LAPACK  3.5.0
LAPACK: Linear Algebra PACKage
 All Classes Files Functions Variables Typedefs Macros
ssytrs.f
Go to the documentation of this file.
1 *> \brief \b SSYTRS
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
8 *> \htmlonly
9 *> Download SSYTRS + dependencies
10 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.tgz?format=tgz&filename=/lapack/lapack_routine/ssytrs.f">
11 *> [TGZ]</a>
12 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.zip?format=zip&filename=/lapack/lapack_routine/ssytrs.f">
13 *> [ZIP]</a>
14 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/ssytrs.f">
15 *> [TXT]</a>
16 *> \endhtmlonly
17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE SSYTRS( UPLO, N, NRHS, A, LDA, IPIV, B, LDB, INFO )
22 *
23 * .. Scalar Arguments ..
24 * CHARACTER UPLO
25 * INTEGER INFO, LDA, LDB, N, NRHS
26 * ..
27 * .. Array Arguments ..
28 * INTEGER IPIV( * )
29 * REAL A( LDA, * ), B( LDB, * )
30 * ..
31 *
32 *
33 *> \par Purpose:
34 * =============
35 *>
36 *> \verbatim
37 *>
38 *> SSYTRS solves a system of linear equations A*X = B with a real
39 *> symmetric matrix A using the factorization A = U*D*U**T or
40 *> A = L*D*L**T computed by SSYTRF.
41 *> \endverbatim
42 *
43 * Arguments:
44 * ==========
45 *
46 *> \param[in] UPLO
47 *> \verbatim
48 *> UPLO is CHARACTER*1
49 *> Specifies whether the details of the factorization are stored
50 *> as an upper or lower triangular matrix.
51 *> = 'U': Upper triangular, form is A = U*D*U**T;
52 *> = 'L': Lower triangular, form is A = L*D*L**T.
53 *> \endverbatim
54 *>
55 *> \param[in] N
56 *> \verbatim
57 *> N is INTEGER
58 *> The order of the matrix A. N >= 0.
59 *> \endverbatim
60 *>
61 *> \param[in] NRHS
62 *> \verbatim
63 *> NRHS is INTEGER
64 *> The number of right hand sides, i.e., the number of columns
65 *> of the matrix B. NRHS >= 0.
66 *> \endverbatim
67 *>
68 *> \param[in] A
69 *> \verbatim
70 *> A is REAL array, dimension (LDA,N)
71 *> The block diagonal matrix D and the multipliers used to
72 *> obtain the factor U or L as computed by SSYTRF.
73 *> \endverbatim
74 *>
75 *> \param[in] LDA
76 *> \verbatim
77 *> LDA is INTEGER
78 *> The leading dimension of the array A. LDA >= max(1,N).
79 *> \endverbatim
80 *>
81 *> \param[in] IPIV
82 *> \verbatim
83 *> IPIV is INTEGER array, dimension (N)
84 *> Details of the interchanges and the block structure of D
85 *> as determined by SSYTRF.
86 *> \endverbatim
87 *>
88 *> \param[in,out] B
89 *> \verbatim
90 *> B is REAL array, dimension (LDB,NRHS)
91 *> On entry, the right hand side matrix B.
92 *> On exit, the solution matrix X.
93 *> \endverbatim
94 *>
95 *> \param[in] LDB
96 *> \verbatim
97 *> LDB is INTEGER
98 *> The leading dimension of the array B. LDB >= max(1,N).
99 *> \endverbatim
100 *>
101 *> \param[out] INFO
102 *> \verbatim
103 *> INFO is INTEGER
104 *> = 0: successful exit
105 *> < 0: if INFO = -i, the i-th argument had an illegal value
106 *> \endverbatim
107 *
108 * Authors:
109 * ========
110 *
111 *> \author Univ. of Tennessee
112 *> \author Univ. of California Berkeley
113 *> \author Univ. of Colorado Denver
114 *> \author NAG Ltd.
115 *
116 *> \date November 2011
117 *
118 *> \ingroup realSYcomputational
119 *
120 * =====================================================================
121  SUBROUTINE ssytrs( UPLO, N, NRHS, A, LDA, IPIV, B, LDB, INFO )
122 *
123 * -- LAPACK computational routine (version 3.4.0) --
124 * -- LAPACK is a software package provided by Univ. of Tennessee, --
125 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
126 * November 2011
127 *
128 * .. Scalar Arguments ..
129  CHARACTER uplo
130  INTEGER info, lda, ldb, n, nrhs
131 * ..
132 * .. Array Arguments ..
133  INTEGER ipiv( * )
134  REAL a( lda, * ), b( ldb, * )
135 * ..
136 *
137 * =====================================================================
138 *
139 * .. Parameters ..
140  REAL one
141  parameter( one = 1.0e+0 )
142 * ..
143 * .. Local Scalars ..
144  LOGICAL upper
145  INTEGER j, k, kp
146  REAL ak, akm1, akm1k, bk, bkm1, denom
147 * ..
148 * .. External Functions ..
149  LOGICAL lsame
150  EXTERNAL lsame
151 * ..
152 * .. External Subroutines ..
153  EXTERNAL sgemv, sger, sscal, sswap, xerbla
154 * ..
155 * .. Intrinsic Functions ..
156  INTRINSIC max
157 * ..
158 * .. Executable Statements ..
159 *
160  info = 0
161  upper = lsame( uplo, 'U' )
162  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
163  info = -1
164  ELSE IF( n.LT.0 ) THEN
165  info = -2
166  ELSE IF( nrhs.LT.0 ) THEN
167  info = -3
168  ELSE IF( lda.LT.max( 1, n ) ) THEN
169  info = -5
170  ELSE IF( ldb.LT.max( 1, n ) ) THEN
171  info = -8
172  END IF
173  IF( info.NE.0 ) THEN
174  CALL xerbla( 'SSYTRS', -info )
175  RETURN
176  END IF
177 *
178 * Quick return if possible
179 *
180  IF( n.EQ.0 .OR. nrhs.EQ.0 )
181  $ RETURN
182 *
183  IF( upper ) THEN
184 *
185 * Solve A*X = B, where A = U*D*U**T.
186 *
187 * First solve U*D*X = B, overwriting B with X.
188 *
189 * K is the main loop index, decreasing from N to 1 in steps of
190 * 1 or 2, depending on the size of the diagonal blocks.
191 *
192  k = n
193  10 CONTINUE
194 *
195 * If K < 1, exit from loop.
196 *
197  IF( k.LT.1 )
198  $ go to 30
199 *
200  IF( ipiv( k ).GT.0 ) THEN
201 *
202 * 1 x 1 diagonal block
203 *
204 * Interchange rows K and IPIV(K).
205 *
206  kp = ipiv( k )
207  IF( kp.NE.k )
208  $ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
209 *
210 * Multiply by inv(U(K)), where U(K) is the transformation
211 * stored in column K of A.
212 *
213  CALL sger( k-1, nrhs, -one, a( 1, k ), 1, b( k, 1 ), ldb,
214  $ b( 1, 1 ), ldb )
215 *
216 * Multiply by the inverse of the diagonal block.
217 *
218  CALL sscal( nrhs, one / a( k, k ), b( k, 1 ), ldb )
219  k = k - 1
220  ELSE
221 *
222 * 2 x 2 diagonal block
223 *
224 * Interchange rows K-1 and -IPIV(K).
225 *
226  kp = -ipiv( k )
227  IF( kp.NE.k-1 )
228  $ CALL sswap( nrhs, b( k-1, 1 ), ldb, b( kp, 1 ), ldb )
229 *
230 * Multiply by inv(U(K)), where U(K) is the transformation
231 * stored in columns K-1 and K of A.
