Is there a difference between the `displaymath` and `equation*` environments?

Question

The amsmath documentation says

Basic LaTeX doesn’t provide an equation* environment, but rather a functionally equivalent environment named displaymath.

Are they really the same or is there a catch? Maybe in interaction with some other packages?

Answer 1

Their effect is similar but they are differently programmed. equation* builds on amsmath macros and is therefore more compatible to other amsmath environments and commands.

For instance, within LaTeX's displaymath environment you cannot use a split environment, even if you loaded amsmath. But within equation* it's possible.

amsmath redefines \displaymath such that it uses equation*. That means, after you loaded amsmath, there won't be a difference any more between those two environments. Further, LaTeX's displaymath is fragile, amsmath makes it robust.

To see that, here are two lines of latex.ltx:

\def\displaymath{\[}
\def\enddisplaymath{\]\@ignoretrue}

And these two lines are at the very end of amsmath.sty:

\DeclareRobustCommand{\[}{\begin{equation*}}
\DeclareRobustCommand{\]}{\end{equation*}}

To sum up, if you use amsmath, which is very recommendable for math texts, you don't need worry about \displaymath.

Answer 2

You can find the relevant definitions in Section 55.2 of source2e.pdf and lines 2523–2667 of amsmath.sty LaTeX aliases \begin{displaymath} to \[ and \end{displaymath} to \], where \[ and \] are wrappers around $$ with some checks. amsmath redefines \[ to \begin{equation*} and \] to \end{equation*}, which in turn wraps around $$ with some checks. So they do the same thing, and as Stefan notes, once you load amsmath, they literally do the same thing.