Calculus of Variations

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A branch of mathematics that is a sort of generalization of calculus. Calculus of variations seeks to find the path, curve, surface, etc., for which a given function has a stationary value (which, in physical problems, is usually a minimum or maximum). Mathematically, this involves finding stationary values of integrals of the form

 I=int_b^af(y,y^.,x)dx.
(1)

I has an extremum only if the Euler-Lagrange differential equation is satisfied, i.e., if

 (partialf)/(partialy)-d/(dx)((partialf)/(partialy^.))=0.
(2)

The fundamental lemma of calculus of variations states that, if

 int_a^bM(x)h(x)dx=0
(3)

for all h(x) with continuous second partial derivatives, then

 M(x)=0
(4)

on (a,b).

A generalization of calculus of variations known as Morse theory (and sometimes called "calculus of variations in the large") uses nonlinear techniques to address variational problems.

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