Positive functions with positive Fourier cosine transforms Are there any real-valued strictly positive functions besides the Gaussian function whose cosine transform is also strictly positive?
For instance with $F_c(e^{-x^2}) = \frac{e^{-\frac{\omega^2}{4}}}{\sqrt{2}}$, both sides are strictly positive, whereas with $f(x) = \begin{cases} 1,  & -1<x<1 \\0, & \text{otherwise}\end{cases}$ which is strictly positive we get $F_c(f(x)) = \frac{\sqrt{\frac{2}{\pi}} \sin(\omega)}{\omega}$, which is not strictly positive.
Edit: by "positive" I actually meant non-negative, as in zero values are welcome too.
 A: How about $f(x)=\exp(-|x|)$?
We have
$$\int_{-\infty}^\infty f(x)\cos tx=
\int_0^\infty (e^{itx}+e^{-itx})e^{-x}=\frac1{1-it}+\frac1{1+it}
=\frac2{1+t^2}.$$
A: By Bochner's theorem, a function $F$ is the Fourier transform of a positive function if and only if it is positive-definite, i.e.
$$ \sum_{1\leq i,j \leq n} \xi_i \bar{\xi}_j F(a_i-a_j) \geq 0 $$
for any $n$, any set of reals $a_i$ and any complex numbers $\xi_i$. One certainly needs $f(0)>0$ and $\lvert f(x) \rvert \leq f(0)$ for this to happen (from the $n=1,2$ cases). Invoking this condition on both $F$ and its Fourier transform implies that $F$ and $\tilde{F}$ are both positive if and only if they are both positive-definite; hence the positive positive-definite functions are closed under Fourier transforms.
A: This might be a little late.. but I just want to put this out here anyway:
All convex functions have positive Fourier-Cosine transform. This was proven in by E.O Tucker 
Here is the link to the paper: On positivity of Fourier transforms