232 *
233  CALL sger( k-2, nrhs, -one, a( 1, k ), 1, b( k, 1 ), ldb,
234  $ b( 1, 1 ), ldb )
235  CALL sger( k-2, nrhs, -one, a( 1, k-1 ), 1, b( k-1, 1 ),
236  $ ldb, b( 1, 1 ), ldb )
237 *
238 * Multiply by the inverse of the diagonal block.
239 *
240  akm1k = a( k-1, k )
241  akm1 = a( k-1, k-1 ) / akm1k
242  ak = a( k, k ) / akm1k
243  denom = akm1*ak - one
244  DO 20 j = 1, nrhs
245  bkm1 = b( k-1, j ) / akm1k
246  bk = b( k, j ) / akm1k
247  b( k-1, j ) = ( ak*bkm1-bk ) / denom
248  b( k, j ) = ( akm1*bk-bkm1 ) / denom
249  20 CONTINUE
250  k = k - 2
251  END IF
252 *
253  go to 10
254  30 CONTINUE
255 *
256 * Next solve U**T *X = B, overwriting B with X.
257 *
258 * K is the main loop index, increasing from 1 to N in steps of
259 * 1 or 2, depending on the size of the diagonal blocks.
260 *
261  k = 1
262  40 CONTINUE
263 *
264 * If K > N, exit from loop.
265 *
266  IF( k.GT.n )
267  $ go to 50
268 *
269  IF( ipiv( k ).GT.0 ) THEN
270 *
271 * 1 x 1 diagonal block
272 *
273 * Multiply by inv(U**T(K)), where U(K) is the transformation
274 * stored in column K of A.
275 *
276  CALL sgemv( 'Transpose', k-1, nrhs, -one, b, ldb, a( 1, k ),
277  $ 1, one, b( k, 1 ), ldb )
278 *
279 * Interchange rows K and IPIV(K).
280 *
281  kp = ipiv( k )
282  IF( kp.NE.k )
283  $ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
284  k = k + 1
285  ELSE
286 *
287 * 2 x 2 diagonal block
288 *
289 * Multiply by inv(U**T(K+1)), where U(K+1) is the transformation
290 * stored in columns K and K+1 of A.
291 *
292  CALL sgemv( 'Transpose', k-1, nrhs, -one, b, ldb, a( 1, k ),
293  $ 1, one, b( k, 1 ), ldb )
294  CALL sgemv( 'Transpose', k-1, nrhs, -one, b, ldb,
295  $ a( 1, k+1 ), 1, one, b( k+1, 1 ), ldb )
296 *
297 * Interchange rows K and -IPIV(K).
298 *
299  kp = -ipiv( k )
300  IF( kp.NE.k )
301  $ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
302  k = k + 2
303  END IF
304 *
305  go to 40
306  50 CONTINUE
307 *
308  ELSE
309 *
310 * Solve A*X = B, where A = L*D*L**T.
311 *
312 * First solve L*D*X = B, overwriting B with X.
313 *
314 * K is the main loop index, increasing from 1 to N in steps of
315 * 1 or 2, depending on the size of the diagonal blocks.
316 *
317  k = 1
318  60 CONTINUE
319 *
320 * If K > N, exit from loop.
321 *
322  IF( k.GT.n )
323  $ go to 80
324 *
325  IF( ipiv( k ).GT.0 ) THEN
326 *
327 * 1 x 1 diagonal block
328 *
329 * Interchange rows K and IPIV(K).
330 *
331  kp = ipiv( k )
332  IF( kp.NE.k )
333  $ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
334 *
335 * Multiply by inv(L(K)), where L(K) is the transformation
336 * stored in column K of A.
337 *
338  IF( k.LT.n )
339  $ CALL sger( n-k, nrhs, -one, a( k+1, k ), 1, b( k, 1 ),
340  $ ldb, b( k+1, 1 ), ldb )
341 *
342 * Multiply by the inverse of the diagonal block.
