I have a doubt and I am not able to prove (or disprove):
- Let $f(x)$ be an odd function with $f(x)>0\,\,\,\forall x\in (0,+\infty)$.
- Let $g(x)$ be a non-negative function: $g(x)\geq 0\;\forall x\in \mathbb{R}.$
- Also suppose $\displaystyle \int_{-\infty}^0g(x)\,dx<\int_{0}^{\infty}g(x)\,dx.$
I wonder if one can assure that:
$$\int_{-\infty}^{\infty}f(x)\,g(x)\,dx>0.$$
EDIT 1: Has been proved (by Adrian Keister) that my thesis is false.
Now I wonder again if is possible add another hypothesis about $g(x)$ to assure my thesis.
EDIT 2:The problem arrives from here:
blue line is $f(x)= \left(e^{-\frac{\cosh ^2(u-1)}{2 }}-e^{-\frac{\cosh ^2(1+u)}{2 }}\right)$ and orange line is $g(x)=e^{-\frac{u^2}{2 }}\cos ^2\left(\frac{\pi (u-1)}{4 }\right)$ and the function $f(x)g(x)$ graphic
As we can see in the graph, the integral $\int_{\mathbb{R}}f(x)g(x)\,dx$ seems to be positive.
We can translate the factor $e^{-u^2/2}$ from $g(x)$ to $f(x)$ (in this case the third hypothesis is not fulfilled):