A collection of LaTeX things that required some searching, so they're gathered here mostly for my own reference:
Multiline subscripts
For example, under a summation symbol:
\sum_{\substack{{y= 1}\\{y \neq 2}}}^{10}
The \substack command is part of the amsmath package.
Double-struck numbers
I like to use a 𝟙A (with a number one in blackboard bold) to denote the characteristic function of a set A. One can get that in LaTeX with the bbm package:
\usepackage{bbm}
\def\1{\mathbbm{1}}
and then \1 gives the symbol 𝟙A in a serif font. There is another package called bbold which contains \mathbb{1} in a sans-serif font, but it also changes all other blackboard bold letters to a sans-serif font.
Using accents and special characters in text
Instead of the special notation \"{o}, \'{a} and such for special characters, one can tell LaTeX that our file contains ö, à and others directly by having
\usepackage[utf8]{inputenc}
in the preamble. The file should then be saved using UTF-8 encoding. Many editors do this by default, but otherwise you can always specify the coding system explicitly (for example, by writing latin1 instead of utf8).
