Published online by Cambridge University Press: 01 March 2016
Consider the first-order retarded differential equation $$\begin{eqnarray}x^{\prime }(t)+p(t)x({\it\tau}(t))=0,\quad t\geqslant t_{0},\end{eqnarray}$$
$p(t)\geqslant 0$ and
${\it\tau}(t)$ is a function of positive real numbers such that
${\it\tau}(t)\leqslant t$ for
$t\geqslant t_{0}$, and
$\lim _{t\rightarrow \infty }{\it\tau}(t)=\infty$. Under the assumption that the retarded argument is non-monotone, a new oscillation criterion, involving
$\liminf$, is established when the well-known oscillation condition
$$\begin{eqnarray}\liminf _{t\rightarrow \infty }\int _{{\it\tau}(t)}^{t}p(s)\,ds>\frac{1}{e}\end{eqnarray}$$