3

enter image description here

How to write this 2? I tried to google it but I cannot find any solution...

3

Based on comments and suggestions below your question, one can use the bbold package to achieve the desired output.

\documentclass{article}

\usepackage{bbold}

\begin{document}
$\mathbb 2^{\mathcal U}$
\end{document}

Running it with pdflatex produces

enter image description here

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