\documentclass[12pt]{article}
\usepackage{amsmath,amssymb,amsfonts}
\begin{document}
Mode coupling takes place in a small, but non-zero, refion around the coupling point. A general formula for the size of this coupling region is represented. This formula is valid for anisotropic absorbing media with the inhomogenity in one direction and for a linear system of wave equations with arbitrary number of fields and of arbitrary order.
\end{document}