1*> \brief \b DPBT05
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 DPBT05( UPLO, N, KD, NRHS, AB, LDAB, B, LDB, X, LDX,
12*                          XACT, LDXACT, FERR, BERR, RESLTS )
13*
14*       .. Scalar Arguments ..
15*       CHARACTER          UPLO
16*       INTEGER            KD, LDAB, LDB, LDX, LDXACT, N, NRHS
17*       ..
18*       .. Array Arguments ..
19*       DOUBLE PRECISION   AB( LDAB, * ), B( LDB, * ), BERR( * ),
20*      $                   FERR( * ), RESLTS( * ), X( LDX, * ),
21*      $                   XACT( LDXACT, * )
22*       ..
23*
24*
25*> \par Purpose:
26*  =============
27*>
28*> \verbatim
29*>
30*> DPBT05 tests the error bounds from iterative refinement for the
31*> computed solution to a system of equations A*X = B, where A is a
32*> symmetric band matrix.
33*>
34*> RESLTS(1) = test of the error bound
35*>           = norm(X - XACT) / ( norm(X) * FERR )
36*>
37*> A large value is returned if this ratio is not less than one.
38*>
39*> RESLTS(2) = residual from the iterative refinement routine
40*>           = the maximum of BERR / ( NZ*EPS + (*) ), where
41*>             (*) = NZ*UNFL / (min_i (abs(A)*abs(X) +abs(b))_i )
42*>             and NZ = max. number of nonzeros in any row of A, plus 1
43*> \endverbatim
44*
45*  Arguments:
46*  ==========
47*
48*> \param[in] UPLO
49*> \verbatim
50*>          UPLO is CHARACTER*1
51*>          Specifies whether the upper or lower triangular part of the
52*>          symmetric matrix A is stored.
53*>          = 'U':  Upper triangular
54*>          = 'L':  Lower triangular
55*> \endverbatim
56*>
57*> \param[in] N
58*> \verbatim
59*>          N is INTEGER
60*>          The number of rows of the matrices X, B, and XACT, and the
61*>          order of the matrix A.  N >= 0.
62*> \endverbatim
63*>
64*> \param[in] KD
65*> \verbatim
66*>          KD is INTEGER
67*>          The number of super-diagonals of the matrix A if UPLO = 'U',
68*>          or the number of sub-diagonals if UPLO = 'L'.  KD >= 0.
69*> \endverbatim
70*>
71*> \param[in] NRHS
72*> \verbatim
73*>          NRHS is INTEGER
74*>          The number of columns of the matrices X, B, and XACT.
75*>          NRHS >= 0.
76*> \endverbatim
77*>
78*> \param[in] AB
79*> \verbatim
80*>          AB is DOUBLE PRECISION array, dimension (LDAB,N)
81*>          The upper or lower triangle of the symmetric band matrix A,
82*>          stored in the first KD+1 rows of the array.  The j-th column
83*>          of A is stored in the j-th column of the array AB as follows:
84*>          if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j;
85*>          if UPLO = 'L', AB(1+i-j,j)    = A(i,j) for j<=i<=min(n,j+kd).
86*> \endverbatim
87*>
88*> \param[in] LDAB
89*> \verbatim
90*>          LDAB is INTEGER
91*>          The leading dimension of the array AB.  LDAB >= KD+1.
92*> \endverbatim
93*>
94*> \param[in] B
95*> \verbatim
96*>          B is DOUBLE PRECISION array, dimension (LDB,NRHS)
97*>          The right hand side vectors for the system of linear
98*>          equations.
99*> \endverbatim
100*>
101*> \param[in] LDB
102*> \verbatim
103*>          LDB is INTEGER
104*>          The leading dimension of the array B.  LDB >= max(1,N).
105*> \endverbatim
106*>
107*> \param[in] X
108*> \verbatim
109*>          X is DOUBLE PRECISION array, dimension (LDX,NRHS)
110*>          The computed solution vectors.  Each vector is stored as a
111*>          column of the matrix X.
