Estimates the reciprocal of the condition number (in the 1-norm) of a real or complex symmetric matrix A using the factorization computed by ?sytrf_rk.

lapack_int LAPACKE_ssycon_3 (int matrix_layout, char uplo, lapack_int n, const float * A, lapack_int lda, const float * e, const lapack_int * ipiv, float anorm, float * rcond);

lapack_int LAPACKE_dsycon_3 (int matrix_layout, char uplo, lapack_int n, const double * A, lapack_int lda, const double * e, const lapack_int * ipiv, double anorm, double * rcond);

lapack_int LAPACKE_csycon_3 (int matrix_layout, char uplo, lapack_int n, const lapack_complex_float * A, lapack_int lda, const lapack_complex_float * e, const lapack_int * ipiv, float anorm, float * rcond);

lapack_int LAPACKE_zsycon_3 (int matrix_layout, char uplo, lapack_int n, const lapack_complex_double * A, lapack_int lda, const lapack_complex_double * e, const lapack_int * ipiv, double anorm, double * rcond);

## Description

?sycon_3 estimates the reciprocal of the condition number (in the 1-norm) of a real or complex symmetric matrix A using the factorization computed by ?sytrf_rk. A = P*U*D*(UT)*(PT) or A = P*L*D*(LT)*(PT), where U (or L) is unit upper (or lower) triangular matrix, UT (or LT) is the transpose of U (or L), P is a permutation matrix, PT is the transpose of P, and D is symmetric and block diagonal with 1-by-1 and 2-by-2 diagonal blocks.

An estimate is obtained for norm(inv(A)), and the reciprocal of the condition number is computed as rcond = 1 / (anorm * norm(inv(A))).

This routine uses BLAS3 solver ?sytrs_3.

## Input Parameters

matrix_layout

Specifies whether matrix storage layout is row major (LAPACK_ROW_MAJOR) or column major (LAPACK_COL_MAJOR).

uplo

Specifies whether the details of the factorization are stored as an upper or lower triangular matrix:

• = 'U': Upper triangular. The form is A = P*U*D*(UT)*(PT).
• = 'L': Lower triangular. The form is A = P*L*D*(LT)*(PT).
n

The order of the matrix A. n ≥ 0.

A

Array of size max(1, lda*n). Diagonal of the block diagonal matrix D and factors U or L as computed by ?sytrf_rk:

• Only diagonal elements of the symmetric block diagonal matrix D on the diagonal of A; that is, D(k,k) = A(k,k). Superdiagonal (or subdiagonal) elements of D should be provided on entry in array e).

—and—

• If uplo = 'U', factor U in the superdiagonal part of A. If uplo = 'L', factor L in the subdiagonal part of A.

lda

The leading dimension of the array A.

e

Array of size n. On entry, contains the superdiagonal (or subdiagonal) elements of the symmetric block diagonal matrix D with 1-by-1 or 2-by-2 diagonal blocks. If uplo = 'U', e(i) = D(i-1,i), i=2:N, and e(1) is not referenced. If uplo = 'L', e(i) = D(i+1,i), i=1:N-1, and e(n) is not referenced.

### Note

For 1-by-1 diagonal block D(k), where 1 ≤ kn, the element e[k-1] is not referenced in both the uplo = 'U' and uplo = 'L' cases.
ipiv

Array of size n. Details of the interchanges and the block structure of D as determined by ?sytrf_rk.

anorm

The 1-norm of the original matrix A.

## Output Parameters

rcond

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.

## Return Values

This function returns a value info.

= 0: Successful exit.

< 0: If info = -i, the ith argument had an illegal value.