Gap-distribution conjecture for Ulam words

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Let π:S[{0,1}]→Z≥0\pi:S[\{0,1\}]\to\mathbb{Z}_{\geq0} interpret a binary word as an integer, and let u1<u2<…<uknu_1<u_2<\ldots<u_{k_n} be the ordered elements of π(Un)\pi(\mathscr{U}_n). Define

pn:Z≥1→[0,∞),g↦#{i∣ui+1−ui=g}kn−1.p_n:\mathbb{Z}_{\geq1}\to[0,\infty),\qquad g\mapsto\frac{\#\{i\mid u_{i+1}-u_i=g\}}{k_n-1}.

Let μg(n)\mu_g(n) be the mean of the probability measure pnp_n. Gap-distribution conjecture. As n→∞n\to\infty, the functions pnp_n converge pointwise to a probability measure p:Z≥1→[0,∞)p:\mathbb{Z}_{\geq1}\to[0,\infty), and

μg(n)=Θ(n3/10);\mu_g(n)=\Theta(n^{3/10});

indeed, there may be a constant c≈1.9c\approx1.9 such that

μg(n)=cn3/10+o(1).\mu_g(n)=cn^{3/10}+o(1).

The conjecture is supported by the paper's numerical data and, if true, implies the conjectured asymptotic density through a proved comparison between the reciprocal density and the mean gap. Its general resolution remains open.

References

Primary source

Paul Adutwum, Hopper Clark, Ro Emerson, Alexandra, Sheydvasser, Arseniy, Sheydvasser and Axelle Tougouma, “Distributions of Ulam Words up to Length 30”, arXiv:2410.01217 (2025).

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