 LAPACK  3.10.1 LAPACK: Linear Algebra PACKage

## ◆ cpbt02()

 subroutine cpbt02 ( character UPLO, integer N, integer KD, integer NRHS, complex, dimension( lda, * ) A, integer LDA, complex, dimension( ldx, * ) X, integer LDX, complex, dimension( ldb, * ) B, integer LDB, real, dimension( * ) RWORK, real RESID )

CPBT02

Purpose:
``` CPBT02 computes the residual for a solution of a Hermitian banded
system of equations  A*x = b:
RESID = norm( B - A*X ) / ( norm(A) * norm(X) * EPS)
where EPS is the machine precision.```
Parameters
 [in] UPLO ``` UPLO is CHARACTER*1 Specifies whether the upper or lower triangular part of the Hermitian matrix A is stored: = 'U': Upper triangular = 'L': Lower triangular``` [in] N ``` N is INTEGER The number of rows and columns of the matrix A. N >= 0.``` [in] KD ``` KD is INTEGER The number of super-diagonals of the matrix A if UPLO = 'U', or the number of sub-diagonals if UPLO = 'L'. KD >= 0.``` [in] NRHS ``` NRHS is INTEGER The number of right hand sides. NRHS >= 0.``` [in] A ``` A is COMPLEX array, dimension (LDA,N) The original Hermitian band matrix A. If UPLO = 'U', the upper triangular part of A is stored as a band matrix; if UPLO = 'L', the lower triangular part of A is stored. The columns of the appropriate triangle are stored in the columns of A and the diagonals of the triangle are stored in the rows of A. See CPBTRF for further details.``` [in] LDA ``` LDA is INTEGER. The leading dimension of the array A. LDA >= max(1,KD+1).``` [in] X ``` X is COMPLEX array, dimension (LDX,NRHS) The computed solution vectors for the system of linear equations.``` [in] LDX ``` LDX is INTEGER The leading dimension of the array X. LDX >= max(1,N).``` [in,out] B ``` B is COMPLEX array, dimension (LDB,NRHS) On entry, the right hand side vectors for the system of linear equations. On exit, B is overwritten with the difference B - A*X.``` [in] LDB ``` LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N).``` [out] RWORK ` RWORK is REAL array, dimension (N)` [out] RESID ``` RESID is REAL The maximum over the number of right hand sides of norm(B - A*X) / ( norm(A) * norm(X) * EPS ).```

Definition at line 134 of file cpbt02.f.

136 *
137 * -- LAPACK test routine --
138 * -- LAPACK is a software package provided by Univ. of Tennessee, --
139 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
140 *
141 * .. Scalar Arguments ..
142  CHARACTER UPLO
143  INTEGER KD, LDA, LDB, LDX, N, NRHS
144  REAL RESID
145 * ..
146 * .. Array Arguments ..
147  REAL RWORK( * )
148  COMPLEX A( LDA, * ), B( LDB, * ), X( LDX, * )
149 * ..
150 *
151 * =====================================================================
152 *
153 * .. Parameters ..
154  REAL ZERO, ONE
155  parameter( zero = 0.0e+0, one = 1.0e+0 )
156  COMPLEX CONE
157  parameter( cone = ( 1.0e+0, 0.0e+0 ) )
158 * ..
159 * .. Local Scalars ..
160  INTEGER J
161  REAL ANORM, BNORM, EPS, XNORM
162 * ..
163 * .. External Functions ..
164  REAL CLANHB, SCASUM, SLAMCH
165  EXTERNAL clanhb, scasum, slamch
166 * ..
167 * .. External Subroutines ..
168  EXTERNAL chbmv
169 * ..
170 * .. Intrinsic Functions ..
171  INTRINSIC max
172 * ..
173 * .. Executable Statements ..
174 *
175 * Quick exit if N = 0 or NRHS = 0.
176 *
177  IF( n.LE.0 .OR. nrhs.LE.0 ) THEN
178  resid = zero
179  RETURN
180  END IF
181 *
182 * Exit with RESID = 1/EPS if ANORM = 0.
183 *
184  eps = slamch( 'Epsilon' )
185  anorm = clanhb( '1', uplo, n, kd, a, lda, rwork )
186  IF( anorm.LE.zero ) THEN
187  resid = one / eps
188  RETURN
189  END IF
190 *
191 * Compute B - A*X
192 *
193  DO 10 j = 1, nrhs
194  CALL chbmv( uplo, n, kd, -cone, a, lda, x( 1, j ), 1, cone,
195  \$ b( 1, j ), 1 )
196  10 CONTINUE
197 *
198 * Compute the maximum over the number of right hand sides of
199 * norm( B - A*X ) / ( norm(A) * norm(X) * EPS )
200 *
201  resid = zero
202  DO 20 j = 1, nrhs
203  bnorm = scasum( n, b( 1, j ), 1 )
204  xnorm = scasum( n, x( 1, j ), 1 )
205  IF( xnorm.LE.zero ) THEN
206  resid = one / eps
207  ELSE
208  resid = max( resid, ( ( bnorm/anorm )/xnorm )/eps )
209  END IF
210  20 CONTINUE
211 *
212  RETURN
213 *
214 * End of CPBT02
215 *
subroutine chbmv(UPLO, N, K, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
CHBMV
Definition: chbmv.f:187
real function clanhb(NORM, UPLO, N, K, AB, LDAB, WORK)
CLANHB returns the value of the 1-norm, or the Frobenius norm, or the infinity norm,...
Definition: clanhb.f:132
real function scasum(N, CX, INCX)
SCASUM
Definition: scasum.f:72
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
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