343 *
344  CALL sscal( nrhs, one / a( k, k ), b( k, 1 ), ldb )
345  k = k + 1
346  ELSE
347 *
348 * 2 x 2 diagonal block
349 *
350 * Interchange rows K+1 and -IPIV(K).
351 *
352  kp = -ipiv( k )
353  IF( kp.NE.k+1 )
354  $ CALL sswap( nrhs, b( k+1, 1 ), ldb, b( kp, 1 ), ldb )
355 *
356 * Multiply by inv(L(K)), where L(K) is the transformation
357 * stored in columns K and K+1 of A.
358 *
359  IF( k.LT.n-1 ) THEN
360  CALL sger( n-k-1, nrhs, -one, a( k+2, k ), 1, b( k, 1 ),
361  $ ldb, b( k+2, 1 ), ldb )
362  CALL sger( n-k-1, nrhs, -one, a( k+2, k+1 ), 1,
363  $ b( k+1, 1 ), ldb, b( k+2, 1 ), ldb )
364  END IF
365 *
366 * Multiply by the inverse of the diagonal block.
367 *
368  akm1k = a( k+1, k )
369  akm1 = a( k, k ) / akm1k
370  ak = a( k+1, k+1 ) / akm1k
371  denom = akm1*ak - one
372  DO 70 j = 1, nrhs
373  bkm1 = b( k, j ) / akm1k
374  bk = b( k+1, j ) / akm1k
375  b( k, j ) = ( ak*bkm1-bk ) / denom
376  b( k+1, j ) = ( akm1*bk-bkm1 ) / denom
377  70 CONTINUE
378  k = k + 2
379  END IF
380 *
381  go to 60
382  80 CONTINUE
383 *
384 * Next solve L**T *X = B, overwriting B with X.
385 *
386 * K is the main loop index, decreasing from N to 1 in steps of
387 * 1 or 2, depending on the size of the diagonal blocks.
388 *
389  k = n
390  90 CONTINUE
391 *
392 * If K < 1, exit from loop.
393 *
394  IF( k.LT.1 )
395  $ go to 100
396 *
397  IF( ipiv( k ).GT.0 ) THEN
398 *
399 * 1 x 1 diagonal block
400 *
401 * Multiply by inv(L**T(K)), where L(K) is the transformation
402 * stored in column K of A.
403 *
404  IF( k.LT.n )
405  $ CALL sgemv( 'Transpose', n-k, nrhs, -one, b( k+1, 1 ),
406  $ ldb, a( k+1, k ), 1, one, b( k, 1 ), ldb )
407 *
408 * Interchange rows K and IPIV(K).
409 *
410  kp = ipiv( k )
411  IF( kp.NE.k )
412  $ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
413  k = k - 1
414  ELSE
415 *
416 * 2 x 2 diagonal block
417 *
418 * Multiply by inv(L**T(K-1)), where L(K-1) is the transformation
419 * stored in columns K-1 and K of A.
420 *
421  IF( k.LT.n ) THEN
422  CALL sgemv( 'Transpose', n-k, nrhs, -one, b( k+1, 1 ),
423  $ ldb, a( k+1, k ), 1, one, b( k, 1 ), ldb )
424  CALL sgemv( 'Transpose', n-k, nrhs, -one, b( k+1, 1 ),
425  $ ldb, a( k+1, k-1 ), 1, one, b( k-1, 1 ),
426  $ ldb )
427  END IF
428 *
429 * Interchange rows K and -IPIV(K).
430 *
431  kp = -ipiv( k )
432  IF( kp.NE.k )
433  $ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
434  k = k - 2
435  END IF
436 *
437  go to 90
438  100 CONTINUE
439  END IF
440 *
441  RETURN
442 *
443 * End of SSYTRS
444 *
445  END