LAPACK  3.8.0 LAPACK: Linear Algebra PACKage
csytrs_3.f
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1 *> \brief \b CSYTRS_3
2 *
3 * =========== DOCUMENTATION ===========
4 *
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16 *> \endhtmlonly
17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE CSYTRS_3( UPLO, N, NRHS, A, LDA, E, IPIV, B, LDB,
22 * INFO )
23 *
24 * .. Scalar Arguments ..
25 * CHARACTER UPLO
26 * INTEGER INFO, LDA, LDB, N, NRHS
27 * ..
28 * .. Array Arguments ..
29 * INTEGER IPIV( * )
30 * COMPLEX A( LDA, * ), B( LDB, * ), E( * )
31 * ..
32 *
33 *
34 *> \par Purpose:
35 * =============
36 *>
37 *> \verbatim
38 *> CSYTRS_3 solves a system of linear equations A * X = B with a complex
39 *> symmetric matrix A using the factorization computed
40 *> by CSYTRF_RK or CSYTRF_BK:
41 *>
42 *> A = P*U*D*(U**T)*(P**T) or A = P*L*D*(L**T)*(P**T),
43 *>
44 *> where U (or L) is unit upper (or lower) triangular matrix,
45 *> U**T (or L**T) is the transpose of U (or L), P is a permutation
46 *> matrix, P**T is the transpose of P, and D is symmetric and block
47 *> diagonal with 1-by-1 and 2-by-2 diagonal blocks.
48 *>
49 *> This algorithm is using Level 3 BLAS.
50 *> \endverbatim
51 *
52 * Arguments:
53 * ==========
54 *
55 *> \param[in] UPLO
56 *> \verbatim
57 *> UPLO is CHARACTER*1
58 *> Specifies whether the details of the factorization are
59 *> stored as an upper or lower triangular matrix:
60 *> = 'U': Upper triangular, form is A = P*U*D*(U**T)*(P**T);
61 *> = 'L': Lower triangular, form is A = P*L*D*(L**T)*(P**T).
62 *> \endverbatim
63 *>
64 *> \param[in] N
65 *> \verbatim
66 *> N is INTEGER
67 *> The order of the matrix A. N >= 0.
68 *> \endverbatim
69 *>
70 *> \param[in] NRHS
71 *> \verbatim
72 *> NRHS is INTEGER
73 *> The number of right hand sides, i.e., the number of columns
74 *> of the matrix B. NRHS >= 0.
75 *> \endverbatim
76 *>
77 *> \param[in] A
78 *> \verbatim
79 *> A is COMPLEX array, dimension (LDA,N)
80 *> Diagonal of the block diagonal matrix D and factors U or L
81 *> as computed by CSYTRF_RK and CSYTRF_BK:
82 *> a) ONLY diagonal elements of the symmetric block diagonal
83 *> matrix D on the diagonal of A, i.e. D(k,k) = A(k,k);
84 *> (superdiagonal (or subdiagonal) elements of D
85 *> should be provided on entry in array E), and
86 *> b) If UPLO = 'U': factor U in the superdiagonal part of A.
87 *> If UPLO = 'L': factor L in the subdiagonal part of A.
88 *> \endverbatim
89 *>
90 *> \param[in] LDA
91 *> \verbatim
92 *> LDA is INTEGER
93 *> The leading dimension of the array A. LDA >= max(1,N).
94 *> \endverbatim
95 *>
96 *> \param[in] E
97 *> \verbatim
98 *> E is COMPLEX array, dimension (N)
99 *> On entry, contains the superdiagonal (or subdiagonal)
100 *> elements of the symmetric block diagonal matrix D
101 *> with 1-by-1 or 2-by-2 diagonal blocks, where
102 *> If UPLO = 'U': E(i) = D(i-1,i),i=2:N, E(1) not referenced;
103 *> If UPLO = 'L': E(i) = D(i+1,i),i=1:N-1, E(N) not referenced.
104 *>
105 *> NOTE: For 1-by-1 diagonal block D(k), where
106 *> 1 <= k <= N, the element E(k) is not referenced in both
107 *> UPLO = 'U' or UPLO = 'L' cases.
108 *> \endverbatim
109 *>
110 *> \param[in] IPIV
111 *> \verbatim
112 *> IPIV is INTEGER array, dimension (N)
113 *> Details of the interchanges and the block structure of D
114 *> as determined by CSYTRF_RK or CSYTRF_BK.
115 *> \endverbatim
116 *>
117 *> \param[in,out] B
118 *> \verbatim
119 *> B is COMPLEX array, dimension (LDB,NRHS)
120 *> On entry, the right hand side matrix B.
121 *> On exit, the solution matrix X.
122 *> \endverbatim
123 *>
124 *> \param[in] LDB
125 *> \verbatim
126 *> LDB is INTEGER
127 *> The leading dimension of the array B. LDB >= max(1,N).
