Notation of an Action Functional in Mathematics and Physics

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Considering the Wikipedia article on the meaning of action in Physics and for example also the first chapter of the book Quantum Mechanics for Mathematicians by Leon A. Takhtajan, one encounters the letter $S$ or $\mathcal{S}$ for action functionals. Are there specific reasons for this? I mean, in pure mathematics one often uses the letter $A$ or its caligraphic version $\mathcal{A}$, which is the first letter of "action". For example, this notation is used in the book Critical Point Theory for Lagrangian Systems by Marco Mazzucchelli.