Developer Reference for Intel® oneAPI Math Kernel Library for Fortran

ID 766686
Date 3/22/2024
Public

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?syequb

Computes row and column scaling factors intended to equilibrate a symmetric indefinite matrix and reduce its condition number.

Syntax

call ssyequb( uplo, n, a, lda, s, scond, amax, work, info )

call dsyequb( uplo, n, a, lda, s, scond, amax, work, info )

call csyequb( uplo, n, a, lda, s, scond, amax, work, info )

call zsyequb( uplo, n, a, lda, s, scond, amax, work, info )

Include Files

  • mkl.fi, lapack.f90

Description

The routine computes row and column scalings intended to equilibrate a symmetric indefinite matrix A and reduce its condition number (with respect to the two-norm).

The array s contains the scale factors, s(i) = 1/sqrt(A(i,i)). These factors are 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.

Input Parameters

uplo

CHARACTER*1. Must be 'U' or 'L'.

Indicates whether the upper or lower triangular part of A is stored:

If uplo = 'U', the array a stores the upper triangular part of the matrix A.

If uplo = 'L', the array a stores the lower triangular part of the matrix A.

n

INTEGER. The order of the matrix A; n 0.

a, work

REAL for ssyequb

DOUBLE PRECISION for dsyequb

COMPLEX for csyequb

DOUBLE COMPLEX for zsyequb.

Array a: lda by *.

Contains the n-by-n symmetric indefinite matrix A whose scaling factors are to be computed. Only the diagonal elements of A are referenced. The second dimension of a must be at least max(1,n).

work(*) is a workspace array. The dimension of work is at least max(1,3*n).

lda

INTEGER. The leading dimension of a; lda max(1, m).

Output Parameters

s

REAL for single precision flavors

DOUBLE PRECISION for double precision flavors.

Array, size (n).

If info = 0, the array s contains the scale factors for A.

scond

REAL for single precision flavors

DOUBLE PRECISION for double precision flavors.

If info = 0, scond 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.

amax

REAL for single precision flavors

DOUBLE PRECISION for double precision flavors.

Absolute value of the largest element of the matrix A. If amax is very close to SMLNUM or BIGNUM, the matrix should be scaled.

info

INTEGER.

If info = 0, the execution is successful.

If info = -i, the i-th parameter had an illegal value.

If info = i, the i-th diagonal element of A is nonpositive.