\documentclass[12pt]{article}
\usepackage{amsmath,amssymb,amsfonts}
\begin{document}
In this paper, we study the problem of the existence of limit
cycles for a predator-prey system with a functional response. It
is assumed that the functional response is positive, increasing,
concave down, and its third derivative has a unique root. A
necessary condition for the nonexistence of limit cycles is
presented. Some conditions are given under which the necessary
condition is also the sufficient condition for the nonexistence of
limit cycles.
\end{document}