1

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
1

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} 

enter image description here

  • 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
0

Another simply solution to your problem. From your own MWE:

\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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.