112*> \endverbatim
113*>
114*> \param[in] LDX
115*> \verbatim
116*>          LDX is INTEGER
117*>          The leading dimension of the array X.  LDX >= max(1,N).
118*> \endverbatim
119*>
120*> \param[in] XACT
121*> \verbatim
122*>          XACT is DOUBLE PRECISION array, dimension (LDX,NRHS)
123*>          The exact solution vectors.  Each vector is stored as a
124*>          column of the matrix XACT.
125*> \endverbatim
126*>
127*> \param[in] LDXACT
128*> \verbatim
129*>          LDXACT is INTEGER
130*>          The leading dimension of the array XACT.  LDXACT >= max(1,N).
131*> \endverbatim
132*>
133*> \param[in] FERR
134*> \verbatim
135*>          FERR is DOUBLE PRECISION array, dimension (NRHS)
136*>          The estimated forward error bounds for each solution vector
137*>          X.  If XTRUE is the true solution, FERR bounds the magnitude
138*>          of the largest entry in (X - XTRUE) divided by the magnitude
139*>          of the largest entry in X.
140*> \endverbatim
141*>
142*> \param[in] BERR
143*> \verbatim
144*>          BERR is DOUBLE PRECISION array, dimension (NRHS)
145*>          The componentwise relative backward error of each solution
146*>          vector (i.e., the smallest relative change in any entry of A
147*>          or B that makes X an exact solution).
148*> \endverbatim
149*>
150*> \param[out] RESLTS
151*> \verbatim
152*>          RESLTS is DOUBLE PRECISION array, dimension (2)
153*>          The maximum over the NRHS solution vectors of the ratios:
154*>          RESLTS(1) = norm(X - XACT) / ( norm(X) * FERR )
155*>          RESLTS(2) = BERR / ( NZ*EPS + (*) )
156*> \endverbatim
157*
158*  Authors:
159*  ========
160*
161*> \author Univ. of Tennessee
162*> \author Univ. of California Berkeley
163*> \author Univ. of Colorado Denver
164*> \author NAG Ltd.
165*
166*> \date November 2011
167*
168*> \ingroup double_lin
169*
170*  =====================================================================
171      SUBROUTINE DPBT05( UPLO, N, KD, NRHS, AB, LDAB, B, LDB, X, LDX,
172     $                   XACT, LDXACT, FERR, BERR, RESLTS )
173*
174*  -- LAPACK test routine (version 3.4.0) --
175*  -- LAPACK is a software package provided by Univ. of Tennessee,    --
176*  -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
177*     November 2011
178*
179*     .. Scalar Arguments ..
180      CHARACTER          UPLO
181      INTEGER            KD, LDAB, LDB, LDX, LDXACT, N, NRHS
182*     ..
183*     .. Array Arguments ..
184      DOUBLE PRECISION   AB( LDAB, * ), B( LDB, * ), BERR( * ),
185     $                   FERR( * ), RESLTS( * ), X( LDX, * ),
186     $                   XACT( LDXACT, * )
187*     ..
188*
189*  =====================================================================
190*
191*     .. Parameters ..
192      DOUBLE PRECISION   ZERO, ONE
193      PARAMETER          ( ZERO = 0.0D+0, ONE = 1.0D+0 )
194*     ..
195*     .. Local Scalars ..
196      LOGICAL            UPPER
197      INTEGER            I, IMAX, J, K, NZ
198      DOUBLE PRECISION   AXBI, DIFF, EPS, ERRBND, OVFL, TMP, UNFL, XNORM
199*     ..
200*     .. External Functions ..
201      LOGICAL            LSAME
202      INTEGER            IDAMAX
203      DOUBLE PRECISION   DLAMCH
204      EXTERNAL           LSAME, IDAMAX, DLAMCH
205*     ..
