# Undefned control sequence problem

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??

• What is \mathds supposed to be? Is that perhaps supposed to be \mathbf. Or perhaps \mathbb which requires the amssymb. – Peter Grill Aug 28 '14 at 1:44
• It is the same as \mathbb or \mathbf. I have the same problem when I uses \mathbb or \mathbf – User Aug 28 '14 at 1:45
• Instead of trying to download a package at a time, I'd recommend you just download the complete TeXLive2014 and go from there. – Peter Grill Aug 28 '14 at 1:48
• What is TeXLive2014? I'm using MikTex – User Aug 28 '14 at 1:53
• @Yk26 You must to put \begin{document} after \title, \author, \date but before \maketitlefor your document works. – Aradnix Aug 28 '14 at 1:56

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}


• Ok. My mistake. But the problem is that I also can't use \mathbb{R} Can you explain this too? As I really just want to use amsmath only. – User Aug 28 '14 at 2:11
• Load the amsfonts package. Bbfonts are not part of basic LaTeX. See my updated answer. – Bernard Aug 28 '14 at 2:16

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

\usepackage{amsmath,amssymb}

%\newcommand{\R}{\mathds{R}} % Anyway it is bad defined

\title{M}
\author{Y}
\date{September 4, 2014}
\begin{document}
\maketitle

\paragraph*{Exercise 1} (Open Balls). Prove \emph{rigorously} that an open ball'' $B_\epsilon(x)$ of radius $\epsilon > 0$ about a point $x$ in $\mathbb{R}$ equipped with the Euclidean topology is, in fact, open.

\end{document}


Notice that I erased few options of the class because they are redundant since are the default settings. I used amssymb instead amsfonts, and it is because long ago someone told me to use both is redundant since amssymbcontains symbols from amsfonts. I don't know if that's true, but in this case it works well.

I also chose to change \texit and \textbf for more suitable options. Not that it is bad in itself, but instead simply highlight the text typographically, you can use \paragraph* to give structure and \emph what it does is highlight the text regardless of the font you use and that makes it more versatile.

I know that \paragraph* is not the best choice, the ideal would have used one of the theorems packages and define a sentence to exercise, but that would complicate the answer too, maybe on a future occasion.