LAPACK  3.6.1 LAPACK: Linear Algebra PACKage
 subroutine csytrf ( character UPLO, integer N, complex, dimension( lda, * ) A, integer LDA, integer, dimension( * ) IPIV, complex, dimension( * ) WORK, integer LWORK, integer INFO )

CSYTRF

Purpose:
``` CSYTRF computes the factorization of a complex symmetric matrix A
using the Bunch-Kaufman diagonal pivoting method.  The form of the
factorization is

A = U*D*U**T  or  A = L*D*L**T

where U (or L) is a product of permutation and unit upper (lower)
triangular matrices, and D is symmetric and block diagonal with
with 1-by-1 and 2-by-2 diagonal blocks.

This is the blocked version of the algorithm, calling Level 3 BLAS.```
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,out] A ``` A is COMPLEX array, dimension (LDA,N) On entry, the symmetric matrix A. If UPLO = 'U', the leading N-by-N upper triangular part of A contains the upper triangular part of the matrix A, and the strictly lower triangular part of A is not referenced. If UPLO = 'L', the leading N-by-N lower triangular part of A contains the lower triangular part of the matrix A, and the strictly upper triangular part of A is not referenced. On exit, the block diagonal matrix D and the multipliers used to obtain the factor U or L (see below for further details).``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,N).``` [out] IPIV ``` IPIV is INTEGER array, dimension (N) Details of the interchanges and the block structure of D. If IPIV(k) > 0, then rows and columns k and IPIV(k) were interchanged and D(k,k) is a 1-by-1 diagonal block. If UPLO = 'U' and IPIV(k) = IPIV(k-1) < 0, then rows and columns k-1 and -IPIV(k) were interchanged and D(k-1:k,k-1:k) is a 2-by-2 diagonal block. If UPLO = 'L' and IPIV(k) = IPIV(k+1) < 0, then rows and columns k+1 and -IPIV(k) were interchanged and D(k:k+1,k:k+1) is a 2-by-2 diagonal block.``` [out] WORK ``` WORK is COMPLEX array, dimension (MAX(1,LWORK)) On exit, if INFO = 0, WORK(1) returns the optimal LWORK.``` [in] LWORK ``` LWORK is INTEGER The length of WORK. LWORK >=1. For best performance LWORK >= N*NB, where NB is the block size returned by ILAENV. If LWORK = -1, then a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued by XERBLA.``` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value > 0: if INFO = i, D(i,i) is exactly zero. The factorization has been completed, but the block diagonal matrix D is exactly singular, and division by zero will occur if it is used to solve a system of equations.```
Date
November 2011
Further Details:
```  If UPLO = 'U', then A = U*D*U**T, where
U = P(n)*U(n)* ... *P(k)U(k)* ...,
i.e., U is a product of terms P(k)*U(k), where k decreases from n to
1 in steps of 1 or 2, and D is a block diagonal matrix with 1-by-1
and 2-by-2 diagonal blocks D(k).  P(k) is a permutation matrix as
defined by IPIV(k), and U(k) is a unit upper triangular matrix, such
that if the diagonal block D(k) is of order s (s = 1 or 2), then

(   I    v    0   )   k-s
U(k) =  (   0    I    0   )   s
(   0    0    I   )   n-k
k-s   s   n-k

If s = 1, D(k) overwrites A(k,k), and v overwrites A(1:k-1,k).
If s = 2, the upper triangle of D(k) overwrites A(k-1,k-1), A(k-1,k),
and A(k,k), and v overwrites A(1:k-2,k-1:k).

If UPLO = 'L', then A = L*D*L**T, where
L = P(1)*L(1)* ... *P(k)*L(k)* ...,
i.e., L is a product of terms P(k)*L(k), where k increases from 1 to
n in steps of 1 or 2, and D is a block diagonal matrix with 1-by-1
and 2-by-2 diagonal blocks D(k).  P(k) is a permutation matrix as
defined by IPIV(k), and L(k) is a unit lower triangular matrix, such
that if the diagonal block D(k) is of order s (s = 1 or 2), then

(   I    0     0   )  k-1
L(k) =  (   0    I     0   )  s
(   0    v     I   )  n-k-s+1
k-1   s  n-k-s+1

If s = 1, D(k) overwrites A(k,k), and v overwrites A(k+1:n,k).
If s = 2, the lower triangle of D(k) overwrites A(k,k), A(k+1,k),
and A(k+1,k+1), and v overwrites A(k+2:n,k:k+1).```

Definition at line 184 of file csytrf.f.