128 *> \endverbatim
129 *>
130 *> \param[out] INFO
131 *> \verbatim
132 *> INFO is INTEGER
133 *> = 0: successful exit
134 *> < 0: if INFO = -i, the i-th argument had an illegal value
135 *> \endverbatim
136 *
137 * Authors:
138 * ========
139 *
140 *> \author Univ. of Tennessee
141 *> \author Univ. of California Berkeley
142 *> \author Univ. of Colorado Denver
143 *> \author NAG Ltd.
144 *
145 *> \date June 2017
146 *
147 *> \ingroup complexSYcomputational
148 *
149 *> \par Contributors:
150 * ==================
151 *>
152 *> \verbatim
153 *>
154 *> June 2017, Igor Kozachenko,
155 *> Computer Science Division,
156 *> University of California, Berkeley
157 *>
158 *> September 2007, Sven Hammarling, Nicholas J. Higham, Craig Lucas,
159 *> School of Mathematics,
160 *> University of Manchester
161 *>
162 *> \endverbatim
163 *
164 * =====================================================================
165  SUBROUTINE csytrs_3( UPLO, N, NRHS, A, LDA, E, IPIV, B, LDB,
166  \$ INFO )
167 *
168 * -- LAPACK computational routine (version 3.7.1) --
169 * -- LAPACK is a software package provided by Univ. of Tennessee, --
170 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
171 * June 2017
172 *
173 * .. Scalar Arguments ..
174  CHARACTER UPLO
175  INTEGER INFO, LDA, LDB, N, NRHS
176 * ..
177 * .. Array Arguments ..
178  INTEGER IPIV( * )
179  COMPLEX A( lda, * ), B( ldb, * ), E( * )
180 * ..
181 *
182 * =====================================================================
183 *
184 * .. Parameters ..
185  COMPLEX ONE
186  parameter( one = ( 1.0e+0,0.0e+0 ) )
187 * ..
188 * .. Local Scalars ..
189  LOGICAL UPPER
190  INTEGER I, J, K, KP
191  COMPLEX AK, AKM1, AKM1K, BK, BKM1, DENOM
192 * ..
193 * .. External Functions ..
194  LOGICAL LSAME
195  EXTERNAL lsame
196 * ..
197 * .. External Subroutines ..
198  EXTERNAL cscal, cswap, ctrsm, xerbla
199 * ..
200 * .. Intrinsic Functions ..
201  INTRINSIC abs, max
202 * ..
203 * .. Executable Statements ..
204 *
205  info = 0
206  upper = lsame( uplo, 'U' )
207  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
208  info = -1
209  ELSE IF( n.LT.0 ) THEN
210  info = -2
211  ELSE IF( nrhs.LT.0 ) THEN
212  info = -3
213  ELSE IF( lda.LT.max( 1, n ) ) THEN
214  info = -5
215  ELSE IF( ldb.LT.max( 1, n ) ) THEN
216  info = -9
217  END IF
218  IF( info.NE.0 ) THEN
219  CALL xerbla( 'CSYTRS_3', -info )
220  RETURN
221  END IF
222 *
223 * Quick return if possible
224 *
225  IF( n.EQ.0 .OR. nrhs.EQ.0 )
226  \$ RETURN
227 *
228  IF( upper ) THEN
229 *
230 * Begin Upper
231 *
232 * Solve A*X = B, where A = U*D*U**T.
233 *
234 * P**T * B
235 *
236 * Interchange rows K and IPIV(K) of matrix B in the same order
237 * that the formation order of IPIV(I) vector for Upper case.
238 *
239 * (We can do the simple loop over IPIV with decrement -1,
240 * since the ABS value of IPIV(I) represents the row index
241 * of the interchange with row i in both 1x1 and 2x2 pivot cases)
242 *
243  DO k = n, 1, -1
244  kp = abs( ipiv( k ) )
245  IF( kp.NE.k ) THEN
246  CALL cswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
247  END IF
248  END DO
249 *
250 * Compute (U \P**T * B) -> B [ (U \P**T * B) ]
251 *
252  CALL ctrsm( 'L', 'U', 'N', 'U', n, nrhs, one, a, lda, b, ldb )
253 *
254 * Compute D \ B -> B [ D \ (U \P**T * B) ]
255 *
256  i = n
257  DO WHILE ( i.GE.1 )
258  IF( ipiv( i ).GT.0 ) THEN
259  CALL cscal( nrhs, one / a( i, i ), b( i, 1 ), ldb )
260  ELSE IF ( i.GT.1 ) THEN
261  akm1k = e( i )
262  akm1 = a( i-1, i-1 ) / akm1k
263  ak = a( i, i ) / akm1k
264  denom = akm1*ak - one
265  DO j = 1, nrhs
266  bkm1 = b( i-1, j ) / akm1k
267  bk = b( i, j ) / akm1k
268  b( i-1, j ) = ( ak*bkm1-bk ) / denom
269  b( i, j ) = ( akm1*bk-bkm1 ) / denom
270  END DO
271  i = i - 1
272  END IF
273  i = i - 1
274  END DO
275 *
276 * Compute (U**T \ B) -> B [ U**T \ (D \ (U \P**T * B) ) ]
277 *
278  CALL ctrsm( 'L', 'U', 'T', 'U', n, nrhs, one, a, lda, b, ldb )
279 *
280 * P * B [ P * (U**T \ (D \ (U \P**T * B) )) ]
281 *
282 * Interchange rows K and IPIV(K) of matrix B in reverse order
283 * from the formation order of IPIV(I) vector for Upper case.
