Extract from the Readme File for the mathrsfs package by Jörg Knappen (c) Copyleft 1995, 1996, 1998, 1999 Jörg Knappen Licence: GNU public licence version 2

Physicists don't like the appearance of the standard calligraphic fonts when speaking about Lagrangian density, Hamiltonian density, or the measure in the path integral. Fortunately, the Ralph Smith Formal Script (rsfs) fonts provide the desired shapes. The mathrsfs package makes them accesible via the command \mathscr{ABC}.

To use this package, just write
\usepackage{mathrsfs}
in the preamble of the document.