 LAPACK  3.10.1 LAPACK: Linear Algebra PACKage

## ◆ ssytrs2()

 subroutine ssytrs2 ( character UPLO, integer N, integer NRHS, real, dimension( lda, * ) A, integer LDA, integer, dimension( * ) IPIV, real, dimension( ldb, * ) B, integer LDB, real, dimension( * ) WORK, integer INFO )

SSYTRS2

Purpose:
``` SSYTRS2 solves a system of linear equations A*X = B with a real
symmetric matrix A using the factorization A = U*D*U**T or
A = L*D*L**T computed by SSYTRF and converted by SSYCONV.```
Parameters
 [in] UPLO ``` UPLO is CHARACTER*1 Specifies whether the details of the factorization are stored as an upper or lower triangular matrix. = 'U': Upper triangular, form is A = U*D*U**T; = 'L': Lower triangular, form is A = L*D*L**T.``` [in] N ``` N is INTEGER The order of the matrix A. N >= 0.``` [in] NRHS ``` NRHS is INTEGER The number of right hand sides, i.e., the number of columns of the matrix B. NRHS >= 0.``` [in,out] A ``` A is REAL array, dimension (LDA,N) The block diagonal matrix D and the multipliers used to obtain the factor U or L as computed by SSYTRF. Note that A is input / output. This might be counter-intuitive, and one may think that A is input only. A is input / output. This is because, at the start of the subroutine, we permute A in a "better" form and then we permute A back to its original form at the end.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,N).``` [in] IPIV ``` IPIV is INTEGER array, dimension (N) Details of the interchanges and the block structure of D as determined by SSYTRF.``` [in,out] B ``` B is REAL array, dimension (LDB,NRHS) On entry, the right hand side matrix B. On exit, the solution matrix X.``` [in] LDB ``` LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N).``` [out] WORK ` WORK is REAL array, dimension (N)` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value```

Definition at line 130 of file ssytrs2.f.

132 *
133 * -- LAPACK computational routine --
134 * -- LAPACK is a software package provided by Univ. of Tennessee, --
135 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
136 *
137 * .. Scalar Arguments ..
138  CHARACTER UPLO
139  INTEGER INFO, LDA, LDB, N, NRHS
140 * ..
141 * .. Array Arguments ..
142  INTEGER IPIV( * )
143  REAL A( LDA, * ), B( LDB, * ), WORK( * )
144 * ..
145 *
146 * =====================================================================
147 *
148 * .. Parameters ..
149  REAL ONE
150  parameter( one = 1.0e+0 )
151 * ..
152 * .. Local Scalars ..
153  LOGICAL UPPER
154  INTEGER I, IINFO, J, K, KP
155  REAL AK, AKM1, AKM1K, BK, BKM1, DENOM
156 * ..
157 * .. External Functions ..
158  LOGICAL LSAME
159  EXTERNAL lsame
160 * ..
161 * .. External Subroutines ..
162  EXTERNAL sscal, ssyconv, sswap, strsm, xerbla
163 * ..
164 * .. Intrinsic Functions ..
165  INTRINSIC max
166 * ..
167 * .. Executable Statements ..
168 *
169  info = 0
170  upper = lsame( uplo, 'U' )
171  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
172  info = -1
173  ELSE IF( n.LT.0 ) THEN
174  info = -2
175  ELSE IF( nrhs.LT.0 ) THEN
176  info = -3
177  ELSE IF( lda.LT.max( 1, n ) ) THEN
178  info = -5
179  ELSE IF( ldb.LT.max( 1, n ) ) THEN
180  info = -8
181  END IF
182  IF( info.NE.0 ) THEN
183  CALL xerbla( 'SSYTRS2', -info )
184  RETURN
185  END IF
186 *
187 * Quick return if possible
188 *
189  IF( n.EQ.0 .OR. nrhs.EQ.0 )
190  \$ RETURN
191 *
192 * Convert A
193 *
194  CALL ssyconv( uplo, 'C', n, a, lda, ipiv, work, iinfo )
195 *
196  IF( upper ) THEN
197 *
198 * Solve A*X = B, where A = U*D*U**T.
199 *
200 * P**T * B
201  k=n
202  DO WHILE ( k .GE. 1 )
203  IF( ipiv( k ).GT.0 ) THEN
204 * 1 x 1 diagonal block
205 * Interchange rows K and IPIV(K).
206  kp = ipiv( k )
207  IF( kp.NE.k )
208  \$ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
209  k=k-1
210  ELSE
211 * 2 x 2 diagonal block
212 * Interchange rows K-1 and -IPIV(K).
213  kp = -ipiv( k )
214  IF( kp.EQ.-ipiv( k-1 ) )
215  \$ CALL sswap( nrhs, b( k-1, 1 ), ldb, b( kp, 1 ), ldb )
