Putman–Wieland conjecture on finite-index subgroups of surface groups

Let Σgp\Sigma^p_g be a smooth surface of genus g2g\ge 2 with p0p\ge 0 punctures, let Σgp\star\in\Sigma^p_g be a basepoint, and let K<π1(Σgp,)K<\pi_1(\Sigma^p_g,\star) be a finite-index subgroup. Let π:(Σ,)(Σgp,)\pi:(\Sigma',\star')\to(\Sigma^p_g,\star) be the corresponding pointed cover, let Σ^\widehat{\Sigma} be the closed surface obtained by filling in the punctures of Σ\Sigma', and let ΓK<Modgp+1\Gamma_K<\operatorname{Mod}^{p+1}_g be the finite-index subgroup preserving KK under the standard action on π1(Σgp,)\pi_1(\Sigma^p_g,\star). The subgroup KK has the Putman–Wieland property if, for every nonzero vector vH1(Σ^,Q)v\in H^1(\widehat{\Sigma},\mathbb{Q}), its ΓK\Gamma_K-orbit is infinite.

Putman–Wieland conjecture. For every g2g\ge 2, every p0p\ge 0, and every finite-index subgroup K<π1(Σgp,)K<\pi_1(\Sigma^p_g,\star), the subgroup KK has the Putman–Wieland property.

The conjecture concerns the dynamics of mapping class groups on the first cohomology of compactified finite covers of punctured surfaces. It was disproved in genus g=2g=2 by Marković, so the conjecture is refuted.

Sources & referencesView supporting material

Primary source

Adam Klukowski and Vladimir Marković, “Tangle free permutations and the Putman-Wieland property of Random covers”, arXiv:2402.19018 (2024).

Additional references

3 papers in this index state this conjecture (2022–2024). The statement above is taken from the most recent of them; the others are arXiv:2209.00717, arXiv:2209.00718.

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