The alternating two-player urn distribution conjecture

Consider two walkers on the three-node segment, both starting at the center, with both edge weights initially equal to 11. The walkers move alternately, walker 11 first, and each center departure chooses an edge with probability proportional to its current edge weight. Let w(n,0)w(n,0) and w(n,1)w(n,1) be the left and right edge weights after time nn, and define

Y:=limnw(n,0)w(n,0)+w(n,1).Y:=\lim_{n\to\infty}\frac{w(n,0)}{w(n,0)+w(n,1)}.

Alternating two-player urn distribution conjecture. The random variable Y[0,1]Y\in[0,1] has a density with respect to Lebesgue measure on [0,1][0,1], is not Beta-distributed, and has a distribution different from that of the limit in the corresponding non-alternating two-player model.

The alternating model has a martingale only at suitable stopping times, but its edge-weight fraction is known to converge almost surely. The density and distributional comparisons are suggested by simulations and remain unproved.

Sources & referencesView supporting material

Primary source

Fabian Michel, “Variations on Reinforced Random Walks”, arXiv:2309.02475 (2023).

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