LAPACK  3.6.0
LAPACK: Linear Algebra PACKage
sgtsvx.f
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1 *> \brief <b> SGTSVX computes the solution to system of linear equations A * X = B for GT matrices <b>
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
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13 *> [ZIP]</a>
14 *> <a href="http://www.netlib.org/cgi-bin/netlibfiles.txt?format=txt&filename=/lapack/lapack_routine/sgtsvx.f">
15 *> [TXT]</a>
16 *> \endhtmlonly
17 *
18 * Definition:
19 * ===========
20 *
21 * SUBROUTINE SGTSVX( FACT, TRANS, N, NRHS, DL, D, DU, DLF, DF, DUF,
22 * DU2, IPIV, B, LDB, X, LDX, RCOND, FERR, BERR,
23 * WORK, IWORK, INFO )
24 *
25 * .. Scalar Arguments ..
26 * CHARACTER FACT, TRANS
27 * INTEGER INFO, LDB, LDX, N, NRHS
28 * REAL RCOND
29 * ..
30 * .. Array Arguments ..
31 * INTEGER IPIV( * ), IWORK( * )
32 * REAL B( LDB, * ), BERR( * ), D( * ), DF( * ),
33 * $ DL( * ), DLF( * ), DU( * ), DU2( * ), DUF( * ),
34 * $ FERR( * ), WORK( * ), X( LDX, * )
35 * ..
36 *
37 *
38 *> \par Purpose:
39 * =============
40 *>
41 *> \verbatim
42 *>
43 *> SGTSVX uses the LU factorization to compute the solution to a real
44 *> system of linear equations A * X = B or A**T * X = B,
45 *> where A is a tridiagonal matrix of order N and X and B are N-by-NRHS
46 *> matrices.
47 *>
48 *> Error bounds on the solution and a condition estimate are also
49 *> provided.
50 *> \endverbatim
51 *
52 *> \par Description:
53 * =================
54 *>
55 *> \verbatim
56 *>
57 *> The following steps are performed:
58 *>
59 *> 1. If FACT = 'N', the LU decomposition is used to factor the matrix A
60 *> as A = L * U, where L is a product of permutation and unit lower
61 *> bidiagonal matrices and U is upper triangular with nonzeros in
62 *> only the main diagonal and first two superdiagonals.
63 *>
64 *> 2. If some U(i,i)=0, so that U is exactly singular, then the routine
65 *> returns with INFO = i. Otherwise, the factored form of A is used
66 *> to estimate the condition number of the matrix A. If the
67 *> reciprocal of the condition number is less than machine precision,
68 *> INFO = N+1 is returned as a warning, but the routine still goes on
69 *> to solve for X and compute error bounds as described below.
70 *>
71 *> 3. The system of equations is solved for X using the factored form
72 *> of A.
73 *>
74 *> 4. Iterative refinement is applied to improve the computed solution
75 *> matrix and calculate error bounds and backward error estimates
76 *> for it.
77 *> \endverbatim
78 *
79 * Arguments:
80 * ==========
81 *
82 *> \param[in] FACT
83 *> \verbatim
84 *> FACT is CHARACTER*1
85 *> Specifies whether or not the factored form of A has been
86 *> supplied on entry.
87 *> = 'F': DLF, DF, DUF, DU2, and IPIV contain the factored
88 *> form of A; DL, D, DU, DLF, DF, DUF, DU2 and IPIV
89 *> will not be modified.
90 *> = 'N': The matrix will be copied to DLF, DF, and DUF
91 *> and factored.
92 *> \endverbatim
93 *>
94 *> \param[in] TRANS
95 *> \verbatim
96 *> TRANS is CHARACTER*1
97 *> Specifies the form of the system of equations:
98 *> = 'N': A * X = B (No transpose)
99 *> = 'T': A**T * X = B (Transpose)
100 *> = 'C': A**H * X = B (Conjugate transpose = Transpose)
101 *> \endverbatim
102 *>
103 *> \param[in] N
104 *> \verbatim
105 *> N is INTEGER
106 *> The order of the matrix A. N >= 0.
107 *> \endverbatim
108 *>
109 *> \param[in] NRHS
110 *> \verbatim
111 *> NRHS is INTEGER
112 *> The number of right hand sides, i.e., the number of columns
113 *> of the matrix B. NRHS >= 0.
