  We interest in the behaviour of the period function for equations of the type
$u'' + g(u) = 0$ and $u'' + f(u)u' + g(u) = 0$ with a center at the origin 0.
$g$ is a function of class $C^k$. For the conservative case, if $k \geq 2$ one
shows that the Opial criterion is the better one among those for which these
the necessary condition $g''(0) = 0$ holds. In the case where $f$ is of class
$C^1$ and $k \geq 3$, the Lienard equations $ u'' + f(u) u'
  + g(u) = 0$ may have a monotonic period function if $g'(0) g^{(3)}(0) - {5/3}
{g''}^{2}(0) - {2/3} {f'}^{2}(0) g'(0) \neq 0$ in a neighborhood of 0. {\it Key
Words and phrases:} period function, monotonicity, isochronicity, Lienard
equation, polynomial systems.
