The reason I ask is that I am trying to understand the "action" in the principle of least action and the author says we must find an extrema point but cannot do so in the normal manner because the input is a function so that the action is a function of a function. He called it calculus of variations.
I do not recall this in my multivariable calculus class that I recently reviewed. Is this something generally not done in multivariable calculus? and if that is the case does anyone have any clues of a simple introduction document or text so I can complete my understanding of the "principle of least action"? Thank you