\documentclass{article}
\usepackage{ifthen}
\usepackage{epsfig}
\usepackage[utf8]{inputenc}
\usepackage{newunicodechar}
  \newunicodechar{⁻}{${}^{-}$}% Superscript minus
  \newunicodechar{²}{${}^{2}$}% Superscript two
  \newunicodechar{³}{${}^{3}$}% Superscript three

\pagestyle{empty}
\begin{document}
\[ H(z) = \frac {b_0 + b_1 z^{-1} + ... + b_N z^{-N}} {1 + a_1 z^{-1} + ... + a_N z^{-N}} \]
\pagebreak

\end{document}
