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

 subroutine zgeequb ( integer M, integer N, complex*16, dimension( lda, * ) A, integer LDA, double precision, dimension( * ) R, double precision, dimension( * ) C, double precision ROWCND, double precision COLCND, double precision AMAX, integer INFO )

ZGEEQUB

Purpose:
``` ZGEEQUB computes row and column scalings intended to equilibrate an
M-by-N matrix A and reduce its condition number.  R returns the row
scale factors and C the column scale factors, chosen to try to make
the largest element in each row and column of the matrix B with
elements B(i,j)=R(i)*A(i,j)*C(j) have an absolute value of at most

R(i) and C(j) are restricted to be a power of the radix between
SMLNUM = smallest safe number and BIGNUM = largest safe number.  Use
of these scaling factors is not guaranteed to reduce the condition
number of A but works well in practice.

This routine differs from ZGEEQU by restricting the scaling factors
to a power of the radix.  Barring over- and underflow, scaling by
these factors introduces no additional rounding errors.  However, the
scaled entries' magnitudes are no longer approximately 1 but lie
Parameters
 [in] M ``` M is INTEGER The number of rows of the matrix A. M >= 0.``` [in] N ``` N is INTEGER The number of columns of the matrix A. N >= 0.``` [in] A ``` A is COMPLEX*16 array, dimension (LDA,N) The M-by-N matrix whose equilibration factors are to be computed.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,M).``` [out] R ``` R is DOUBLE PRECISION array, dimension (M) If INFO = 0 or INFO > M, R contains the row scale factors for A.``` [out] C ``` C is DOUBLE PRECISION array, dimension (N) If INFO = 0, C contains the column scale factors for A.``` [out] ROWCND ``` ROWCND is DOUBLE PRECISION If INFO = 0 or INFO > M, ROWCND contains the ratio of the smallest R(i) to the largest R(i). If ROWCND >= 0.1 and AMAX is neither too large nor too small, it is not worth scaling by R.``` [out] COLCND ``` COLCND is DOUBLE PRECISION If INFO = 0, COLCND contains the ratio of the smallest C(i) to the largest C(i). If COLCND >= 0.1, it is not worth scaling by C.``` [out] AMAX ``` AMAX is DOUBLE PRECISION Absolute value of largest matrix element. If AMAX is very close to overflow or very close to underflow, the matrix should be scaled.``` [out] INFO ``` INFO is INTEGER = 0: successful exit < 0: if INFO = -i, the i-th argument had an illegal value > 0: if INFO = i, and i is <= M: the i-th row of A is exactly zero > M: the (i-M)-th column of A is exactly zero```

Definition at line 145 of file zgeequb.f.

147*
148* -- LAPACK computational routine --
149* -- LAPACK is a software package provided by Univ. of Tennessee, --
150* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
151*
152* .. Scalar Arguments ..
153 INTEGER INFO, LDA, M, N
154 DOUBLE PRECISION AMAX, COLCND, ROWCND
155* ..
156* .. Array Arguments ..
157 DOUBLE PRECISION C( * ), R( * )
158 COMPLEX*16 A( LDA, * )
159* ..
160*
161* =====================================================================
162*
163* .. Parameters ..
164 DOUBLE PRECISION ONE, ZERO
165 parameter( one = 1.0d+0, zero = 0.0d+0 )
166* ..
167* .. Local Scalars ..
168 INTEGER I, J
169 DOUBLE PRECISION BIGNUM, RCMAX, RCMIN, SMLNUM, RADIX, LOGRDX
170 COMPLEX*16 ZDUM
171* ..
172* .. External Functions ..
173 DOUBLE PRECISION DLAMCH