114 *> \endverbatim
115 *>
116 *> \param[in] DL
117 *> \verbatim
118 *> DL is REAL array, dimension (N-1)
119 *> The (n-1) subdiagonal elements of A.
120 *> \endverbatim
121 *>
122 *> \param[in] D
123 *> \verbatim
124 *> D is REAL array, dimension (N)
125 *> The n diagonal elements of A.
126 *> \endverbatim
127 *>
128 *> \param[in] DU
129 *> \verbatim
130 *> DU is REAL array, dimension (N-1)
131 *> The (n-1) superdiagonal elements of A.
132 *> \endverbatim
133 *>
134 *> \param[in,out] DLF
135 *> \verbatim
136 *> DLF is REAL array, dimension (N-1)
137 *> If FACT = 'F', then DLF is an input argument and on entry
138 *> contains the (n-1) multipliers that define the matrix L from
139 *> the LU factorization of A as computed by SGTTRF.
140 *>
141 *> If FACT = 'N', then DLF is an output argument and on exit
142 *> contains the (n-1) multipliers that define the matrix L from
143 *> the LU factorization of A.
144 *> \endverbatim
145 *>
146 *> \param[in,out] DF
147 *> \verbatim
148 *> DF is REAL array, dimension (N)
149 *> If FACT = 'F', then DF is an input argument and on entry
150 *> contains the n diagonal elements of the upper triangular
151 *> matrix U from the LU factorization of A.
152 *>
153 *> If FACT = 'N', then DF is an output argument and on exit
154 *> contains the n diagonal elements of the upper triangular
155 *> matrix U from the LU factorization of A.
156 *> \endverbatim
157 *>
158 *> \param[in,out] DUF
159 *> \verbatim
160 *> DUF is REAL array, dimension (N-1)
161 *> If FACT = 'F', then DUF is an input argument and on entry
162 *> contains the (n-1) elements of the first superdiagonal of U.
163 *>
164 *> If FACT = 'N', then DUF is an output argument and on exit
165 *> contains the (n-1) elements of the first superdiagonal of U.
166 *> \endverbatim
167 *>
168 *> \param[in,out] DU2
169 *> \verbatim
170 *> DU2 is REAL array, dimension (N-2)
171 *> If FACT = 'F', then DU2 is an input argument and on entry
172 *> contains the (n-2) elements of the second superdiagonal of
173 *> U.
174 *>
175 *> If FACT = 'N', then DU2 is an output argument and on exit
176 *> contains the (n-2) elements of the second superdiagonal of
177 *> U.
178 *> \endverbatim
179 *>
180 *> \param[in,out] IPIV
181 *> \verbatim
182 *> IPIV is INTEGER array, dimension (N)
183 *> If FACT = 'F', then IPIV is an input argument and on entry
184 *> contains the pivot indices from the LU factorization of A as
185 *> computed by SGTTRF.
186 *>
187 *> If FACT = 'N', then IPIV is an output argument and on exit
188 *> contains the pivot indices from the LU factorization of A;
189 *> row i of the matrix was interchanged with row IPIV(i).
190 *> IPIV(i) will always be either i or i+1; IPIV(i) = i indicates
191 *> a row interchange was not required.
192 *> \endverbatim
193 *>
194 *> \param[in] B
195 *> \verbatim
196 *> B is REAL array, dimension (LDB,NRHS)
197 *> The N-by-NRHS right hand side matrix B.
198 *> \endverbatim
199 *>
200 *> \param[in] LDB
201 *> \verbatim
202 *> LDB is INTEGER
203 *> The leading dimension of the array B. LDB >= max(1,N).
204 *> \endverbatim
205 *>
206 *> \param[out] X
207 *> \verbatim
208 *> X is REAL array, dimension (LDX,NRHS)
209 *> If INFO = 0 or INFO = N+1, the N-by-NRHS solution matrix X.
210 *> \endverbatim
211 *>
212 *> \param[in] LDX
213 *> \verbatim
214 *> LDX is INTEGER
215 *> The leading dimension of the array X. LDX >= max(1,N).
216 *> \endverbatim
217 *>
218 *> \param[out] RCOND
219 *> \verbatim
220 *> RCOND is REAL
221 *> The estimate of the reciprocal condition number of the matrix
222 *> A. If RCOND is less than the machine precision (in
223 *> particular, if RCOND = 0), the matrix is singular to working
224 *> precision. This condition is indicated by a return code of
225 *> INFO > 0.
226 *> \endverbatim
227 *>
228 *> \param[out] FERR
229 *> \verbatim
230 *> FERR is REAL array, dimension (NRHS)
231 *> The estimated forward error bound for each solution vector
232 *> X(j) (the j-th column of the solution matrix X).
233 *> If XTRUE is the true solution corresponding to X(j), FERR(j)
234 *> is an estimated upper bound for the magnitude of the largest
235 *> element in (X(j) - XTRUE) divided by the magnitude of the
236 *> largest element in X(j). The estimate is as reliable as
237 *> the estimate for RCOND, and is almost always a slight
238 *> overestimate of the true error.
239 *> \endverbatim
240 *>
241 *> \param[out] BERR
242 *> \verbatim
243 *> BERR is REAL array, dimension (NRHS)
244 *> The componentwise relative backward error of each solution
245 *> vector X(j) (i.e., the smallest relative change in
246 *> any element of A or B that makes X(j) an exact solution).
247 *> \endverbatim
248 *>
249 *> \param[out] WORK
250 *> \verbatim
251 *> WORK is REAL array, dimension (3*N)
252 *> \endverbatim
253 *>
254 *> \param[out] IWORK
255 *> \verbatim
256 *> IWORK is INTEGER array, dimension (N)
257 *> \endverbatim
258 *>
259 *> \param[out] INFO
260 *> \verbatim
261 *> INFO is INTEGER
262 *> = 0: successful exit
263 *> < 0: if INFO = -i, the i-th argument had an illegal value
264 *> > 0: if INFO = i, and i is
265 *> <= N: U(i,i) is exactly zero. The factorization
266 *> has not been completed unless i = N, but the
267 *> factor U is exactly singular, so the solution
268 *> and error bounds could not be computed.
269 *> RCOND = 0 is returned.
270 *> = N+1: U is nonsingular, but RCOND is less than machine
271 *> precision, meaning that the matrix is singular
272 *> to working precision. Nevertheless, the
273 *> solution and error bounds are computed because
274 *> there are a number of situations where the
275 *> computed solution can be more accurate than the
276 *> value of RCOND would suggest.
277 *> \endverbatim
278 *
279 * Authors:
280 * ========
281 *
282 *> \author Univ. of Tennessee
283 *> \author Univ. of California Berkeley
284 *> \author Univ. of Colorado Denver
285 *> \author NAG Ltd.
286 *
287 *> \date September 2012
288 *
289 *> \ingroup realGTsolve
290 *
291 * =====================================================================
292  SUBROUTINE sgtsvx( FACT, TRANS, N, NRHS, DL, D, DU, DLF, DF, DUF,
293  $ du2, ipiv, b, ldb, x, ldx, rcond, ferr, berr,
294  $ work, iwork, info )
295 *
296 * -- LAPACK driver routine (version 3.4.2) --
297 * -- LAPACK is a software package provided by Univ. of Tennessee, --
298 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
299 * September 2012
300 *
301 * .. Scalar Arguments ..
302  CHARACTER FACT, TRANS
303  INTEGER INFO, LDB, LDX, N, NRHS
304  REAL RCOND
305 * ..
306 * .. Array Arguments ..
307  INTEGER IPIV( * ), IWORK( * )
308  REAL B( ldb, * ), BERR( * ), D( * ), DF( * ),
309  $ dl( * ), dlf( * ), du( * ), du2( * ), duf( * ),
310  $ ferr( * ), work( * ), x( ldx, * )
311 * ..
312 *
313 * =====================================================================
314 *
315 * .. Parameters ..
316  REAL ZERO
317  parameter( zero = 0.0e+0 )
318 * ..
319 * .. Local Scalars ..
320  LOGICAL NOFACT, NOTRAN
321  CHARACTER NORM
322  REAL ANORM
323 * ..
324 * .. External Functions ..
325  LOGICAL LSAME
326  REAL SLAMCH, SLANGT
327  EXTERNAL lsame, slamch, slangt
328 * ..
329 * .. External Subroutines ..
330  EXTERNAL scopy, sgtcon, sgtrfs, sgttrf, sgttrs, slacpy,
331  $ xerbla
332 * ..
333 * .. Intrinsic Functions ..
334  INTRINSIC max
335 * ..
