amsmathsymbols
I am looking for a 'double' brace symbol (aka Blackboard or Doublestroke) like this – but for curly braces instead of square brackets.
The symbol is used to denote the average in discontinuous Galerkin methods.
(I have tried de-texify and can't identify it)
It should look similar to:
$$\{\hspace{-4.9pt}\{ u_x \}\hspace{-5pt}\}$$
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}
Since it is not exactly clear if you want a normal "Y" or a fraktur "Y", I provide both... (note that these are designed to take up the same space as an unadulterated "Y" or \mathfrak{Y})
The stoke elevation can be adjusted with the first argument of \stackengine.
\documentclass{article}
\usepackage{amssymb}
\usepackage{stackengine}
\def\strokeY{\ensurestackMath{\stackengine{0pt}{-\,}{Y}{O}{c}{F}{T}{L}}}
\def\strokefracY{\ensurestackMath{\stackengine{0pt}{\,-}{\mathfrak{Y}}{O}{c}{F}{T}{L}}}
\begin{document}
$\strokeY = X$
$\strokefracY = X$
\end{document}
Best Answer
My eyes bleed, but here it is:
If I change
.625into.45and.7into.5, the result seems better: