Multiset notation in LaTeX - latex

Question

Does anyone know how to make (nice looking) double bracket multiset notation in LaTeX, i.e something like (\binom{n}{k}) where there are two outer brackets instead of 1 as in binomial? You can see an example of what I mean in http://en.wikipedia.org/wiki/Multiset under the heading "Multiset coefficients" with the double brackets.
In Wikipedia they typeset it as:
\left(\!\!{n\choose k}\!\!\right)
but although this works well for LaTeX in maths mode, with inline equations the outer bracket becomes much larger than the inner bracket.
I have also tried using
\genfrac{((}{))}{0pt}{}{n}{k}
but it has an error with the double brackets.
I am using \binom as well in my document, so I would like the bracket sizes to be similar for \binom and \multiset.

Answer 1

You can explicitly specify the size of the brackets via
\big( \Big( \bigg( or \Bigg(
Then use \! for negative space to get the brackets closer to each other.

Answer 2

One can use the e-TeX \middle command as follows:
\newcommand{\multibinom}[2]{
\left(\!\middle(\genfrac{}{}{0pt}{}{#1}{#2}\middle)\!\right)
}
This assumes that you are using the AMSmath package. If not, replace \genfrac with the appropriate construct using \atop.
(Of course this is a hack: the proper solution would be scalable glyphs for the doubled parenthesis, but I can't find any fonts that provide it.)
I'm surprised it wasn't googlable either, so I'll provide a solution here for posterity's sake.

Answer 3

It is also possible to define two different new commands, using \tbinom and \dbinom (section 4.11.2 of the User's Guide for the amsmath Package):
\documentclass{article}
\usepackage{amsmath}
\newcommand{\inlinebnm}[2]{\ensuremath{\big(\!\tbinom{#1}{#2}\!\big)}}
\newcommand{\displybnm}[2]{\bigg(\!\!\dbinom{#1}{#2}\!\!\bigg)}
\begin{document}
Text $\inlinebnm{a}{b}$ text. %% inline
Text \inlinebnm{a}{b} text. %% inline (also ok thanks to ensuremath)
\[
\displybnm{a}{b} %% display-style
\]
\end{document}