How to type # in math mode?

Question

How to type this symbol

in math mode?

I googled it, but I got nothing;(

Answer 1

This mostly depends on what kind of symbol you need. If it's an ordinary symbol, just

\#

will do, using the hash symbol from the main text font. But if you need a relation symbol or an operation symbol, you need, respectively

\mathrel\#
\mathbin\#

Note the difference:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
$a # b$
$a \mathrel# b$
$a \mathbin# b$
\end{document}

The difference between the last two is subtle, but noticeable.

Of course, the best is, after you have decided the type, to define your command, for instance

\newcommand{\mathhash}{\mathrel\#}

Choose a more semantic name.

You might want to import \hash from mathabx. Here I define a \mathrel, change to \mathord or \mathbin if you want an ordinary or operation symbol respectively.

\documentclass{article}
\usepackage{amsmath}
\DeclareFontFamily{U}{matha}{}
\DeclareFontShape{U}{matha}{m}{n}{
 <-5.5> matha5
 <5.5-6.5> matha6
 <6.5-7.5> matha7
 <7.5-8.5> matha8
 <8.5-9.5> matha9
 <9.5-11> matha10
 <11-> matha12
}{}
\DeclareSymbolFont{matha}{U}{matha}{m}{n}
\DeclareMathSymbol{\hash}{\mathrel}{matha}{"23}
\begin{document}
$a \hash b$
\end{document}