The noncongruence-surjection conjecture for finite simple groups

From papers

Let F2F_2 be the free group of rank 22, let GG be a nonabelian finite simple group, and let φ:F2G\varphi:F_2\to G be a surjection. Denote by Γφ\Gamma_\varphi the stabilizer of the corresponding point under the relevant SL2(Z)\operatorname{SL}_2(\mathbb{Z}) action.

Noncongruence-surjection conjecture. If GG is a nonabelian finite simple group, then there exists a surjection φ:F2G\varphi:F_2\to G such that Γφ\Gamma_\varphi is noncongruence. Equivalently, the kernel of the SL2(Z)\operatorname{SL}_2(\mathbb{Z}) action on Epi(F2,G)/Inn(G)\operatorname{Epi}(F_2,G)/\operatorname{Inn}(G) is noncongruence.

This is proposed as a weakening of an earlier conjecture after the paper finds many noncongruence surjections and infinitely many counterexamples to that earlier statement. The proposed weakening remains open in the supplied text.

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Sources & referencesView supporting material

Primary source

William Y. Chen, Alex Lubotzky and Pham Huu Tiep, “Finite simple characteristic quotients of the free group of rank 2”, arXiv:2308.14302 (2023).

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