equationsmath-modesyntax
I've heard that you should use \[ ... \] for displayed equations instead of $$ ... $$, but why is that?
I'd assumed that it's so that you can more easily tell which are starting and which are ending delimiters, but if I always use a syntax-highlighting text editor, I can see that easily based on the color of the symbols.
Is there any reason for this suggestion?
I think it's a survival of the fittest thing. First of all, math notation isn't specific to LaTeX, it's inherent in the TeX engine.
So what do we have:
I don't know what troff did for formulas (which might have been a competitor at the time), and I don't know whether Knuth invented it all by himself (was'nt there a system named scribe he used as a blueprint?), but I can imagine there was not that much pressure to ever develop an alternative aspiring for all these features at once.
You can look at MathML for the result what happens when such a thing is attempted by a committee ;-)
In the displaymath mode, you can not use any paragraph triggering commands such as blank lines or \par commands. (Not a good idea, but you can use it inside the cases environment). Also you don't need to switch to text mode and then again to math mode. You can just use text on the text and leave the rest as it is.
Another point is the use of & characters which is the column delimiter that should be used inside the cases environment. This would be apparent if one of the cases starts with 1250 and the other is 1 which would lead to a bad alignment (try without the & characters!).
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
X(\omega) = \begin{cases}
1 &\text{se $\omega\in A$}\\
1250 &\text{se $\omega \in A^c$}
\end{cases}
\end{equation*}
\end{document}
Also mathtoolspackage provides some nice extensions and bugfixes of amsmath, so here is an example of dcases* environment which automatically switches to text mode in the second entry of the case declaration:
\documentclass{article}
\usepackage{mathtools}
\begin{document}
\[
X(\omega) = \begin{dcases*}
1 & se $\omega\in A$\\
0 & se $\omega \in A^c$
\end{dcases*}
\]
\end{document}
If you are using the standalone class (which gives errors) then you have to use it as
\documentclass[preview]{standalone}
Best Answer
$$is TeX primitive syntax, which, as others have commented is hard to redefine (in classic TeX there is no command name which triggers entering or leaving display math).LaTeX doesn't officially support
$$. The most noticeable failure if you use the syntax is that thefleqnoption will no longer affect the display of the mathematics, it will remain centered rather than being set flush left.However if you are using
\[it is best to use theamsmathpackage (it is best to load this package if doing any serious mathematical layout with LaTeX).\[is a short form of\begin{displaymath}which one might expect to act like an un-numbered form of\begin{equation}. Theamsmathpackage redefines\[to be\begin{equation*}which is exactly an un-numbered form of theequationenvironment as defined by that package. In the core LaTeX definition\[has the definitionwhich means that the spacing if used between paragraphs is better that a primitive
$$(which causes a blank paragraph just consisting of the indentation and parfillskip glue to be inserted before the display) but it is inconsistent withequationand always uses\abovedisplayskiprather than\abovedisplayshortskip. In the AMS versions the spacing is more consistent,