ZPPEQU(l) LAPACK routine (version 1.1) ZPPEQU(l)
NAME
ZPPEQU - compute row and column scalings intended to equilibrate a Hermi-
tian positive definite matrix A in packed storage and reduce its condition
number (with respect to the two-norm)
SYNOPSIS
SUBROUTINE ZPPEQU( UPLO, N, AP, S, SCOND, AMAX, INFO )
CHARACTER UPLO
INTEGER INFO, N
DOUBLE PRECISION AMAX, SCOND
DOUBLE PRECISION S( * )
COMPLEX*16 AP( * )
PURPOSE
ZPPEQU 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 condi-
tion number over all possible diagonal scalings.
ARGUMENTS
UPLO (input) CHARACTER*1
= 'U': Upper triangle of A is stored;
= 'L': Lower triangle of A is stored.
N (input) INTEGER
The order of the matrix A. N >= 0.
AP (input) COMPLEX*16 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.
S (output) DOUBLE PRECISION array, dimension (N)
If INFO = 0, S contains the scale factors for A.
SCOND (output) DOUBLE PRECISION
If INFO = 0, S contains the ratio of the smallest S(i) to the larg-
est S(i). If SCOND >= 0.1 and AMAX is neither too large nor too
small, it is not worth scaling by S.
AMAX (output) 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.
INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value
> 0: if INFO = i, the i-th diagonal entry is nonpositive.
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