LAPACK  3.8.0
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 ).
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.
Date
December 2016

Definition at line 138 of file cpbt02.f.

138 *
139 * -- LAPACK test routine (version 3.7.0) --
140 * -- LAPACK is a software package provided by Univ. of Tennessee, --
141 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
142 * December 2016
143 *
144 * .. Scalar Arguments ..
145  CHARACTER uplo
146  INTEGER kd, lda, ldb, ldx, n, nrhs
147  REAL resid
148 * ..
149 * .. Array Arguments ..
150  REAL rwork( * )
151  COMPLEX a( lda, * ), b( ldb, * ), x( ldx, * )
152 * ..
153 *
154 * =====================================================================
155 *
156 * .. Parameters ..
157  REAL zero, one
158  parameter( zero = 0.0e+0, one = 1.0e+0 )
159  COMPLEX cone
160  parameter( cone = ( 1.0e+0, 0.0e+0 ) )
161 * ..
162 * .. Local Scalars ..
163  INTEGER j
164  REAL anorm, bnorm, eps, xnorm
165 * ..
166 * .. External Functions ..
167  REAL clanhb, scasum, slamch
168  EXTERNAL clanhb, scasum, slamch
169 * ..
170 * .. External Subroutines ..
171  EXTERNAL chbmv
172 * ..
173 * .. Intrinsic Functions ..
174  INTRINSIC max
175 * ..
176 * .. Executable Statements ..
177 *
178 * Quick exit if N = 0 or NRHS = 0.
179 *
180  IF( n.LE.0 .OR. nrhs.LE.0 ) THEN
181  resid = zero
182  RETURN
183  END IF
184 *
185 * Exit with RESID = 1/EPS if ANORM = 0.
186 *
187  eps = slamch( 'Epsilon' )
188  anorm = clanhb( '1', uplo, n, kd, a, lda, rwork )
189  IF( anorm.LE.zero ) THEN
190  resid = one / eps
191  RETURN
192  END IF
193 *
194 * Compute B - A*X
195 *
196  DO 10 j = 1, nrhs
197  CALL chbmv( uplo, n, kd, -cone, a, lda, x( 1, j ), 1, cone,
198  $ b( 1, j ), 1 )
199  10 CONTINUE
200 *
201 * Compute the maximum over the number of right hand sides of
202 * norm( B - A*X ) / ( norm(A) * norm(X) * EPS )
203 *
204  resid = zero
205  DO 20 j = 1, nrhs
206  bnorm = scasum( n, b( 1, j ), 1 )
207  xnorm = scasum( n, x( 1, j ), 1 )
208  IF( xnorm.LE.zero ) THEN
209  resid = one / eps
210  ELSE
211  resid = max( resid, ( ( bnorm/anorm )/xnorm )/eps )
212  END IF
213  20 CONTINUE
214 *
215  RETURN
216 *
217 * End of CPBT02
218 *
subroutine chbmv(UPLO, N, K, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
CHBMV
Definition: chbmv.f:189
real function scasum(N, CX, INCX)
SCASUM
Definition: scasum.f:74
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, or the element of largest absolute value of a Hermitian band matrix.
Definition: clanhb.f:134
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:69
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