Lower limits and equivalences for convolution tails
  Suppose $F$ is a distribution on the half-line $[0,\infty)$. We study the limits of the ratios of tails $\bar{F*F}(x)/\bar{F}(x)$ as $x\to\infty$. We also discuss the classes of distributions ${\mathcal{S}}$, ${\mathcal{S}}(\gamma)$ and ${\mathcal{S}}^*$. 
Sergey Foss, Dmitry Korshunov