184 *
185 * -- LAPACK computational routine (version 3.4.0) --
186 * -- LAPACK is a software package provided by Univ. of Tennessee, --
187 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
188 * November 2011
189 *
190 * .. Scalar Arguments ..
191  CHARACTER uplo
192  INTEGER info, lda, lwork, n
193 * ..
194 * .. Array Arguments ..
195  INTEGER ipiv( * )
196  COMPLEX a( lda, * ), work( * )
197 * ..
198 *
199 * =====================================================================
200 *
201 * .. Local Scalars ..
202  LOGICAL lquery, upper
203  INTEGER iinfo, iws, j, k, kb, ldwork, lwkopt, nb, nbmin
204 * ..
205 * .. External Functions ..
206  LOGICAL lsame
207  INTEGER ilaenv
208  EXTERNAL lsame, ilaenv
209 * ..
210 * .. External Subroutines ..
211  EXTERNAL clasyf, csytf2, xerbla
212 * ..
213 * .. Intrinsic Functions ..
214  INTRINSIC max
215 * ..
216 * .. Executable Statements ..
217 *
218 * Test the input parameters.
219 *
220  info = 0
221  upper = lsame( uplo, 'U' )
222  lquery = ( lwork.EQ.-1 )
223  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
224  info = -1
225  ELSE IF( n.LT.0 ) THEN
226  info = -2
227  ELSE IF( lda.LT.max( 1, n ) ) THEN
228  info = -4
229  ELSE IF( lwork.LT.1 .AND. .NOT.lquery ) THEN
230  info = -7
231  END IF
232 *
233  IF( info.EQ.0 ) THEN
234 *
235 * Determine the block size
236 *
237  nb = ilaenv( 1, 'CSYTRF', uplo, n, -1, -1, -1 )
238  lwkopt = n*nb
239  work( 1 ) = lwkopt
240  END IF
241 *
242  IF( info.NE.0 ) THEN
243  CALL xerbla( 'CSYTRF', -info )
244  RETURN
245  ELSE IF( lquery ) THEN
246  RETURN
247  END IF
248 *
249  nbmin = 2
250  ldwork = n
251  IF( nb.GT.1 .AND. nb.LT.n ) THEN
252  iws = ldwork*nb
253  IF( lwork.LT.iws ) THEN
254  nb = max( lwork / ldwork, 1 )
255  nbmin = max( 2, ilaenv( 2, 'CSYTRF', uplo, n, -1, -1, -1 ) )
256  END IF
257  ELSE
258  iws = 1
259  END IF
260  IF( nb.LT.nbmin )
261  \$ nb = n
262 *
263  IF( upper ) THEN
264 *
265 * Factorize A as U*D*U**T using the upper triangle of A
266 *
267 * K is the main loop index, decreasing from N to 1 in steps of
268 * KB, where KB is the number of columns factorized by CLASYF;
269 * KB is either NB or NB-1, or K for the last block
270 *
271  k = n
272  10 CONTINUE
273 *
274 * If K < 1, exit from loop
275 *
276  IF( k.LT.1 )
277  \$ GO TO 40
278 *
279  IF( k.GT.nb ) THEN
280 *
281 * Factorize columns k-kb+1:k of A and use blocked code to
282 * update columns 1:k-kb
283 *
284  CALL clasyf( uplo, k, nb, kb, a, lda, ipiv, work, n, iinfo )
285  ELSE
286 *
287 * Use unblocked code to factorize columns 1:k of A
288 *
289  CALL csytf2( uplo, k, a, lda, ipiv, iinfo )
290  kb = k
291  END IF
292 *
293 * Set INFO on the first occurrence of a zero pivot
294 *
295  IF( info.EQ.0 .AND. iinfo.GT.0 )
296  \$ info = iinfo
297 *
298 * Decrease K and return to the start of the main loop
299 *
300  k = k - kb
301  GO TO 10
302 *
303  ELSE
304 *
305 * Factorize A as L*D*L**T using the lower triangle of A
306 *
307 * K is the main loop index, increasing from 1 to N in steps of
308 * KB, where KB is the number of columns factorized by CLASYF;
309 * KB is either NB or NB-1, or N-K+1 for the last block
310 *
311  k = 1
312  20 CONTINUE
313 *
314 * If K > N, exit from loop
315 *
316  IF( k.GT.n )
317  \$ GO TO 40
318 *
319  IF( k.LE.n-nb ) THEN
320 *
321 * Factorize columns k:k+kb-1 of A and use blocked code to
322 * update columns k+kb:n
323 *
324  CALL clasyf( uplo, n-k+1, nb, kb, a( k, k ), lda, ipiv( k ),
325  \$ work, n, iinfo )
326  ELSE
327 *
328 * Use unblocked code to factorize columns k:n of A
329 *
330  CALL csytf2( uplo, n-k+1, a( k, k ), lda, ipiv( k ), iinfo )
331  kb = n - k + 1
332  END IF
333 *
334 * Set INFO on the first occurrence of a zero pivot
335 *
336  IF( info.EQ.0 .AND. iinfo.GT.0 )
337  \$ info = iinfo + k - 1
338 *
340 *
341  DO 30 j = k, k + kb - 1
342  IF( ipiv( j ).GT.0 ) THEN
343  ipiv( j ) = ipiv( j ) + k - 1
344  ELSE
345  ipiv( j ) = ipiv( j ) - k + 1
346  END IF
347  30 CONTINUE
348 *
349 * Increase K and return to the start of the main loop
350 *
351  k = k + kb
352  GO TO 20
353 *
354  END IF
355 *
356  40 CONTINUE
357  work( 1 ) = lwkopt
358  RETURN
359 *
360 * End of CSYTRF
361 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:62
subroutine clasyf(UPLO, N, NB, KB, A, LDA, IPIV, W, LDW, INFO)
CLASYF computes a partial factorization of a complex symmetric matrix using the Bunch-Kaufman diagona...
Definition: clasyf.f:179
integer function ilaenv(ISPEC, NAME, OPTS, N1, N2, N3, N4)
Definition: tstiee.f:83
subroutine csytf2(UPLO, N, A, LDA, IPIV, INFO)
CSYTF2 computes the factorization of a real symmetric indefinite matrix, using the diagonal pivoting ...
Definition: csytf2.f:193
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:55

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