Limiting root-measure conjecture for random positive 3-braids

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Let Br3\mathrm{Br}_3 be the 3-strand braid group, let Ω={σ1,σ2}N\Omega=\{\sigma_1,\sigma_2\}^{\mathbb{N}} with the product measure assigning probability 1/21/2 to each generator, and set wn=g1⋯gnw_n=g_1\cdots g_n. For a braid ww, let ZwZ_w be the multiset of zeros of the Alexander polynomial of its closure and let

νw=1#Zw∑z∈Zwδz.\nu_w=\frac{1}{\# Z_w}\sum_{z\in Z_w}\delta_z.

Limiting root-measure conjecture. There is a compactly supported measure ν∞\nu_\infty on C\mathbb{C} such that, for almost every sequence (gi)i∈N∈Ω(g_i)_{i\in\mathbb{N}}\in\Omega, νwn\nu_{w_n} converges weakly to ν∞\nu_\infty. Its support is contained in the union of the unit circle, a continuous curve from ζ3\zeta_3 to ζ‾3\overline{\zeta}_3, and the image of that curve under z↦1/zz\mapsto1/z; the curve crosses the real axis at about −0.78-0.78. Writing

AL={eiθ:∣θ−π∣<π/3},AR={eiθ:∣θ∣<2π/3},\mathcal A_L=\{e^{i\theta}:|\theta-\pi|<\pi/3\},\qquad \mathcal A_R=\{e^{i\theta}:|\theta|<2\pi/3\},

exactly 2/32/3 of the mass lies on AR\mathcal A_R, where the measure is a multiple of Lebesgue measure; its mass on AL\mathcal A_L is (7−35)/12≈0.024316(7-3\sqrt5)/12\approx0.024316, so the total mass on the unit circle is (5−5)/4≈0.690983(5-\sqrt5)/4\approx0.690983. The measures along the curves joining ζ3\zeta_3 to ζ‾3\overline{\zeta}_3 are absolutely continuous with respect to Lebesgue measure. These predictions refine the experimentally observed concentration and equidistribution of roots; the convergence and the detailed support and density assertions remain conjectural.

References

Primary source

Nathan M. Dunfield and Giulio Tiozzo, “Roots of Alexander polynomials of random positive 3-braids”, arXiv:2402.06771 (2025).

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