LAPACK 3.11.0 LAPACK: Linear Algebra PACKage
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## ◆ cpbcon()

 subroutine cpbcon ( character UPLO, integer N, integer KD, complex, dimension( ldab, * ) AB, integer LDAB, real ANORM, real RCOND, complex, dimension( * ) WORK, real, dimension( * ) RWORK, integer INFO )

CPBCON

Purpose:
``` CPBCON estimates the reciprocal of the condition number (in the
1-norm) of a complex Hermitian positive definite band matrix using
the Cholesky factorization A = U**H*U or A = L*L**H computed by
CPBTRF.

An estimate is obtained for norm(inv(A)), and the reciprocal of the
condition number is computed as RCOND = 1 / (ANORM * norm(inv(A))).```
Parameters
 [in] UPLO ``` UPLO is CHARACTER*1 = 'U': Upper triangular factor stored in AB; = 'L': Lower triangular factor stored in AB.``` [in] N ``` N is INTEGER The order of the matrix A. N >= 0.``` [in] KD ``` KD is INTEGER The number of superdiagonals of the matrix A if UPLO = 'U', or the number of sub-diagonals if UPLO = 'L'. KD >= 0.``` [in] AB ``` AB is COMPLEX array, dimension (LDAB,N) The triangular factor U or L from the Cholesky factorization A = U**H*U or A = L*L**H of the band matrix A, stored in the first KD+1 rows of the array. The j-th column of U or L is stored in the j-th column of the array AB as follows: if UPLO ='U', AB(kd+1+i-j,j) = U(i,j) for max(1,j-kd)<=i<=j; if UPLO ='L', AB(1+i-j,j) = L(i,j) for j<=i<=min(n,j+kd).``` [in] LDAB ``` LDAB is INTEGER The leading dimension of the array AB. LDAB >= KD+1.``` [in] ANORM ``` ANORM is REAL The 1-norm (or infinity-norm) of the Hermitian band matrix A.``` [out] RCOND ``` RCOND is REAL The reciprocal of the condition number of the matrix A, computed as RCOND = 1/(ANORM * AINVNM), where AINVNM is an estimate of the 1-norm of inv(A) computed in this routine.``` [out] WORK ` WORK is COMPLEX array, dimension (2*N)` [out] RWORK ` RWORK is REAL array, dimension (N)` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value```

Definition at line 131 of file cpbcon.f.

133*
134* -- LAPACK computational routine --
135* -- LAPACK is a software package provided by Univ. of Tennessee, --
136* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
137*
138* .. Scalar Arguments ..
139 CHARACTER UPLO
140 INTEGER INFO, KD, LDAB, N
141 REAL ANORM, RCOND
142* ..
143* .. Array Arguments ..
144 REAL RWORK( * )
145 COMPLEX AB( LDAB, * ), WORK( * )
146* ..
147*
148* =====================================================================
149*
150* .. Parameters ..
151 REAL ONE, ZERO
152 parameter( one = 1.0e+0, zero = 0.0e+0 )
153* ..
154* .. Local Scalars ..
155 LOGICAL UPPER
156 CHARACTER NORMIN
157 INTEGER IX, KASE
158 REAL AINVNM, SCALE, SCALEL, SCALEU, SMLNUM
159 COMPLEX ZDUM
160* ..
161* .. Local Arrays ..
162 INTEGER ISAVE( 3 )
163* ..
164* .. External Functions ..
165 LOGICAL LSAME
166 INTEGER ICAMAX
167 REAL SLAMCH
168 EXTERNAL lsame, icamax, slamch
169* ..
170* .. External Subroutines ..
171 EXTERNAL clacn2, clatbs, csrscl, xerbla
172* ..
173* .. Intrinsic Functions ..
174 INTRINSIC abs, aimag, real
175* ..
176* .. Statement Functions ..
177 REAL CABS1
178* ..
179* .. Statement Function definitions ..
180 cabs1( zdum ) = abs( real( zdum ) ) + abs( aimag( zdum ) )
181* ..
182* .. Executable Statements ..
183*
184* Test the input parameters.
185*
186 info = 0
187 upper = lsame( uplo, 'U' )
188 IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
189 info = -1
190 ELSE IF( n.LT.0 ) THEN
191 info = -2
192 ELSE IF( kd.LT.0 ) THEN
193 info = -3
194 ELSE IF( ldab.LT.kd+1 ) THEN
195 info = -5
196 ELSE IF( anorm.LT.zero ) THEN
197 info = -6
198 END IF
199 IF( info.NE.0 ) THEN
200 CALL xerbla( 'CPBCON', -info )
201 RETURN
202 END IF
203*
204* Quick return if possible
205*
206 rcond = zero
207 IF( n.EQ.0 ) THEN
208 rcond = one
209 RETURN
210 ELSE IF( anorm.EQ.zero ) THEN
211 RETURN
212 END IF
213*
214 smlnum = slamch( 'Safe minimum' )
215*
216* Estimate the 1-norm of the inverse.
217*
218 kase = 0
219 normin = 'N'
220 10 CONTINUE
221 CALL clacn2( n, work( n+1 ), work, ainvnm, kase, isave )
222 IF( kase.NE.0 ) THEN
223 IF( upper ) THEN
224*
225* Multiply by inv(U**H).
226*
227 CALL clatbs( 'Upper', 'Conjugate transpose', 'Non-unit',
228 \$ normin, n, kd, ab, ldab, work, scalel, rwork,
229 \$ info )
230 normin = 'Y'
231*
232* Multiply by inv(U).
233*
234 CALL clatbs( 'Upper', 'No transpose', 'Non-unit', normin, n,
235 \$ kd, ab, ldab, work, scaleu, rwork, info )
236 ELSE
237*
238* Multiply by inv(L).
239*
240 CALL clatbs( 'Lower', 'No transpose', 'Non-unit', normin, n,
241 \$ kd, ab, ldab, work, scalel, rwork, info )
242 normin = 'Y'
243*
244* Multiply by inv(L**H).
245*
246 CALL clatbs( 'Lower', 'Conjugate transpose', 'Non-unit',
247 \$ normin, n, kd, ab, ldab, work, scaleu, rwork,
248 \$ info )
249 END IF
250*
251* Multiply by 1/SCALE if doing so will not cause overflow.
252*
253 scale = scalel*scaleu
254 IF( scale.NE.one ) THEN
255 ix = icamax( n, work, 1 )
256 IF( scale.LT.cabs1( work( ix ) )*smlnum .OR. scale.EQ.zero )
257 \$ GO TO 20
258 CALL csrscl( n, scale, work, 1 )
259 END IF
260 GO TO 10
261 END IF
262*
263* Compute the estimate of the reciprocal condition number.
264*
265 IF( ainvnm.NE.zero )
266 \$ rcond = ( one / ainvnm ) / anorm
267*
268 20 CONTINUE
269*
270 RETURN
271*
272* End of CPBCON
273*
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
integer function icamax(N, CX, INCX)
ICAMAX
Definition: icamax.f:71
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
subroutine csrscl(N, SA, SX, INCX)
CSRSCL multiplies a vector by the reciprocal of a real scalar.
Definition: csrscl.f:84
subroutine clatbs(UPLO, TRANS, DIAG, NORMIN, N, KD, AB, LDAB, X, SCALE, CNORM, INFO)
CLATBS solves a triangular banded system of equations.
Definition: clatbs.f:243
subroutine clacn2(N, V, X, EST, KASE, ISAVE)
CLACN2 estimates the 1-norm of a square matrix, using reverse communication for evaluating matrix-vec...
Definition: clacn2.f:133
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
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