216  k=k-2
217  END IF
218  END DO
219 *
220 * Compute (U \P**T * B) -> B [ (U \P**T * B) ]
221 *
222  CALL strsm('L','U','N','U',n,nrhs,one,a,lda,b,ldb)
223 *
224 * Compute D \ B -> B [ D \ (U \P**T * B) ]
225 *
226  i=n
227  DO WHILE ( i .GE. 1 )
228  IF( ipiv(i) .GT. 0 ) THEN
229  CALL sscal( nrhs, one / a( i, i ), b( i, 1 ), ldb )
230  ELSEIF ( i .GT. 1) THEN
231  IF ( ipiv(i-1) .EQ. ipiv(i) ) THEN
232  akm1k = work(i)
233  akm1 = a( i-1, i-1 ) / akm1k
234  ak = a( i, i ) / akm1k
235  denom = akm1*ak - one
236  DO 15 j = 1, nrhs
237  bkm1 = b( i-1, j ) / akm1k
238  bk = b( i, j ) / akm1k
239  b( i-1, j ) = ( ak*bkm1-bk ) / denom
240  b( i, j ) = ( akm1*bk-bkm1 ) / denom
241  15 CONTINUE
242  i = i - 1
243  ENDIF
244  ENDIF
245  i = i - 1
246  END DO
247 *
248 * Compute (U**T \ B) -> B [ U**T \ (D \ (U \P**T * B) ) ]
249 *
250  CALL strsm('L','U','T','U',n,nrhs,one,a,lda,b,ldb)
251 *
252 * P * B [ P * (U**T \ (D \ (U \P**T * B) )) ]
253 *
254  k=1
255  DO WHILE ( k .LE. n )
256  IF( ipiv( k ).GT.0 ) THEN
257 * 1 x 1 diagonal block
258 * Interchange rows K and IPIV(K).
259  kp = ipiv( k )
260  IF( kp.NE.k )
261  \$ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
262  k=k+1
263  ELSE
264 * 2 x 2 diagonal block
265 * Interchange rows K-1 and -IPIV(K).
266  kp = -ipiv( k )
267  IF( k .LT. n .AND. kp.EQ.-ipiv( k+1 ) )
268  \$ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
269  k=k+2
270  ENDIF
271  END DO
272 *
273  ELSE
274 *
275 * Solve A*X = B, where A = L*D*L**T.
276 *
277 * P**T * B
278  k=1
279  DO WHILE ( k .LE. n )
280  IF( ipiv( k ).GT.0 ) THEN
281 * 1 x 1 diagonal block
282 * Interchange rows K and IPIV(K).
283  kp = ipiv( k )
284  IF( kp.NE.k )
285  \$ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
286  k=k+1
287  ELSE
288 * 2 x 2 diagonal block
289 * Interchange rows K and -IPIV(K+1).
290  kp = -ipiv( k+1 )
291  IF( kp.EQ.-ipiv( k ) )
292  \$ CALL sswap( nrhs, b( k+1, 1 ), ldb, b( kp, 1 ), ldb )
293  k=k+2
294  ENDIF
295  END DO
296 *
297 * Compute (L \P**T * B) -> B [ (L \P**T * B) ]
298 *
299  CALL strsm('L','L','N','U',n,nrhs,one,a,lda,b,ldb)
300 *
301 * Compute D \ B -> B [ D \ (L \P**T * B) ]
302 *
303  i=1
304  DO WHILE ( i .LE. n )
305  IF( ipiv(i) .GT. 0 ) THEN
306  CALL sscal( nrhs, one / a( i, i ), b( i, 1 ), ldb )
307  ELSE
308  akm1k = work(i)
309  akm1 = a( i, i ) / akm1k
310  ak = a( i+1, i+1 ) / akm1k
311  denom = akm1*ak - one
312  DO 25 j = 1, nrhs
313  bkm1 = b( i, j ) / akm1k
314  bk = b( i+1, j ) / akm1k
315  b( i, j ) = ( ak*bkm1-bk ) / denom
316  b( i+1, j ) = ( akm1*bk-bkm1 ) / denom
317  25 CONTINUE
318  i = i + 1
319  ENDIF
320  i = i + 1
321  END DO
322 *
323 * Compute (L**T \ B) -> B [ L**T \ (D \ (L \P**T * B) ) ]
324 *
325  CALL strsm('L','L','T','U',n,nrhs,one,a,lda,b,ldb)
326 *
327 * P * B [ P * (L**T \ (D \ (L \P**T * B) )) ]
328 *
329  k=n
330  DO WHILE ( k .GE. 1 )
331  IF( ipiv( k ).GT.0 ) THEN
332 * 1 x 1 diagonal block
333 * Interchange rows K and IPIV(K).
334  kp = ipiv( k )
335  IF( kp.NE.k )
336  \$ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
337  k=k-1
338  ELSE
339 * 2 x 2 diagonal block
340 * Interchange rows K-1 and -IPIV(K).
341  kp = -ipiv( k )
342  IF( k.GT.1 .AND. kp.EQ.-ipiv( k-1 ) )
343  \$ CALL sswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
344  k=k-2
345  ENDIF
346  END DO
347 *
348  END IF
349 *
350 * Revert A
351 *
352  CALL ssyconv( uplo, 'R', n, a, lda, ipiv, work, iinfo )
353 *
354  RETURN
355 *
356 * End of SSYTRS2
357 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine ssyconv(UPLO, WAY, N, A, LDA, IPIV, E, INFO)
SSYCONV
Definition: ssyconv.f:114
subroutine sswap(N, SX, INCX, SY, INCY)
SSWAP
Definition: sswap.f:82
subroutine sscal(N, SA, SX, INCX)
SSCAL
Definition: sscal.f:79
subroutine strsm(SIDE, UPLO, TRANSA, DIAG, M, N, ALPHA, A, LDA, B, LDB)
STRSM
Definition: strsm.f:181
Here is the call graph for this function:
Here is the caller graph for this function: