I am typing

```
\documentclass[12pt,letterpaper,fleqn,leqno,oneside]{article}
\usepackage{amsmath}
\newcommand{\R}{\mathds{R}}
\begin{document}
\title{M}
\author{Y}
\date{September 4, 2014}
\maketitle
\textbf{Exercise 1} (Open Balls). Prove \textit{rigorously} that an "open ball" $B_\epsilon(x)$ of radius $\epsilon > 0$ about a point $x$ in $\mathds{R}$ equipped with the Euclidean topology is, in fact, open.
\end{document}
```

And I'm getting the error

Undefined control sequence problem. …epsilon>0$ about a point $x$ in $\mathds

What is the problem?

I suspect that I didn't download completely the package `amsmath`

. But, I downloaded in my desktop. Is there a problem with this??

## Best Answer

You didn't load the

`dsfont`

package: