*absest
357  END IF
358  tmp = real( sqrt( sine * conjg( sine )
359  $ + cosine * conjg( cosine ) ) )
360  s = sine / tmp
361  c = cosine / tmp
362  RETURN
363 *
364  END IF
365  END IF
366  RETURN
367 *
368 * End of CLAIC1
369 *
complex function cdotc(N, CX, INCX, CY, INCY)
CDOTC
Definition: cdotc.f:83
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
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