Calculus of variations - Wikipedia
https://en.wikipedia.org/wiki/Calculus_of_variationsThe calculus of variations is a field of mathematical analysis that uses variations, which are small changes in functions and functionals, to find maxima and minima of functionals: mappings from a set of functions to the real numbers. Functionals are often expressed as definite integrals involving functions and their derivatives.Functions that maximize or minimize functionals may …