I A concise "proof" of the Riemann-Lebesgue lemma

psie
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Roughly, at least in the text I'm currently reading, the Riemann-Lebesgue lemma states that the Fourier transform ##\hat f## of an ##L^1(\mathbb R,\mathcal B(\mathbb R),\lambda)## function ##f## vanishes at infinity. I have trouble understanding a concise proof of this lemma.
In the Riemann-Lebesgue lemma, the author says it suffices to prove $$\hat{f}(\xi)\underset{|\xi|\to\infty}{\to}0$$for step functions on ##\mathbb R## only (step functions are simple functions where the sets of the indicator functions are intervals in ##\mathbb R##). This is because the step functions are dense in ##L^1(\mathbb R,\mathcal B(\mathbb R),\lambda)## and so let ##(\varphi_n)## be a sequence of step functions that approximate ##f## in the ##L^1## norm, i.e. ##\|f-\varphi_n\|_1\to0## as ##n\to\infty##. Then we just have to observe \begin{align*}\sup_{\xi\in\mathbb R}|\hat{f}(\xi)-\hat{\varphi}_n(\xi)|&=\sup_{\xi\in\mathbb R}\left|\int f(x)e^{\mathrm{i}x\xi}\,\mathrm{d}x-\int \varphi_n(x)e^{\mathrm{i}x\xi}\,\mathrm{d}x\right|\\ &\leq\|f-\varphi_n\|_1\end{align*}

That's basically the end of the "proof". The above inequality shows ##\hat\varphi_n\to\hat f## in ##L^\infty## (since almost uniform convergence is equivalent to convergence in ##L^\infty##), but how does the proof of the lemma proceed (assuming we've shown the lemma holds for step functions)?
 
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I think I get it now (and it wasn't so hard, just use the triangle inequality :smile: ). $$|\hat f(\xi)|\leq |\hat{f}(\xi)-\hat{\varphi}_n(\xi)|+|\hat{\varphi}_n(\xi)|\leq\|\hat{f}-\hat{\varphi}_n\|_\infty+|\hat{\varphi}_n(\xi)|.$$Now pick ##n## so large that ##\|\hat{f}-\hat{\varphi}_n\|_\infty<\epsilon## and then for those ##n##, let ##|\xi|\to \infty##.
 
A sphere as topological manifold can be defined by gluing together the boundary of two disk. Basically one starts assigning each disk the subspace topology from ##\mathbb R^2## and then taking the quotient topology obtained by gluing their boundaries. Starting from the above definition of 2-sphere as topological manifold, shows that it is homeomorphic to the "embedded" sphere understood as subset of ##\mathbb R^3## in the subspace topology.
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