\documentclass{article}

\documentclass{article}
\usepackage{amsmath} % pkg required by EasyLatex
\usepackage{amsfonts} % pkg required by EasyLatex
\usepackage{amssymb} % pkg required by EasyLatex
\usepackage{psfrag} % pkg required by EasyLatex
\usepackage[dvips]{graphicx} % pkg required by EasyLatex

\begin{document}

\begin{align*}
\left[ \begin{array}{lll}
3 & 4 & 5
\\ 5 & 7 & 9
\\ 13 & 29 & 61
\end{array} \right]
\end{align*}

Function "Ack" (Ackermann's function) copied from Doug Bagley.

\end{document}