nag_zppcon (f07guc) estimates the condition number of a complex Hermitian positive definite matrix
$A$, where
$A$ has been factorized by
nag_zpptrf (f07grc), using packed storage.
nag_zppcon (f07guc) estimates the condition number (in the
$1$-norm) of a complex Hermitian positive definite matrix
$A$:
Since
$A$ is Hermitian,
${\kappa}_{1}\left(A\right)={\kappa}_{\infty}\left(A\right)={\Vert A\Vert}_{\infty}{\Vert {A}^{-1}\Vert}_{\infty}$.
The function should be preceded by a call to
nag_zhp_norm (f16udc) to compute
${\Vert A\Vert}_{1}$ and a call to
nag_zpptrf (f07grc) to compute the Cholesky factorization of
$A$. The function then uses Higham's implementation of Hager's method (see
Higham (1988)) to estimate
${\Vert {A}^{-1}\Vert}_{1}$.
Higham N J (1988) FORTRAN codes for estimating the one-norm of a real or complex matrix, with applications to condition estimation ACM Trans. Math. Software 14 381–396
The computed estimate
rcond is never less than the true value
$\rho $, and in practice is nearly always less than
$10\rho $, although examples can be constructed where
rcond is much larger.
Please consult the
x06 Chapter Introduction for information on how to control and interrogate the OpenMP environment used within this function. Please also consult the
Users' Note for your implementation for any additional implementation-specific information.
A call to
nag_zppcon (f07guc) involves solving a number of systems of linear equations of the form
$Ax=b$; the number is usually
$5$ and never more than
$11$. Each solution involves approximately
$8{n}^{2}$ real floating-point operations but takes considerably longer than a call to
nag_zpptrs (f07gsc) with one right-hand side, because extra care is taken to avoid overflow when
$A$ is approximately singular.
The real analogue of this function is
nag_dppcon (f07ggc).
This example estimates the condition number in the
$1$-norm (or
$\infty $-norm) of the matrix
$A$, where
Here
$A$ is Hermitian positive definite, stored in packed form, and must first be factorized by
nag_zpptrf (f07grc). The true condition number in the
$1$-norm is
$201.92$.