Two-part criterion for odd-characteristic subgroup rigidity

Let FF be a free group and let pp be an odd prime. Let HH be a finitely generated subgroup of FF, and let H~\widetilde H be its L2L^2-closure. The two-part odd-characteristic criterion.

β0Fp[F](IFp[F]/FIFp[H])=β0Q[F](IQ[F]/FIQ[H]).\beta_0^{\mathbb{F}_p[F]}(I_{\mathbb{F}_p[F]}/{}^FI_{\mathbb{F}_p[H]})=\beta_0^{\mathbb{Q}[F]}(I_{\mathbb{Q}[F]}/{}^FI_{\mathbb{Q}[H]}).
  1. The left Fp[F]\mathbb{F}_p[F]-module IFp[F]/FIFp[H~]I_{\mathbb{F}_p[F]}/{}^FI_{\mathbb{F}_p[\widetilde H]} is DFp[F]\mathcal{D}_{\mathbb{F}_p[F]}-torsion-free.

These are presented as the two subproblems underlying odd-characteristic subgroup rigidity of free groups. They remain open in the supplied text.

Sources & referencesView supporting material

Primary source

Andrei Jaikin-Zapirain, “Free groups are L^2-subgroup rigid”, arXiv:2403.09515 (2026).

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