\documentclass{article}
\usepackage{amsmath}
\title{Complete Statistical Mechanics: Boltzmann Distribution}
\begin{document}
\maketitle

\section{Boltzmann Distribution}

When a system exchanges energy with a heat bath, a state of energy $E_i$ has
weight

$$
p_i=\frac{e^{-\beta E_i}}{Z}.
$$

The exponential weight is the basic bridge from microscopic energy levels to
thermal equilibrium.

\end{document}
