LAPACK  3.4.2 LAPACK: Linear Algebra PACKage
sgbt01.f
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1 *> \brief \b SGBT01
2 *
3 * =========== DOCUMENTATION ===========
4 *
5 * Online html documentation available at
6 * http://www.netlib.org/lapack/explore-html/
7 *
8 * Definition:
9 * ===========
10 *
11 * SUBROUTINE SGBT01( M, N, KL, KU, A, LDA, AFAC, LDAFAC, IPIV, WORK,
12 * RESID )
13 *
14 * .. Scalar Arguments ..
15 * INTEGER KL, KU, LDA, LDAFAC, M, N
16 * REAL RESID
17 * ..
18 * .. Array Arguments ..
19 * INTEGER IPIV( * )
20 * REAL A( LDA, * ), AFAC( LDAFAC, * ), WORK( * )
21 * ..
22 *
23 *
24 *> \par Purpose:
25 * =============
26 *>
27 *> \verbatim
28 *>
29 *> SGBT01 reconstructs a band matrix A from its L*U factorization and
30 *> computes the residual:
31 *> norm(L*U - A) / ( N * norm(A) * EPS ),
32 *> where EPS is the machine epsilon.
33 *>
34 *> The expression L*U - A is computed one column at a time, so A and
35 *> AFAC are not modified.
36 *> \endverbatim
37 *
38 * Arguments:
39 * ==========
40 *
41 *> \param[in] M
42 *> \verbatim
43 *> M is INTEGER
44 *> The number of rows of the matrix A. M >= 0.
45 *> \endverbatim
46 *>
47 *> \param[in] N
48 *> \verbatim
49 *> N is INTEGER
50 *> The number of columns of the matrix A. N >= 0.
51 *> \endverbatim
52 *>
53 *> \param[in] KL
54 *> \verbatim
55 *> KL is INTEGER
56 *> The number of subdiagonals within the band of A. KL >= 0.
57 *> \endverbatim
58 *>
59 *> \param[in] KU
60 *> \verbatim
61 *> KU is INTEGER
62 *> The number of superdiagonals within the band of A. KU >= 0.
63 *> \endverbatim
64 *>
65 *> \param[in,out] A
66 *> \verbatim
67 *> A is REAL array, dimension (LDA,N)
68 *> The original matrix A in band storage, stored in rows 1 to
69 *> KL+KU+1.
70 *> \endverbatim
71 *>
72 *> \param[in] LDA
73 *> \verbatim
74 *> LDA is INTEGER.
75 *> The leading dimension of the array A. LDA >= max(1,KL+KU+1).
76 *> \endverbatim
77 *>
78 *> \param[in] AFAC
79 *> \verbatim
80 *> AFAC is REAL array, dimension (LDAFAC,N)
81 *> The factored form of the matrix A. AFAC contains the banded
82 *> factors L and U from the L*U factorization, as computed by
83 *> SGBTRF. U is stored as an upper triangular band matrix with
84 *> KL+KU superdiagonals in rows 1 to KL+KU+1, and the
85 *> multipliers used during the factorization are stored in rows
86 *> KL+KU+2 to 2*KL+KU+1. See SGBTRF for further details.
87 *> \endverbatim
88 *>
89 *> \param[in] LDAFAC
90 *> \verbatim
91 *> LDAFAC is INTEGER
92 *> The leading dimension of the array AFAC.
93 *> LDAFAC >= max(1,2*KL*KU+1).
94 *> \endverbatim
95 *>
96 *> \param[in] IPIV
97 *> \verbatim
98 *> IPIV is INTEGER array, dimension (min(M,N))
99 *> The pivot indices from SGBTRF.
100 *> \endverbatim
101 *>
102 *> \param[out] WORK
103 *> \verbatim
104 *> WORK is REAL array, dimension (2*KL+KU+1)
105 *> \endverbatim
106 *>
107 *> \param[out] RESID
108 *> \verbatim
109 *> RESID is REAL
110 *> norm(L*U - A) / ( N * norm(A) * EPS )
111 *> \endverbatim
112 *
113 * Authors:
114 * ========
115 *
116 *> \author Univ. of Tennessee
117 *> \author Univ. of California Berkeley
118 *> \author Univ. of Colorado Denver
119 *> \author NAG Ltd.
