Cutoff in the Bernoulli-Laplace Model With Unequal Colors and Urn Sizes

Abstract

We consider a generalization of the Bernoulli-Laplace model in which there are two urns and $n$ total balls, of which $r$ are red and $n - r$ white, and where the left urn holds $m$ balls. At each time increment, $k$ balls are chosen uniformly at random from each urn and then swapped. This system can be used to model phenomena such as gas particle interchange between containers or card shuffling. Under a reasonable set of assumptions, we bound the mixing time of the resulting Markov chain asymptotically in $n$ with cutoff at $\log{n}$ and constant window. Among other techniques, we employ the spectral analysis of arXiv:0906.4242 on the Markov transition kernel and the chain coupling tools of arXiv:2203.08647 and arXiv:1606.01437