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In my combinatorics homework I use double parenthesis to represent a multiset. I have referred to all the answers under Multiset notation in LaTeX on how to build such a notation.

However, the double parenthesis looks good alone but when put together with a binomial, it looks very inconsistent.

I used the code that come from this answer https://tex.stackexchange.com/a/5825/91144, which utilizes a \genfrac with left(/right).

\def\multiset#1#2{\ensuremath{\left(\kern-.3em\left(\genfrac{}{}{0pt}{}{#1}{#2}\right)\kern-.3em\right)}}

Here are four problematic examples

\documentclass{article}

\def\multiset#1#2{\ensuremath{\left(\kern-.3em\left(\genfrac{}{}{0pt}{}{#1}{#2}\right)\kern-.3em\right)}}

\begin{document}

\verb~$\multiset{5}{k}+\multiset{5}{k-1}+\multiset{5}{k-4}$~ \\
$\multiset{5}{k}+\multiset{5}{k-1}+\multiset{5}{k-4}$

\verb~$\sum_{\substack{i+j=15\\i,j\geq0}}\binom{5}{i}\multiset{3}{j}$~ \\
$\sum_{\substack{i+j=15\\i,j\geq0}}\binom{5}{i}\multiset{3}{j}$

\verb~\[ ab\binom{5}{i}123\multiset{3}{j}cd \]~ \\
\[ ab\binom{5}{i}123\multiset{3}{j}cd \]

\verb~\[ \sum{\multiset{5}{k}\multiset{5}{k}} \]~ \\
\[ \sum_{i=0}^{k}{\multiset{m}{i}\multiset{n}{k-i}} \]

\end{document}

examples

  1. inconsistent height (inline mode)
  2. inconsistent height with \binom (inline mode)
  3. incorrect spacing, comparing to \binom (display mode)
  4. inconsistent sizes (display mode)

What I wanted is a notation that looks just similar to \binom{n}{k}, i.e. size, shape, spacing, etc, while having two pair of parenthesis around. And I expect it to work both inline and in display mode. Thank you :)

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  • 1
    Source code for the examples?
    – egreg
    Nov 1, 2015 at 21:36
  • But this image of source code! Please make it so, that it can be copied in editor and test it.
    – Zarko
    Nov 1, 2015 at 22:25
  • @Zarko: here is it: lpaste.net/6732560716236062720
    – shouya
    Nov 1, 2015 at 22:32
  • @ShouYa, ok, why you not copy in your question as minimal working example (without line numbers ...) by this you will make to people here eaiery to help you.
    – Zarko
    Nov 1, 2015 at 22:35
  • The rules for the delimiter size in generalized fractions are different from those for \left and \right; this is particularly evident in text style. The difference in case 4 is because the - has a depth (you'd see the same with +).
    – egreg
    Nov 1, 2015 at 23:11

2 Answers 2

Reset to default
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Try this:

\documentclass[preview,border=5mm]{standalone}
    \usepackage{mathtools}
\newcommand{\textmultiset}[2]{\bigl(\!{\binom{#1}{#2}}\!\bigr)}
\newcommand{\displaymultiset}[2]{\left(\!{\binom{#1}{#2}}\!\right)}
\newcommand\multiset[2]{\mathchoice{\displaymultiset{#1}{#2}}
                                {\textmultiset{#1}{#2}}
                                {\textmultiset{#1}{#2}}
                                {\textmultiset{#1}{#2}}}
    \begin{document}
some text $ \multiset{5}{k}+\binom{5}{k-1}+\multiset{5}{k{+}1}$

\bigskip 
some text $\sum_{\substack{i+j=15\\i,j\geq0}}\binom{5}{i}\multiset{3}{j}\multiset{3}{j+1}$

\[ ab\multiset{5}{i+1}123\binom{3}{j+1}cd \]

\[ \sum_{i=0}^{k}{\multiset{m}{i}\binom{n}{k-i}} \]
    \end{document}

![enter image description here

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  • thank you for your answer. I'm still wondering if the spacing around the \multiset can be better. e.g. ab␣((5,i+1))␣123 looks wider than 123␣(3,j+1)␣cd. Should I just add some \! around the command or is there a better way to do this?
    – shouya
    Nov 1, 2015 at 23:57
  • Instead of \! you can use TeX primitive \kern -<value>, where you set <value> according to your taste. For example, for display version of \multset you can try with \newcommand{\displaymultiset}[2]{\left(\kern-5pt{\binom{#1}{#2}}\!\!\right)} and see, which distance (left or right) looks nicer to you.
    – Zarko
    Nov 2, 2015 at 0:39
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I get somewhat better results by tweaking @Zarko's answer to

\newcommand\multiset[2]%
{\mathchoice{\left(\kern-0.5em{\binom{#1}{#2}}\kern-0.5em\right)}
            {\bigl(\kern-0.3em{\binom{#1}{#2}}\kern-0.3em\bigr)}
            {\bigl(\kern-0.3em{\binom{#1}{#2}}\kern-0.3em\bigr)}
            {\bigl(\kern-0.3em{\binom{#1}{#2}}\kern-0.3em\bigr)}}

But e.g. 2^{\multiset{2}{3}} looks ugly (the inner () are a bit too low).

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