# Some LaTeX constructs and symbols I had to look up

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 *𝟙 _{A}* (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 *𝟙 _{A}* 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`

).