?sycon

Estimates the reciprocal of the condition number of a symmetric matrix.

Syntax

FORTRAN 77:

call ssycon( uplo, n, a, lda, ipiv, anorm, rcond, work, iwork, info )

call dsycon( uplo, n, a, lda, ipiv, anorm, rcond, work, iwork, info )

call csycon( uplo, n, a, lda, ipiv, anorm, rcond, work, info )

call zsycon( uplo, n, a, lda, ipiv, anorm, rcond, work, info )

FORTRAN 95:

call sycon( a, ipiv, anorm, rcond [,uplo] [,info] )

lapack_int LAPACKE_ssycon( int matrix_order, char uplo, lapack_int n, const float* a, lapack_int lda, const lapack_int* ipiv, float anorm, float* rcond );

lapack_int LAPACKE_dsycon( int matrix_order, char uplo, lapack_int n, const double* a, lapack_int lda, const lapack_int* ipiv, double anorm, double* rcond );

lapack_int LAPACKE_csycon( int matrix_order, char uplo, lapack_int n, const lapack_complex_float* a, lapack_int lda, const lapack_int* ipiv, float anorm, float* rcond );

lapack_int LAPACKE_zsycon( int matrix_order, char uplo, lapack_int n, const lapack_complex_double* a, lapack_int lda, const lapack_int* ipiv, double anorm, double* rcond );

Include Files

Fortran: mkl_lapack.fi and mkl_lapack.h
Fortran 95: lapack.f90
C: mkl_lapacke.h

Description

The routine estimates the reciprocal of the condition number of a symmetric matrix A:

κ₁(A) = ||A||₁ ||A^-1||₁ (since A is symmetric, κ_∞(A) = κ₁(A)).

Before calling this routine:

compute anorm (either ||A||₁ = max_j Σ_i |a_ij| or ||A||_∞ = max_i Σ_j |a_ij|)
call ?sytrf to compute the factorization of A.

Input Parameters

The data types are given for the Fortran interface. A <datatype> placeholder, if present, is used for the C interface data types in the C interface section above. See C Interface Conventions for the C interface principal conventions and type definitions.

uplo	CHARACTER1. Must be 'U' or 'L'. Indicates how the input matrix `A` has been factored: If `uplo = 'U'`, the array `a` stores the upper triangular factor `U` of the factorization `A` = `PUDU^TP^T`. If `uplo = 'L'`, the array `a` stores the lower triangular factor `L` of the factorization `A` = `PLDL^T*P^T`.
n	INTEGER. The order of matrix `A`; `n` ≥ 0.
a, work	REAL for ssycon DOUBLE PRECISION for dsycon COMPLEX for csycon DOUBLE COMPLEX for zsycon. Arrays: `a(lda,)`, `work()`. The array `a` contains the factored matrix `A`, as returned by ?sytrf. The second dimension of `a` must be at least `max(1,n)`. The array `work` is a workspace for the routine. The dimension of `work` must be at least `max(1, 2*n)`.
lda	INTEGER. The leading dimension of `a`; `lda ≥ max(1, n)`.
ipiv	INTEGER. Array, DIMENSION at least `max(1, n)`. The array `ipiv`, as returned by ?sytrf.
anorm	REAL for single precision flavors. DOUBLE PRECISION for double precision flavors. The norm of the original matrix `A` (see Description).
iwork	INTEGER. Workspace array, DIMENSION at least `max(1, n)`.

Output Parameters

rcond

REAL for single precision flavors

DOUBLE PRECISION for double precision flavors.

An estimate of the reciprocal of the condition number. The routine sets rcond =0 if the estimate underflows; in this case the matrix is singular (to working precision). However, anytime rcond is small compared to 1.0, for the working precision, the matrix may be poorly conditioned or even singular.

info

INTEGER. If info = 0, the execution is successful.

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

Fortran 95 Interface Notes

Routines in Fortran 95 interface have fewer arguments in the calling sequence than their FORTRAN 77 counterparts. For general conventions applied to skip redundant or reconstructible arguments, see Fortran 95 Interface Conventions.

Specific details for the routine sycon interface are as follows:

a	Holds the matrix `A` of size (`n`, `n`).
ipiv	Holds the vector of length `n`.
uplo	Must be 'U' or 'L'. The default value is 'U'.

Application Notes

The computed rcond is never less than r (the reciprocal of the true condition number) and in practice is nearly always less than 10r. A call to this routine involves solving a number of systems of linear equations A*x = b; the number is usually 4 or 5 and never more than 11. Each solution requires approximately 2n² floating-point operations for real flavors and 8n² for complex flavors.