Ranking-based rich-get-richer processes

We study a discrete-time Markov process $X_n\in {\mathbb{R}}^d$, for which the distribution of the future increments depends only on the relative ranking of its components (descending order by value). We endow the process with a rich-get-richer assumption and show that, together with a finite second moments assumption, it is enough to guarantee almost sure convergence of $X_n$ / $n$. We characterize the possible limits if one is free to choose the initial state, and give a condition under which the initial state is irrelevant. Finally, we show how our framework can account for ranking-based P'olya urns and can be used to study ranking-algorithms for web interfaces.