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Recently in a question on Math.SE, I have seen for the first time in my life an unknown symbol of double binomial coefficient. It is simply possible to make it with a code of this style:

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}

\begin{document}
\begin{equation}
    \left(!!\binom{n}{k}!!\right)
\end{equation}


\end{document}

enter image description here

Is there a direct command that produces the double round bracket symbol? I have seen this same answer very old but I not like :-( Multiset notation in LaTeX.

After almost 10 years is there a command in some package?

Sebastiano
  • 54,118

1 Answers1

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You could of course construct it with two ( but Unicode has double parenthesis U+2e28 and U+2e29 which looks like

⸨x⸩

which looks like enter image description here in this browser at least.

But this is in the Supplemental Punctuation block so not considered mathematical and indeed Stix Two and Cambria math fonts (at least) don't have that character.

David Carlisle
  • 757,742