Visible to Intel only — GUID: GUID-C84FC6A9-EA43-4390-96D6-FC497737E234
Visible to Intel only — GUID: GUID-C84FC6A9-EA43-4390-96D6-FC497737E234
?sycon
Estimates the reciprocal of the condition number of a symmetric matrix.
Syntax
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 )
call sycon( a, ipiv, anorm, rcond [,uplo] [,info] )
Include Files
- mkl.fi, lapack.f90
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)).
An estimate is obtained for ||A-1||, and the reciprocal of the condition number is computed as rcond = 1 / (||A|| ||A-1||).
Before calling this routine:
compute anorm (either ||A||1 = maxjΣi |aij| or ||A||∞ = maxiΣj |aij|)
call ?sytrf to compute the factorization of A.
Input Parameters
uplo |
CHARACTER*1. 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 = U*D*UT. If uplo = 'L', the array a stores the lower triangular factor L of the factorization A = L*D*LT. |
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, size 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, size 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. |
LAPACK 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 LAPACK 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.