It's well known that the Fourier series converges uniformly when a function is $C^2$ and periodic on say $[0,2\pi]$. If the function is not periodic you can have lack of uniform convergence near the endpoints due to "Gibbs" phenomenon. However I would like to understand if and when you still have uniform convergence on compact subsets of $[0,2\pi]$? This seems to be geometrically obvious but I can't find a good source for it. Any suggestions would be appreciated. For instance $f(x) = x$ on $[0,1]$. It would appear that the Fourier series converges uniformly on compact subsets of $[0,2\pi]$ but this doesn't seem to follow from analysis of the Fourier coefficients. Isn't it the same as if I had done an odd extension of $f(x)$ so that I have $-|x|$ on $[-2\pi,2\pi]$ and find the fourier series for this new domain? In the latter case I clearly get uniform convergence and I don't think this should depend on the fact that I reflected and chose a different domain.
[Math] Fourier series of non-periodic functions
fourier analysis
Best Answer
I assume you mean "...if and when you still have uniform convergence on compact subsets of (0,2π)? " This is in the nature of what is called a localization theorem. These go back to Riemann who proved that the convergence of the Fourier series of an $L^1$ function $f$ AT a point $x$ depends only on the behavior of $f$ in any small neighborhood of $x$. I'll stick my neck out a little and say that I wouldn't be too surprised if there is a generalization of Riemann's Localization Theorem that says that local uniform convergence near $x$ likewise only depends on the behavior of $f$ near $x$ (but that is pure conjecture on my part).