Gap-distribution conjecture for Ulam words

Let π:S[{0,1}]Z0\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:Z1[0,),g#{iui+1ui=g}kn1.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 nn\to\infty, the functions pnp_n converge pointwise to a probability measure p:Z1[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 c1.9c\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.

Sources & referencesView supporting material

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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