Suppose we want to input the following sentence in LaTeX:
Therefore, the approximation ratio is 42.
Here is the obvious way:
Therefore, the approximation ratio is 42.
It works well, hmm, until that one day when you decide to change your math font. The correct way should be:
Therefore, the approximation ratio is $42$.
The dollar signs have a semantic meaning: “42″ is in the mathematical context, just as how we would input $x^2$. So if you intend to change the math font, then you want this occurrence to use that new font as well.
In case you want to see it in effect, here is a small example that you can try:
\documentclass{article}
\usepackage{ccfonts}
\usepackage[euler-digits]{eulervm}
\begin{document}
\begin{enumerate}
\item 1 $1$ -1 $-1$ one
\item 2 $2$ -2 $-2$ two
\end{enumerate}
\end{document}
This example also shows the importance of the dollar signs when dealing with negative numbers. Without the dollar signs, the minus sign is actually a hyphen. This difference is very noticeable in the output. You can refer to this post if you want a reminder on hyphens (-), number ranges(–) and dashes(—).
