1

This question already has an answer here:

How to write the following symbol in LaTeX

enter image description here

that's used for Banach algebras.

marked as duplicate by egreg symbols Sep 27 '18 at 12:51

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2

It is a fraktur "E".

\documentclass{article}
\usepackage{amssymb}
\begin{document}
$\mathfrak{E}_p(I)$
\end{document}

enter image description here

Interestingly, while http://detexify.kirelabs.org/classify.html is usually very good with this, I was not able to get auto-recognition of the fraktur E.

enter image description here

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