174 EXTERNAL dlamch
175* ..
176* .. External Subroutines ..
177 EXTERNAL xerbla
178* ..
179* .. Intrinsic Functions ..
180 INTRINSIC abs, max, min, log, dble, dimag
181* ..
182* .. Statement Functions ..
183 DOUBLE PRECISION CABS1
184* ..
185* .. Statement Function definitions ..
186 cabs1( zdum ) = abs( dble( zdum ) ) + abs( dimag( zdum ) )
187* ..
188* .. Executable Statements ..
189*
190* Test the input parameters.
191*
192 info = 0
193 IF( m.LT.0 ) THEN
194 info = -1
195 ELSE IF( n.LT.0 ) THEN
196 info = -2
197 ELSE IF( lda.LT.max( 1, m ) ) THEN
198 info = -4
199 END IF
200 IF( info.NE.0 ) THEN
201 CALL xerbla( 'ZGEEQUB', -info )
202 RETURN
203 END IF
204*
205* Quick return if possible.
206*
207 IF( m.EQ.0 .OR. n.EQ.0 ) THEN
208 rowcnd = one
209 colcnd = one
210 amax = zero
211 RETURN
212 END IF
213*
214* Get machine constants. Assume SMLNUM is a power of the radix.
215*
216 smlnum = dlamch( 'S' )
217 bignum = one / smlnum
218 radix = dlamch( 'B' )
219 logrdx = log( radix )
220*
221* Compute row scale factors.
222*
223 DO 10 i = 1, m
224 r( i ) = zero
225 10 CONTINUE
226*
227* Find the maximum element in each row.
228*
229 DO 30 j = 1, n
230 DO 20 i = 1, m
231 r( i ) = max( r( i ), cabs1( a( i, j ) ) )
232 20 CONTINUE
233 30 CONTINUE
234 DO i = 1, m
235 IF( r( i ).GT.zero ) THEN
236 r( i ) = radix**int( log(r( i ) ) / logrdx )
237 END IF
238 END DO
239*
240* Find the maximum and minimum scale factors.
241*
242 rcmin = bignum
243 rcmax = zero
244 DO 40 i = 1, m
245 rcmax = max( rcmax, r( i ) )
246 rcmin = min( rcmin, r( i ) )
247 40 CONTINUE
248 amax = rcmax
249*
250 IF( rcmin.EQ.zero ) THEN
251*
252* Find the first zero scale factor and return an error code.
253*
254 DO 50 i = 1, m
255 IF( r( i ).EQ.zero ) THEN
256 info = i
257 RETURN
258 END IF
259 50 CONTINUE
260 ELSE
261*
262* Invert the scale factors.
263*
264 DO 60 i = 1, m
265 r( i ) = one / min( max( r( i ), smlnum ), bignum )
266 60 CONTINUE
267*
268* Compute ROWCND = min(R(I)) / max(R(I)).
269*
270 rowcnd = max( rcmin, smlnum ) / min( rcmax, bignum )
271 END IF
272*
273* Compute column scale factors.
274*
275 DO 70 j = 1, n
276 c( j ) = zero
277 70 CONTINUE
278*
279* Find the maximum element in each column,
280* assuming the row scaling computed above.
281*
282 DO 90 j = 1, n
283 DO 80 i = 1, m
284 c( j ) = max( c( j ), cabs1( a( i, j ) )*r( i ) )
285 80 CONTINUE
286 IF( c( j ).GT.zero ) THEN
287 c( j ) = radix**int( log( c( j ) ) / logrdx )
288 END IF
289 90 CONTINUE
290*
291* Find the maximum and minimum scale factors.
292*
293 rcmin = bignum
294 rcmax = zero
295 DO 100 j = 1, n
296 rcmin = min( rcmin, c( j ) )
297 rcmax = max( rcmax, c( j ) )
298 100 CONTINUE
299*
300 IF( rcmin.EQ.zero ) THEN
301*
302* Find the first zero scale factor and return an error code.
303*
304 DO 110 j = 1, n
305 IF( c( j ).EQ.zero ) THEN
306 info = m + j
307 RETURN
308 END IF
309 110 CONTINUE
310 ELSE
311*
312* Invert the scale factors.
313*
314 DO 120 j = 1, n
315 c( j ) = one / min( max( c( j ), smlnum ), bignum )
316 120 CONTINUE
317*
318* Compute COLCND = min(C(J)) / max(C(J)).
319*
320 colcnd = max( rcmin, smlnum ) / min( rcmax, bignum )
321 END IF
322*
323 RETURN
324*
325* End of ZGEEQUB
326*
double precision function dlamch(CMACH)
DLAMCH
Definition: dlamch.f:69
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
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