# Why does this error keep popping up? Undefined control sequence. \end{frame}

This is my code:

\begin{frame}{Ponavljanje - područje definicije}
\section{Ponavljanje - područje definicije}
\subsection{Polinomi}
\begin{enumerate}[1.]
\item Polinomi
\end{enumerate}
\centering
\includegraphics[width=50mm]{polinomi (1)}
$$D_{f} = \mathbb{R} , R_{f} = \begin{cases} \mathbb{R} & n  neparan \\ [o, + \infty > & n  paran \end{cases}$$

\end{frame}


Error: Undefined control sequence. \end{frame}

LaTeX uses $...$ instead of $$...$$ for displayed math equations, see: Why is $$ preferable to $$$$?

\mathbb and environment cases are supported by package amsmath. The following works without error:

\documentclass[hyperref={pdfencoding=auto}]{beamer}
\usepackage[utf8]{inputenc}
\usepackage[croatian]{babel}
\usepackage[T1]{fontenc}

\usepackage{amsmath}

\begin{document}
\begin{frame}{Ponavljanje - područje definicije}
\section{Ponavljanje - područje definicije}
\subsection{Polinomi}
\begin{enumerate}[1.]
\item Polinomi
\end{enumerate}
\centering
\includegraphics[width=50mm]{example-image-a}
$D_{f} = \mathbb{R} , R_{f} = \begin{cases} \mathbb{R} & n  neparan \\ [o, + \infty > & n  paran \end{cases}$
\end{frame}
\end{document}


BTW, TeX shows the undefined command sequence in the error message at the end of the line that starts with the line number.

• Why are \section{...} and \subsection{...} inside the frame environment? – user94293 Nov 12 '16 at 6:45
• @user94293 Merely copied from the question. – Heiko Oberdiek Nov 12 '16 at 9:09