[Tex/LaTex] Undefned control sequence problem

errors

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:

\documentclass[12pt,letterpaper,fleqn,leqno,oneside]{article}

\usepackage{amsmath, amsfonts}
\usepackage{dsfont}

\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. Or will $ x $ live in $ \mathbb R$?

\end{document} 

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