336 * .. Executable Statements ..
337 *
338  info = 0
339  nofact = lsame( fact, 'N' )
340  notran = lsame( trans, 'N' )
341  IF( .NOT.nofact .AND. .NOT.lsame( fact, 'F' ) ) THEN
342  info = -1
343  ELSE IF( .NOT.notran .AND. .NOT.lsame( trans, 'T' ) .AND. .NOT.
344  $ lsame( trans, 'C' ) ) THEN
345  info = -2
346  ELSE IF( n.LT.0 ) THEN
347  info = -3
348  ELSE IF( nrhs.LT.0 ) THEN
349  info = -4
350  ELSE IF( ldb.LT.max( 1, n ) ) THEN
351  info = -14
352  ELSE IF( ldx.LT.max( 1, n ) ) THEN
353  info = -16
354  END IF
355  IF( info.NE.0 ) THEN
356  CALL xerbla( 'SGTSVX', -info )
357  RETURN
358  END IF
359 *
360  IF( nofact ) THEN
361 *
362 * Compute the LU factorization of A.
363 *
364  CALL scopy( n, d, 1, df, 1 )
365  IF( n.GT.1 ) THEN
366  CALL scopy( n-1, dl, 1, dlf, 1 )
367  CALL scopy( n-1, du, 1, duf, 1 )
368  END IF
369  CALL sgttrf( n, dlf, df, duf, du2, ipiv, info )
370 *
371 * Return if INFO is non-zero.
372 *
373  IF( info.GT.0 )THEN
374  rcond = zero
375  RETURN
376  END IF
377  END IF
378 *
379 * Compute the norm of the matrix A.
380 *
381  IF( notran ) THEN
382  norm = '1'
383  ELSE
384  norm = 'I'
385  END IF
386  anorm = slangt( norm, n, dl, d, du )
387 *
388 * Compute the reciprocal of the condition number of A.
389 *
390  CALL sgtcon( norm, n, dlf, df, duf, du2, ipiv, anorm, rcond, work,
391  $ iwork, info )
392 *
393 * Compute the solution vectors X.
394 *
395  CALL slacpy( 'Full', n, nrhs, b, ldb, x, ldx )
396  CALL sgttrs( trans, n, nrhs, dlf, df, duf, du2, ipiv, x, ldx,
397  $ info )
398 *
399 * Use iterative refinement to improve the computed solutions and
400 * compute error bounds and backward error estimates for them.
401 *
402  CALL sgtrfs( trans, n, nrhs, dl, d, du, dlf, df, duf, du2, ipiv,
403  $ b, ldb, x, ldx, ferr, berr, work, iwork, info )
404 *
405 * Set INFO = N+1 if the matrix is singular to working precision.
406 *
407  IF( rcond.LT.slamch( 'Epsilon' ) )
408  $ info = n + 1
409 *
410  RETURN
411 *
412 * End of SGTSVX
413 *
414  END
subroutine scopy(N, SX, INCX, SY, INCY)
SCOPY
Definition: scopy.f:53
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:62
subroutine sgtrfs(TRANS, N, NRHS, DL, D, DU, DLF, DF, DUF, DU2, IPIV, B, LDB, X, LDX, FERR, BERR, WORK, IWORK, INFO)
SGTRFS
Definition: sgtrfs.f:211
subroutine sgtsvx(FACT, TRANS, N, NRHS, DL, D, DU, DLF, DF, DUF, DU2, IPIV, B, LDB, X, LDX, RCOND, FERR, BERR, WORK, IWORK, INFO)
SGTSVX computes the solution to system of linear equations A * X = B for GT matrices ...
Definition: sgtsvx.f:295
subroutine sgttrf(N, DL, D, DU, DU2, IPIV, INFO)
SGTTRF
Definition: sgttrf.f:126
subroutine sgtcon(NORM, N, DL, D, DU, DU2, IPIV, ANORM, RCOND, WORK, IWORK, INFO)
SGTCON
Definition: sgtcon.f:148
subroutine slacpy(UPLO, M, N, A, LDA, B, LDB)
SLACPY copies all or part of one two-dimensional array to another.
Definition: slacpy.f:105
subroutine sgttrs(TRANS, N, NRHS, DL, D, DU, DU2, IPIV, B, LDB, INFO)
SGTTRS
Definition: sgttrs.f:140