# Error Missing $inserted, Help! Buen día tengo un trabajo y quiero poner algunas cosas, soy nuevo en esto asi que me surgieron muchas dudas, este es mi codigo: \documentclass{beamer} \usepackage[spanish]{babel} \usetheme{Madrid} \title{4.2 Funciones Integrables} \author{J.~Zacarias\inst{} \and P.~Rodr\'iguez\inst{} \and H.~Dom\'inguez \inst{}} \institute[] % (optional, but mostly needed) { \inst{}% Universidad Aut\'onoma de Yucat\'an\\ Facultad de Matem\'aticas} \date{Viernes 4 de diciembre de 2015} \subject{} \AtBeginSubsection[] { \begin{frame}<beamer>{Funciones Integrables} \tableofcontents[currentsection,currentsubsection] \end{frame} } % Let's get started \begin{document} \begin{frame} \titlepage \end{frame} \begin{frame}{Fundamentos y conceptos b\'asicos de integraci\'on} \begin{block}{Definici\'on 4.2.1.} Sea$f : A \subset \mathbb{R}^n \rightarrow \mathbb{R}\ $acotada .Si$B$\subset\ A \$definimos$: \\ \item \centering$M_B(f)=sup \{f(x) : x\in B\}\\
\item
\centering $m_B(f) = inf\{f(x) : x\in B\} \end{block} \end{frame} \end{document}  • Welcome to TeX.SX! You can have a look at our starter guide to familiarize yourself further with our format. My Spanish is pretty bad, but I think your first equation should be Sea$f: A \subset \mathbb{R}^n \rightarrow \mathbb{R}$acotada. Si$B \subset A$definimos:. Since TeX ignores single line breaks, you may want to split regular text and math expressions onto their own lines, making it easier for you to keep the $ or $$ and $$ symbols balanced. Dec 2 '15 at 4:23
• Also after both \items you have $... you need $ ... $, e.g. $M_B(f)=sup \{f(x) : x\in B\} should be $M_B(f)=sup \{f(x) : x\in B\}$. In other news, sup ---> \sup, inf ---> \inf Dec 2 '15 at 4:35

Your input has some glitches; you are mainly forgetting to balance opening and closing $ for formulas. Here's a part of your code. \begin{block}{Definici\'on 4.2.1.} Sea$f : A \subset \mathbb{R}^n \rightarrow \mathbb{R}\ $acotada .Si$B$\subset\ A \$definimos$: \\ \item \centering$M_B(f)=sup \{f(x) : x\in B\}\\
\item
\centering $m_B(f) = inf\{f(x) : x\in B\} \end{block}  1. In general, the closing $ is followed either by a space or punctuation; don't put the space before it.

2. The sentence ending period must be next to the final word

3. A formula contains everything that's math: $B\subset A$

4. Names of common functions such as logarithm, supremum, infimum and so on, should be written in upright font.

5. Don't substitute display math mode with \centering

6. Recall that \centering continues until the end of the current group

Fixed version

\begin{block}{Definici\'on 4.2.1.}
Sea $f\colon A \subset \mathbb{R}^n \rightarrow \mathbb{R}$ acotada.
Si $B \subset A$ definimos:
\begin{gather*}
M_B(f)=sup \{f(x) : x\in B\}\\
m_B(f) = inf\{f(x) : x\in B\}
\end{gather*}
\end{block}
`