LAPACK  3.10.0
LAPACK: Linear Algebra PACKage

◆ cppequ()

subroutine cppequ ( character  UPLO,
integer  N,
complex, dimension( * )  AP,
real, dimension( * )  S,
real  SCOND,
real  AMAX,
integer  INFO 
)

CPPEQU

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Purpose:
 CPPEQU computes row and column scalings intended to equilibrate a
 Hermitian positive definite matrix A in packed storage and reduce
 its condition number (with respect to the two-norm).  S contains the
 scale factors, S(i)=1/sqrt(A(i,i)), chosen so that the scaled matrix
 B with elements B(i,j)=S(i)*A(i,j)*S(j) has ones on the diagonal.
 This choice of S puts the condition number of B within a factor N of
 the smallest possible condition number over all possible diagonal
 scalings.
Parameters
[in]UPLO
          UPLO is CHARACTER*1
          = 'U':  Upper triangle of A is stored;
          = 'L':  Lower triangle of A is stored.
[in]N
          N is INTEGER
          The order of the matrix A.  N >= 0.
[in]AP
          AP is COMPLEX array, dimension (N*(N+1)/2)
          The upper or lower triangle of the Hermitian matrix A, packed
          columnwise in a linear array.  The j-th column of A is stored
          in the array AP as follows:
          if UPLO = 'U', AP(i + (j-1)*j/2) = A(i,j) for 1<=i<=j;
          if UPLO = 'L', AP(i + (j-1)*(2n-j)/2) = A(i,j) for j<=i<=n.
[out]S
          S is REAL array, dimension (N)
          If INFO = 0, S contains the scale factors for A.
[out]SCOND
          SCOND is REAL
          If INFO = 0, S contains the ratio of the smallest S(i) to
          the largest S(i).  If SCOND >= 0.1 and AMAX is neither too
          large nor too small, it is not worth scaling by S.
[out]AMAX
          AMAX is REAL
          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, the i-th diagonal element is nonpositive.
Author
Univ. of Tennessee
Univ. of California Berkeley
Univ. of Colorado Denver
NAG Ltd.

Definition at line 116 of file cppequ.f.

117 *
118 * -- LAPACK computational routine --
119 * -- LAPACK is a software package provided by Univ. of Tennessee, --
120 * -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
121 *
122 * .. Scalar Arguments ..
123  CHARACTER UPLO
124  INTEGER INFO, N
125  REAL AMAX, SCOND
126 * ..
127 * .. Array Arguments ..
128  REAL S( * )
129  COMPLEX AP( * )
130 * ..
131 *
132 * =====================================================================
133 *
134 * .. Parameters ..
135  REAL ONE, ZERO
136  parameter( one = 1.0e+0, zero = 0.0e+0 )
137 * ..
138 * .. Local Scalars ..
139  LOGICAL UPPER
140  INTEGER I, JJ
141  REAL SMIN
142 * ..
143 * .. External Functions ..
144  LOGICAL LSAME
145  EXTERNAL lsame
146 * ..
147 * .. External Subroutines ..
148  EXTERNAL xerbla
149 * ..
150 * .. Intrinsic Functions ..
151  INTRINSIC max, min, real, sqrt
152 * ..
153 * .. Executable Statements ..
154 *
155 * Test the input parameters.
156 *
157  info = 0
158  upper = lsame( uplo, 'U' )
159  IF( .NOT.upper .AND. .NOT.lsame( uplo, 'L' ) ) THEN
160  info = -1
161  ELSE IF( n.LT.0 ) THEN
162  info = -2
163  END IF
164  IF( info.NE.0 ) THEN
165  CALL xerbla( 'CPPEQU', -info )
166  RETURN
167  END IF
168 *
169 * Quick return if possible
170 *
171  IF( n.EQ.0 ) THEN
172  scond = one
173  amax = zero
174  RETURN
175  END IF
176 *
177 * Initialize SMIN and AMAX.
178 *
179  s( 1 ) = real( ap( 1 ) )
180  smin = s( 1 )
181  amax = s( 1 )
182 *
183  IF( upper ) THEN
184 *
185 * UPLO = 'U': Upper triangle of A is stored.
186 * Find the minimum and maximum diagonal elements.
187 *
188  jj = 1
189  DO 10 i = 2, n
190  jj = jj + i
191  s( i ) = real( ap( jj ) )
192  smin = min( smin, s( i ) )
193  amax = max( amax, s( i ) )
194  10 CONTINUE
195 *
196  ELSE
197 *
198 * UPLO = 'L': Lower triangle of A is stored.
199 * Find the minimum and maximum diagonal elements.
200 *
201  jj = 1
202  DO 20 i = 2, n
203  jj = jj + n - i + 2
204  s( i ) = real( ap( jj ) )
205  smin = min( smin, s( i ) )
206  amax = max( amax, s( i ) )
207  20 CONTINUE
208  END IF
209 *
210  IF( smin.LE.zero ) THEN
211 *
212 * Find the first non-positive diagonal element and return.
213 *
214  DO 30 i = 1, n
215  IF( s( i ).LE.zero ) THEN
216  info = i
217  RETURN
218  END IF
219  30 CONTINUE
220  ELSE
221 *
222 * Set the scale factors to the reciprocals
223 * of the diagonal elements.
224 *
225  DO 40 i = 1, n
226  s( i ) = one / sqrt( s( i ) )
227  40 CONTINUE
228 *
229 * Compute SCOND = min(S(I)) / max(S(I))
230 *
231  scond = sqrt( smin ) / sqrt( amax )
232  END IF
233  RETURN
234 *
235 * End of CPPEQU
236 *
subroutine xerbla(SRNAME, INFO)
XERBLA
Definition: xerbla.f:60
logical function lsame(CA, CB)
LSAME
Definition: lsame.f:53
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