# Is gather* equivalent to displaymath?

Is gather* with one line of math equivalent to displaymath?

No, it is not: the vertical spaces before and after the display are different.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\noindent Text
\begin{gather*}
a=b
\end{gather*}
Text
\begin{displaymath}
a=b
\end{displaymath}
Text
\end{document}


Also, if you have a very long equation in a $...$, the space between the binary operators (like + and \times) will shrink to make the equation break the margin as little as possible (as happens with an equation in an ordinary paragraph; I'm assuming you're using Computer Modern fonts here). This space never shrinks inside a \begin...\end{gather*}.

This space also shrinks when necessary in an equation in \begin..., and never in an equation in \begin...\end{gather}.

By the way, very old versions of amsmath did define $...$ and \begin... as a one-line \begin...\end{gather*} and \begin...\end{gather}, respectively. See "Changes in amsmath 2.0" here. The first edition (1994) of The LaTeX Companion is no longer correct about $...$ and \begin... (see page 243).