I am completely new in LaTeX. I wrote this below code in TeXworks but it doesn't work:

```
\documentclass{article}
\usepackage{amsthm}
\begin{document}
\section{Introduction}
In this section I introduce some concepts which we need to fully understand the Ellipsoid method.
\begin{definition}
A set $S\in \mathbb R^n$ is said affine set if for every $x,y \in S$ and every $\lambda in \mathbb R$ we have $\lambda x+(1-\lambda )y\in S$.
\end{definition}
\end{document}
```

Could you please help me what is the problem?

## Best Answer

Very simple: there is no

`definition`

theorem-like environment by default. There's only a ‘definition’`theorem style`

. So here is how to define it, numbered by section: