1

Please consider the following file:

\documentclass{article}
\newcommand{\ineq}[2][$0$]{#2<#1}
\[\ineq x\]
\end{document}

If I compile it, I get an error message, as it would be expected when one uses dollar signs within a \[…\] pair. The correct definition for the \ineq command should be

\newcommand{\ineq}[2][0]{#2<#1}

However, if I compile this file:

\documentclass{article}
\usepackage{amsmath}
\newcommand{\abc}[3][$0$]{\begin{cases}
        #2& \text{ if } x \neq #1\\
        #3&\text{ if } x= #1
                             \end{cases}}
\begin{document}
\[\abc ab\]
\end{document}

then I get no error message and everything works as if I had typed [0] instead of [$0$]. Why is it so?

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1 Answer 1

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3

Inside an alignment you are not in $$ ... $$ but $\displaystyle...$ so single dollars just stop and start the math.

$\displaystyle...  $0$ ... $

is not as intended but not an error. It is easier to see with letters

enter image description here


\documentclass{article}
\usepackage{amsmath}
\newcommand{\abc}[3][abc$xyz0$]{\begin{cases}
        #2& \text{ if } x \neq #1\\
        #3&\text{ if } x= #1
                             \end{cases}}
\begin{document}
\[\abc ab\]
\end{document}
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