Using amsthm environment, using mathscr command, by the way, Mathtype is simply a dirty math software, and now no one reads the papers written in Mathtype. LaTex is needed to write articles in international magazines and papers.

The result of writing code \mathscr{L} in LaTex is

There are many cursive characters in function, and it is most convenient to write with latex. Of course, you can also type it out with Mathtype and choose the font in the picture.