Liénard–Chipart criterion
# Liénard–Chipart criterion

In control system theory, the Liénard–Chipart criterion is a stability criterion modified from the Routh–Hurwitz stability criterion, proposed by A. Liénard and M. H. Chipart. This criterion has a computational advantage over the Routh–Hurwitz criterion because it involves only about half the number of determinant computations.

## Algorithm

The Routh–Hurwitz stability criterion says that a necessary and sufficient condition for all the roots of the polynomial with real coefficients

       $f(z)=a_{0}z^{n}+a_{1}z^{n-1}+\cdots +a_{n}\,(a_{0}>0)$

to have negative real parts (i.e. $f$ is Hurwitz stable) is that

       $\Delta _{1}>0,\,\Delta _{2}>0,\ldots ,\Delta _{n}>0,$

where $\Delta _{i}$ is the i-th leading principal minor of the Hurwitz matrix associated with $f$.

Using the same notation as above, the Liénard–Chipart criterion is that $f$ is Hurwitz stable if and only if any one of the four conditions is satisfied:

* $a_{n}>0,a_{n-2}>0,\ldots ;\,\Delta _{1}>0,\Delta _{3}>0,\ldots$
* $a_{n}>0,a_{n-2}>0,\ldots ;\,\Delta _{2}>0,\Delta _{4}>0,\ldots$
* $a_{n}>0,a_{n-1}>0,a_{n-3}>0,\ldots ;\,\Delta _{1}>0,\Delta _{3}>0,\ldots$
* $a_{n}>0,a_{n-1}>0,a_{n-3}>0,\ldots ;\,\Delta _{2}>0,\Delta _{4}>0,\ldots$

Hence one can see that by choosing one of these conditions, the number of determinants required to be evaluated is reduced.

Alternatively Fuller formulated this as follows for (noticing that $\Delta _{1}>0$ is never needed to be checked):

$a_{n}>0,a_{1}>0,a_{3}>0,a_{5}>0,\ldots ;$

$\Delta _{n-1}>0,\Delta _{n-3}>0,\Delta _{n-5}>0,\ldots ,\{\Delta _{3}>0\ (n\ even)\,\Delta _{2}>0\ (n\ odd)\}.$

This means if n is even, the second line ends in $\Delta _{3}>0$ and if n is odd, it ends in $\Delta _{2}>0$ and so this is just 1. condition for odd n and 4. condition for even n from above. The first line always ends in $a_{n}$, but $a_{n-1}>0$ is also needed for even n.

