I was looking for a function $f$ which verifies (I do not know if it exists) :
- $f:\mathbb{R}\rightarrow\mathbb{R}$
- $\forall x\in\mathbb{R},\;f(x)\geq 0$
- $\lim\limits_{x\rightarrow-\infty}f(x)=\lim\limits_{x\rightarrow+\infty}f(x)=0$
- $\int\limits_{-\infty}^{+\infty}f(x)\textrm{d}x=+\infty$
Here is a candidate I tried, but I cannot prove it diverges :
$$\int\limits_{-\infty}^{+\infty}\frac{1}{\ln(x^2+1)}\textrm{d}x$$