\[ ...\] in place of $$ ...$$?LaTeX defines inline- and display-maths commands, apparently analagous to those that derive from the TeX primitive maths sequences bracketing maths commands with single dollar signs (or pairs of dollar signs).
As it turns out, LaTeX's inline maths grouping,
\( ... \), has precisely the same effect as the
TeX primitive version $ ... $. (Except that
the LaTeX version checks to ensure you don't put \( and
\) the wrong way round.)
In this circumstance, one often finds LaTeX users, who have some
experience of using Plain TeX, merely assuming that LaTeX's
display maths grouping \[ ... \] may be replaced by
the TeX primitive display maths $$ ... $$.
Unfortunately, they are wrong: if LaTeX code is going to patch display
maths, it can only do so by patching \[ and \]. The most
obvious way this turns up, is that the class option fleqn
simply does not work for equations coded using
$$ ... $$, whether you're using the standard classes
alone, or using package amsmath.
There are more subtle effects (especially with package
amsmath), and the simple rule is \[ ... \] whenever
unadorned displayed maths is needed in LaTeX.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=dolldoll