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 wikpedia they typset 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.

Thanks for any help. I'm surprised I can't find a solution on google.

Answer 1

I got the definition below after some tinkering. It seems to look fine at any size.

\documentclass[a4paper,12pt]{article}
\pagestyle{empty}
\usepackage{mathtools}
\def\multiset#1#2{\ensuremath{\left(\kern-.3em\left(\genfrac{}{}{0pt}{}{#1}{#2}\right)\kern-.3em\right)}}

\begin{document}
Consider an inline multiset, \multiset{n}{k}, or a displayed one,
\[ \multiset{n}{k}, \]
or perhaps even a displayed one with something big up- and downstairs,
\[\multiset{
    {\displaystyle \sum_{j=1}^n n^j}}
    {{\displaystyle \sum_{j=1}^n (-1)^j n^j}}_{n \text{ even}}. 
\]
\end{document}

Answer 2

\def\multiset#1#2{\left(\!\left({#1\atopwithdelims..#2}\right)\!\right)} maybe? Looks ok in inline equations.

Answer 3

Old post, but I ran into issues with the other answers, so here's mine:

\newcommand{\mch}[2]{
\left.\mathchoice
  {\left(\kern-0.48em\binom{#1}{#2}\kern-0.48em\right)}
  {\big(\kern-0.30em\binom{\smash{#1}}{\smash{#2}}\kern-0.30em\big)}
  {\left(\kern-0.30em\binom{\smash{#1}}{\smash{#2}}\kern-0.30em\right)}
  {\left(\kern-0.30em\binom{\smash{#1}}{\smash{#2}}\kern-0.30em\right)}
\right.}

With Kahen's answer I had issues if a non-display-mode line had both \binom and \mch (the name \multiset seems a bit off too; I read the symbol as "multichoose"), namely the outer and inner parens weren't of equal size. This answer incorporates jjh's comment above without making you use two separate macros. I'm sure there are minor issues with it, but it works for me in a paper that uses it a bunch of times.

Answer 4

bmatrix from the amsmath package gives simple, effective results for displayed equations. It does not work well for inline equations, however.

\documentclass[12pt]{article}
\pagestyle{empty}
\usepackage{amsmath}

\begin{document}
\begin{bmatrix}
    \begin{bmatrix}
        n \\
        k
    \end{bmatrix}
\end{bmatrix}
\end{document}