I need to determine all functions u(x) that extremise the functional: I[u]=∫∞−∞[(u′)22+(1−cosu)]dx
subject to the boundary conditions
lim and \lim_{x \to \infty} u(x) = 2\pi
I used the standard approach for finding the stationary points of a functional; that is, attempting to solve the Euler-Lagrange equation, but assuming I've attempted this correctly I arrive at
\frac{d^2u}{dx^2} = \sin u
which I believe is not (easily) directly solvable, so I'm presuming there's either another way to approach this problem or I've messed up somewhere. A point in the right direction would be great, thanks in advance
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