206*     .. Intrinsic Functions ..
207      INTRINSIC          ABS, MAX, MIN
208*     ..
209*     .. Executable Statements ..
210*
211*     Quick exit if N = 0 or NRHS = 0.
212*
213      IF( N.LE.0 .OR. NRHS.LE.0 ) THEN
214         RESLTS( 1 ) = ZERO
215         RESLTS( 2 ) = ZERO
216         RETURN
217      END IF
218*
219      EPS = DLAMCH( 'Epsilon' )
220      UNFL = DLAMCH( 'Safe minimum' )
221      OVFL = ONE / UNFL
222      UPPER = LSAME( UPLO, 'U' )
223      NZ = 2*MAX( KD, N-1 ) + 1
224*
225*     Test 1:  Compute the maximum of
226*        norm(X - XACT) / ( norm(X) * FERR )
227*     over all the vectors X and XACT using the infinity-norm.
228*
229      ERRBND = ZERO
230      DO 30 J = 1, NRHS
231         IMAX = IDAMAX( N, X( 1, J ), 1 )
232         XNORM = MAX( ABS( X( IMAX, J ) ), UNFL )
233         DIFF = ZERO
234         DO 10 I = 1, N
235            DIFF = MAX( DIFF, ABS( X( I, J )-XACT( I, J ) ) )
236   10    CONTINUE
237*
238         IF( XNORM.GT.ONE ) THEN
239            GO TO 20
240         ELSE IF( DIFF.LE.OVFL*XNORM ) THEN
241            GO TO 20
242         ELSE
243            ERRBND = ONE / EPS
244            GO TO 30
245         END IF
246*
247   20    CONTINUE
248         IF( DIFF / XNORM.LE.FERR( J ) ) THEN
249            ERRBND = MAX( ERRBND, ( DIFF / XNORM ) / FERR( J ) )
250         ELSE
251            ERRBND = ONE / EPS
252         END IF
253   30 CONTINUE
254      RESLTS( 1 ) = ERRBND
255*
256*     Test 2:  Compute the maximum of BERR / ( NZ*EPS + (*) ), where
257*     (*) = NZ*UNFL / (min_i (abs(A)*abs(X) +abs(b))_i )
258*
259      DO 90 K = 1, NRHS
260         DO 80 I = 1, N
261            TMP = ABS( B( I, K ) )
262            IF( UPPER ) THEN
263               DO 40 J = MAX( I-KD, 1 ), I
264                  TMP = TMP + ABS( AB( KD+1-I+J, I ) )*ABS( X( J, K ) )
265   40          CONTINUE
266               DO 50 J = I + 1, MIN( I+KD, N )
267                  TMP = TMP + ABS( AB( KD+1+I-J, J ) )*ABS( X( J, K ) )
268   50          CONTINUE
269            ELSE
270               DO 60 J = MAX( I-KD, 1 ), I - 1
271                  TMP = TMP + ABS( AB( 1+I-J, J ) )*ABS( X( J, K ) )
272   60          CONTINUE
273               DO 70 J = I, MIN( I+KD, N )
274                  TMP = TMP + ABS( AB( 1+J-I, I ) )*ABS( X( J, K ) )
275   70          CONTINUE
276            END IF
277            IF( I.EQ.1 ) THEN
278               AXBI = TMP
279            ELSE
280               AXBI = MIN( AXBI, TMP )
281            END IF
282   80    CONTINUE
283         TMP = BERR( K ) / ( NZ*EPS+NZ*UNFL / MAX( AXBI, NZ*UNFL ) )
284         IF( K.EQ.1 ) THEN
285            RESLTS( 2 ) = TMP
286         ELSE
287            RESLTS( 2 ) = MAX( RESLTS( 2 ), TMP )
288         END IF
289   90 CONTINUE
290*
291      RETURN
292*
293*     End of DPBT05
294*
295      END
296