# Connection between the Fourier transform of f and |f|

If $f\in L^p(R)$ with $1\leq p\leq 2$, then Hausdorff-Young inequality implies that the Fourier transform $\widehat{f}\in L^{p'}$, $p'$ is the dual exponent of $p$, and $$\|\widehat{f}\|_{L^{p'}}\lesssim \|f\|_{L^p}.$$ We note that $f\in L^p$ is equivalent to $|f|\in L^p$, so we find $\widehat{|f|}\in L^{p'}$ and $$\|\widehat{|f|}\|_{L^{p'}}\lesssim \|f\|_{L^p}.$$ So my question arises, does there exist connection between $\|\widehat{f}\|_{L^{p'}}$ and $\|\widehat{|f|}\|_{L^{p'}}$ ? Precisely, does the following $$\|\widehat{f}\|_{L^{p'}}\lesssim \|\widehat{|f|}\|_{L^{p'}}$$ holds? Note that $p=2$ it is a trivial by Plancherel theorem.

• Have you tried looking in Katznelson's book, in the remarks and examples surrounding his discussion of the Hausdorff-Young inequality? – Yemon Choi Jan 2 '15 at 1:19
• @YemonChoi Thanks for your comment. Unfortunately, I don't find the answer there. – Wang Ming Jan 2 '15 at 2:08
• Have you tried writing $|f(x)|=s(x)f(x)$ (where $s:\mathbb R\to\{-1,1\}$ is the sign of $f$) and studying the convolution that arises from the Fourier transform of $|f|$? I'm not sure if it takes you anywhere, but it might be worth a try. – Joonas Ilmavirta Jan 2 '15 at 9:33

## 1 Answer

No. The inequality $||\hat{f}||_{L^p} \lesssim || \hat{|f|} ||_{L^{p}}$ does not hold for $p \neq 2$. This is, perhaps, easier to see in the case of Fourier series on the circle. A sketch of a contstruction is given in my answer to another mathoverflow quesion here. This example can be modified to work in Euclidean space as well.