Let $f\in L^1(\mathbb{R})$. Suppose that $\hat{f}$, the Fourier transform of $f$, is a positive function in $C_0(\mathbb{R})$. Does there exists any function $g\in L^1(\mathbb{R})$ with $(\hat{g})^2=\hat{f}$?
The inversion formula for the square roof of a positive function
ABB
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