Multiset notation in 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.