\documentclass[12pt]{article}
\usepackage{amsmath,amssymb,amsfonts}
\begin{document}
By using a relativistic fluid model, a nonlinear theory for the
propagation of an intense laser pulse in an inhomogeneous cold
plasma is developed. Assuming that the radiation spot size is
larger than the plasma wavelength, we derive an envelope equation
for the momentum of the electron fluid, taking into account
relativistic electron mass variation and finite amplitude electron
density perturbations that are driven by the relativistic
ponderomotive force of light. Localized solutions of the envelope
equation are discussed from an energy integral containing an
effective potential. Numerical results for envelope solitons are
obtained in a quasistationary approximation. The dependency of
these localized solutions on the amplitude and the group velocity
of the laser pulse is discussed. Also derived is an equation that
governs the dynamics of the pulse center.
\end{document}