The phase transition for domination in two interacting urns with power reinforcement

Abstract

We study two interacting urns with power reinforcement $W(n)=n^\alpha$, for $\alpha>1$. Using Qin's stochastic-approximation theorem as input, we reduce the phase transition problem for domination to the sign analysis of a single analytic function $\Phi_p$. This deterministic reduction identifies the sharp threshold and proves that the two natural critical parameters for domination coincide. We determine the first-order behavior of the critical interaction parameter as $\alpha\downarrow1$ and show that it is not nondecreasing in the reinforcement exponent, answering two questions posed by Qin.