284 *
285 * (We can do the simple loop over IPIV with increment 1,
286 * since the ABS value of IPIV( I ) represents the row index
287 * of the interchange with row i in both 1x1 and 2x2 pivot cases)
288 *
289  DO k = 1, n, 1
290  kp = abs( ipiv( k ) )
291  IF( kp.NE.k ) THEN
292  CALL cswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
293  END IF
294  END DO
295 *
296  ELSE
297 *
298 * Begin Lower
299 *
300 * Solve A*X = B, where A = L*D*L**T.
301 *
302 * P**T * B
303 * Interchange rows K and IPIV(K) of matrix B in the same order
304 * that the formation order of IPIV(I) vector for Lower case.
305 *
306 * (We can do the simple loop over IPIV with increment 1,
307 * since the ABS value of IPIV(I) represents the row index
308 * of the interchange with row i in both 1x1 and 2x2 pivot cases)
309 *
310  DO k = 1, n, 1
311  kp = abs( ipiv( k ) )
312  IF( kp.NE.k ) THEN
313  CALL cswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
314  END IF
315  END DO
316 *
317 * Compute (L \P**T * B) -> B [ (L \P**T * B) ]
318 *
319  CALL ctrsm( 'L', 'L', 'N', 'U', n, nrhs, one, a, lda, b, ldb )
320 *
321 * Compute D \ B -> B [ D \ (L \P**T * B) ]
322 *
323  i = 1
324  DO WHILE ( i.LE.n )
325  IF( ipiv( i ).GT.0 ) THEN
326  CALL cscal( nrhs, one / a( i, i ), b( i, 1 ), ldb )
327  ELSE IF( i.LT.n ) THEN
328  akm1k = e( i )
329  akm1 = a( i, i ) / akm1k
330  ak = a( i+1, i+1 ) / akm1k
331  denom = akm1*ak - one
332  DO j = 1, nrhs
333  bkm1 = b( i, j ) / akm1k
334  bk = b( i+1, j ) / akm1k
335  b( i, j ) = ( ak*bkm1-bk ) / denom
336  b( i+1, j ) = ( akm1*bk-bkm1 ) / denom
337  END DO
338  i = i + 1
339  END IF
340  i = i + 1
341  END DO
342 *
343 * Compute (L**T \ B) -> B [ L**T \ (D \ (L \P**T * B) ) ]
344 *
345  CALL ctrsm('L', 'L', 'T', 'U', n, nrhs, one, a, lda, b, ldb )
346 *
347 * P * B [ P * (L**T \ (D \ (L \P**T * B) )) ]
348 *
349 * Interchange rows K and IPIV(K) of matrix B in reverse order
350 * from the formation order of IPIV(I) vector for Lower case.
351 *
352 * (We can do the simple loop over IPIV with decrement -1,
353 * since the ABS value of IPIV(I) represents the row index
354 * of the interchange with row i in both 1x1 and 2x2 pivot cases)
355 *
356  DO k = n, 1, -1
357  kp = abs( ipiv( k ) )
358  IF( kp.NE.k ) THEN
359  CALL cswap( nrhs, b( k, 1 ), ldb, b( kp, 1 ), ldb )
360  END IF
361  END DO
362 *
363 * END Lower
364 *
365  END IF
366 *
367  RETURN
368 *
369 * End of CSYTRS_3
370 *
371  END
subroutine cscal(N, CA, CX, INCX)
CSCAL
Definition: cscal.f:80
subroutine ctrsm(SIDE, UPLO, TRANSA, DIAG, M, N, ALPHA, A, LDA, B, LDB)
CTRSM
Definition: ctrsm.f:182
subroutine csytrs_3(UPLO, N, NRHS, A, LDA, E, IPIV, B, LDB, INFO)
CSYTRS_3
Definition: csytrs_3.f:167
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:62
subroutine cswap(N, CX, INCX, CY, INCY)
CSWAP
Definition: cswap.f:83