The Frog Model on $\mathbb{Z}$ with General Random Survival Parameter

Abstract

We study the frog model on $\mathbb{Z}$ with particle-wise random geometric lifetimes: each particle has a survival parameter $\pi\in(0,1)$ sampled i.i.d., whose density near $1$ satisfies $f_\pi(u)\sim (1-u)^{\beta-1}L\big((1-u)^{-1}\big)$ with $\beta>0$, and $L$ slowly varying. This strictly extends the $\mathrm{Beta}(\alpha,\beta)$ case. Let $\eta$ denote the common law of the i.i.d.\ initial number of particles $\{\eta_x\}_{x\in\mathbb{Z}}$. Using a percolation comparison and sharp one-particle displacement tails, we obtain a universal threshold at $\beta=\tfrac12$. If $\beta>\tfrac12$ and $E(\eta)<\infty$, extinction occurs almost surely. If $\beta<\tfrac12$ and $\mathbb{P}(\eta=0)<1$, survival has positive probability. At the boundary $\beta=\tfrac12$ we give sharp criteria: extinction if $E(\eta)<\infty$ and $8\,\limsup_{n\to\infty}L(n^2)<1/E(\eta)$; survival if $\mathbb{P}(\eta=0)<1$ and $\sqrt{2}\,\liminf_{n\to\infty}L(n^2)>1/E(\eta)$. These results recover the Carvalho-Machado threshold for Beta laws and show that only the exponent $\beta$ governs the phase transition, while $L$ impacts the critical regime.