LAPACK  3.10.1 LAPACK: Linear Algebra PACKage

## ◆ ctbt02()

 subroutine ctbt02 ( character UPLO, character TRANS, character DIAG, integer N, integer KD, integer NRHS, complex, dimension( ldab, * ) AB, integer LDAB, complex, dimension( ldx, * ) X, integer LDX, complex, dimension( ldb, * ) B, integer LDB, complex, dimension( * ) WORK, real, dimension( * ) RWORK, real RESID )

CTBT02

Purpose:
``` CTBT02 computes the residual for the computed solution to a
triangular system of linear equations op(A)*X = B, when A is a
triangular band matrix. The test ratio is the maximum over
norm(b - op(A)*x) / ( ||op(A)||_1 * norm(x) * EPS ),
where op(A) = A, A**T, or A**H, b is the column of B, x is the
solution vector, and EPS is the machine epsilon.```
Parameters
 [in] UPLO ``` UPLO is CHARACTER*1 Specifies whether the matrix A is upper or lower triangular. = 'U': Upper triangular = 'L': Lower triangular``` [in] TRANS ``` TRANS is CHARACTER*1 Specifies the operation applied to A. = 'N': A * X = B (No transpose) = 'T': A**T * X = B (Transpose) = 'C': A**H * X = B (Conjugate transpose)``` [in] DIAG ``` DIAG is CHARACTER*1 Specifies whether or not the matrix A is unit triangular. = 'N': Non-unit triangular = 'U': Unit triangular``` [in] N ``` N is INTEGER The order of the matrix A. N >= 0.``` [in] KD ``` KD is INTEGER The number of superdiagonals or subdiagonals of the triangular band matrix A. KD >= 0.``` [in] NRHS ``` NRHS is INTEGER The number of right hand sides, i.e., the number of columns of the matrices X and B. NRHS >= 0.``` [in] AB ``` AB is COMPLEX array, dimension (LDA,N) The upper or lower triangular band matrix A, stored in the first kd+1 rows of the array. The j-th column of A is stored in the j-th column of the array AB as follows: if UPLO = 'U', AB(kd+1+i-j,j) = A(i,j) for max(1,j-kd)<=i<=j; if UPLO = 'L', AB(1+i-j,j) = A(i,j) for j<=i<=min(n,j+kd).``` [in] LDAB ``` LDAB is INTEGER The leading dimension of the array AB. LDAB >= 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] B ``` B is COMPLEX array, dimension (LDB,NRHS) The right hand side vectors for the system of linear equations.``` [in] LDB ``` LDB is INTEGER The leading dimension of the array B. LDB >= max(1,N).``` [out] WORK ` WORK is COMPLEX array, dimension (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(op(A)*x - b) / ( norm(op(A)) * norm(x) * EPS ).```

Definition at line 157 of file ctbt02.f.

159 *
160 * -- LAPACK test routine --
161 * -- LAPACK is a software package provided by Univ. of Tennessee, --
162 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
163 *
164 * .. Scalar Arguments ..
165  CHARACTER DIAG, TRANS, UPLO
166  INTEGER KD, LDAB, LDB, LDX, N, NRHS
167  REAL RESID
168 * ..
169 * .. Array Arguments ..
170  REAL RWORK( * )
171  COMPLEX AB( LDAB, * ), B( LDB, * ), WORK( * ),
172  \$ X( LDX, * )
173 * ..
174 *
175 * =====================================================================
176 *
177 * .. Parameters ..
178  REAL ZERO, ONE
179  parameter( zero = 0.0e+0, one = 1.0e+0 )
180 * ..
181 * .. Local Scalars ..
182  INTEGER J
183  REAL ANORM, BNORM, EPS, XNORM
184 * ..
185 * .. External Functions ..
186  LOGICAL LSAME
187  REAL CLANTB, SCASUM, SLAMCH
188  EXTERNAL lsame, clantb, scasum, slamch
189 * ..
190 * .. External Subroutines ..
191  EXTERNAL caxpy, ccopy, ctbmv
192 * ..
193 * .. Intrinsic Functions ..
194  INTRINSIC cmplx, max
195 * ..
196 * .. Executable Statements ..
197 *
198 * Quick exit if N = 0 or NRHS = 0
199 *
200  IF( n.LE.0 .OR. nrhs.LE.0 ) THEN
201  resid = zero
202  RETURN
203  END IF
204 *
205 * Compute the 1-norm of op(A).
206 *
207  IF( lsame( trans, 'N' ) ) THEN
208  anorm = clantb( '1', uplo, diag, n, kd, ab, ldab, rwork )
209  ELSE
210  anorm = clantb( 'I', uplo, diag, n, kd, ab, ldab, rwork )
211  END IF
212 *
213 * Exit with RESID = 1/EPS if ANORM = 0.
214 *
215  eps = slamch( 'Epsilon' )
216  IF( anorm.LE.zero ) THEN
217  resid = one / eps
218  RETURN
219  END IF
220 *
221 * Compute the maximum over the number of right hand sides of
222 * norm(op(A)*x - b) / ( norm(op(A)) * norm(x) * EPS ).
223 *
224  resid = zero
225  DO 10 j = 1, nrhs
226  CALL ccopy( n, x( 1, j ), 1, work, 1 )
227  CALL ctbmv( uplo, trans, diag, n, kd, ab, ldab, work, 1 )
228  CALL caxpy( n, cmplx( -one ), b( 1, j ), 1, work, 1 )
229  bnorm = scasum( n, work, 1 )
230  xnorm = scasum( n, x( 1, j ), 1 )
231  IF( xnorm.LE.zero ) THEN
232  resid = one / eps
233  ELSE
234  resid = max( resid, ( ( bnorm / anorm ) / xnorm ) / eps )
235  END IF
236  10 CONTINUE
237 *
238  RETURN
239 *
240 * End of CTBT02
241 *
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine ccopy(N, CX, INCX, CY, INCY)
CCOPY
Definition: ccopy.f:81
subroutine caxpy(N, CA, CX, INCX, CY, INCY)
CAXPY
Definition: caxpy.f:88
subroutine ctbmv(UPLO, TRANS, DIAG, N, K, A, LDA, X, INCX)
CTBMV
Definition: ctbmv.f:186
real function clantb(NORM, UPLO, DIAG, N, K, AB, LDAB, WORK)
CLANTB returns the value of the 1-norm, or the Frobenius norm, or the infinity norm,...
Definition: clantb.f:141
real function scasum(N, CX, INCX)
SCASUM
Definition: scasum.f:72
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
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