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Consider the Gaussian $G(x):=e^{-x^2}$ on the real line, and localize it to the region $|x|\sim 2^k$ by multiplying it by an appropriate smooth cut-off. More precisely, take $\phi\in C_0^\infty(\mathbb{R})$ supported in the region $\left \{x\in\mathbb{R}: \frac{1}{2}<|x|\leq2 \right\}$ such that $0\leq\phi\leq 1,$ and consider $G_k(x):=\phi(2^{-k}x)G(x).$ It is straightforward to check that $\|G_k\|_{L^1}\lesssim 2^ke^{-4^k}$. My question is: what can be said about $\|\widehat{G_k}\|_{L^1}$? In particular, what decay (if any) do you get in terms of $k$?

Thank you.

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    @Dilip: Edited, thanks!2011-10-13

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Comment converted to answer as suggested by Jonas Teuwen:

Hint: If we write $\phi_k(x) = \phi(2^{-k} x)$, then note that $ \widehat{G_k}(x) = \widehat{\phi_k}(x) \ast \widehat G(x). $ Applying Young's inequality for convolutions gives $ \| \widehat{G_k}\|_{1} \leq \| \widehat{\phi_k} \|_1 \| \widehat{G} \|_1 \lesssim \| \widehat{\phi_k} \|_1. $ Note that $\| \widehat{G} \|_1 \lesssim 1$ since $G$ is a Schwarz function and hence $\widehat{G}$ is too. It remains to find a good bound for $\| \widehat{\phi_k} \|_1$, which I will leave up to you.

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    Right, so this argument gives no decay in $k$, but that still doesn't answer my second question. It just shows that Young's inequality is too crude for our purposes (unless one could somehow show that the first inequality is actually an equality, which is in general far from being true). Any ideas?2011-10-13