How to type the bold script in latex?

Question

Hello, everyone, I want to ask, how to type this formula in latex? Thank you So much!

Answer 1

The font usually used for math script does not have a bold variant, but you can emulate something like bold by using the \pmb command from the amsbsy package, as suggested in this answer to a similar question.

(There are other solutions in that thread you might also consider.)

\documentclass{article}
\usepackage{mathrsfs}
\usepackage{amsbsy}
\begin{document}
$\pmb{\mathscr{E}} = \mathscr{E}\negthinspace\mathbf{e}_{x}$
\end{document}