Double brackets {{ }}

Question

How to type the double bracket '{{b, a}}' in tex just as the picture below?

Answer 1

You need negative spaces \! to get the braces close to each other. There may be a better way.

\documentclass{article}
\begin{document}
( {!!{a,b}!!} )
\end{document}

Answer 2

\documentclass{article}
\newcommand\doubleBrackets[1]{${!{ #1 }!}$}
\begin{document}
These are double { } brackets \doubleBrackets{b, a}.
\end{document}