120 *
121 *> \date November 2011
122 *
123 *> \ingroup single_lin
124 *
125 * =====================================================================
126  SUBROUTINE sgbt01( M, N, KL, KU, A, LDA, AFAC, LDAFAC, IPIV, WORK,
127  \$ resid )
128 *
129 * -- LAPACK test routine (version 3.4.0) --
130 * -- LAPACK is a software package provided by Univ. of Tennessee, --
131 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
132 * November 2011
133 *
134 * .. Scalar Arguments ..
135  INTEGER kl, ku, lda, ldafac, m, n
136  REAL resid
137 * ..
138 * .. Array Arguments ..
139  INTEGER ipiv( * )
140  REAL a( lda, * ), afac( ldafac, * ), work( * )
141 * ..
142 *
143 * =====================================================================
144 *
145 * .. Parameters ..
146  REAL zero, one
147  parameter( zero = 0.0e+0, one = 1.0e+0 )
148 * ..
149 * .. Local Scalars ..
150  INTEGER i, i1, i2, il, ip, iw, j, jl, ju, jua, kd, lenj
151  REAL anorm, eps, t
152 * ..
153 * .. External Functions ..
154  REAL sasum, slamch
155  EXTERNAL sasum, slamch
156 * ..
157 * .. External Subroutines ..
158  EXTERNAL saxpy, scopy
159 * ..
160 * .. Intrinsic Functions ..
161  INTRINSIC max, min, real
162 * ..
163 * .. Executable Statements ..
164 *
165 * Quick exit if M = 0 or N = 0.
166 *
167  resid = zero
168  IF( m.LE.0 .OR. n.LE.0 )
169  \$ return
170 *
171 * Determine EPS and the norm of A.
172 *
173  eps = slamch( 'Epsilon' )
174  kd = ku + 1
175  anorm = zero
176  DO 10 j = 1, n
177  i1 = max( kd+1-j, 1 )
178  i2 = min( kd+m-j, kl+kd )
179  IF( i2.GE.i1 )
180  \$ anorm = max( anorm, sasum( i2-i1+1, a( i1, j ), 1 ) )
181  10 continue
182 *
183 * Compute one column at a time of L*U - A.
184 *
185  kd = kl + ku + 1
186  DO 40 j = 1, n
187 *
188 * Copy the J-th column of U to WORK.
189 *
190  ju = min( kl+ku, j-1 )
191  jl = min( kl, m-j )
192  lenj = min( m, j ) - j + ju + 1
193  IF( lenj.GT.0 ) THEN
194  CALL scopy( lenj, afac( kd-ju, j ), 1, work, 1 )
195  DO 20 i = lenj + 1, ju + jl + 1
196  work( i ) = zero
197  20 continue
198 *
199 * Multiply by the unit lower triangular matrix L. Note that L
200 * is stored as a product of transformations and permutations.
201 *
202  DO 30 i = min( m-1, j ), j - ju, -1
203  il = min( kl, m-i )
204  IF( il.GT.0 ) THEN
205  iw = i - j + ju + 1
206  t = work( iw )
207  CALL saxpy( il, t, afac( kd+1, i ), 1, work( iw+1 ),
208  \$ 1 )
209  ip = ipiv( i )
210  IF( i.NE.ip ) THEN
211  ip = ip - j + ju + 1
212  work( iw ) = work( ip )
213  work( ip ) = t
214  END IF
215  END IF
216  30 continue
217 *
218 * Subtract the corresponding column of A.
219 *
220  jua = min( ju, ku )
221  IF( jua+jl+1.GT.0 )
222  \$ CALL saxpy( jua+jl+1, -one, a( ku+1-jua, j ), 1,
223  \$ work( ju+1-jua ), 1 )
224 *
225 * Compute the 1-norm of the column.
226 *
227  resid = max( resid, sasum( ju+jl+1, work, 1 ) )
228  END IF
229  40 continue
230 *
231 * Compute norm( L*U - A ) / ( N * norm(A) * EPS )
232 *
233  IF( anorm.LE.zero ) THEN
234  IF( resid.NE.zero )
235  \$ resid = one / eps
236  ELSE
237  resid = ( ( resid / REAL( N ) ) / anorm ) / eps
238  END IF
239 *
240  return
241 *
242 * End of SGBT01
243 *
244  END