How to write this X=1 to L with curvy brackets in LaTeX? (image included)

Question

is what I need to write. This is from Introduciton to Statistical Learning textbook, which in in pdf format. Here's what I now have, $\{X_l\}^{L}$, I have not been able to get to display 1 right and this is how it is displayed currently.

Answer 1

Use \ell for the loopy l and _{1} for the subscript 1

\documentclass{article}

\begin{document}

$
\{X_{\ell}\}_{1}^{L}
$
\end{document}