[Tex/LaTex] Special sign \pounds changed to dollar sign in alignment surrounding

Question

I am writing a beamer and use

\begin{align*}
\pounds^i_M   etc.
\end{align*}

on a slide. The pounds sign then changes to a dollar sign. How do I write to get the pounds sign?

\documentclass{beamer}
\usepackage{amssymb}
\usepackage{apalike}
\usepackage{multicol}
\usepackage[all]{xy}
\usepackage[ngerman]{babel}
%\usepackage[applemac]{inputenc}
\usepackage{graphicx}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{endnotes}
\def\L{\mathcal{L}}
\setlength{\unitlength} {1mm}
\usetheme{Madrid}

\begin{document}

\frame { \maketitle}

\frame{ \frametitle{Prescriptions of \pounds \ I: Classical logical maxims} \begin{align*}
\pounds^i_M & \qquad A\rightarrow (B\rightarrow A)
\end{align*}
}
\end{document}

Answer 1

I have also this problem when using the fonts Beamer loads by default (Computer Modern sans serif). If I use their Latin Modern equivalent, it works fine:

\documentclass{beamer}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\begin{document}
\begin{frame}
\begin{align*} \pounds^i_M  \end{align*}
\end{frame}
\end{document}