Equation environment definition

Question

Where is the full definition of the equation environment? I cannot find it in the .cls files. Is this in latex.ltx? Or something?

Answer 1

In texdoc source2e it starts on p214. It's in ltmath.dtx.

Here it is:

\@definecounter{equation}
\def\equation{$$\refstepcounter{equation}}
\def\endequation{\eqno \hbox{\@eqnnum}$$\@ignoretrue}

Answer 2

Yes, you can find it in latex.ltx. Seamus pointet you already to the source in ltmath.dtx and noted the three lines.

If you use equations, you very probably use the amsmath package because that's very recommendable for writing math texts. This package redefines the equation environment! It even defines a starred environment equation*.

So, if you use amsmath, you can find the definition of the equation environment in amsmath.sty:

\renewenvironment{equation}{%
  \incr@eqnum
  \mathdisplay@push
  \st@rredfalse \global\@eqnswtrue
  \mathdisplay{equation}%
}{%
  \endmathdisplay{equation}%
  \mathdisplay@pop
  \ignorespacesafterend
}
\newenvironment{equation*}{%
  \mathdisplay@push
  \st@rredtrue \global\@eqnswfalse
  \mathdisplay{equation*}%
}{%
  \endmathdisplay{equation*}%
  \mathdisplay@pop
  \ignorespacesafterend
}

Even \[ ... \] is redefined to use this equation*. For more information regarding the used macros look at amsmath.sty. Or just at latex.ltx, if you decide to to without amsmath.