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Andreadakis Conjecture


The Andreadakis conjecture compares two filtrations of the automorphism group of a free group F_n. Write gamma_c(F_n) for its lower central series, and let A_c be the kernel of the action of Aut(F_n) on F_n/gamma_(c+1)(F_n). Then A_1=IA(F_n) consists of the automorphisms acting trivially on the Abelianization. The conjectured equality is gamma_c(IA(F_n))=A_c (Andreadakis 1965).

Bartholdi (2013, 2016) gave an earlier counterexample in rank three and degree seven. Kofinas (2026) proved the strict inclusion

 gamma_c(IA(F_n))gamma_(c-1)(IA(F_n)) intersection A_c

for every n>=3 and c>=4. Thus the conjectured equality fails throughout these ranks and degrees. The notation A_c here corresponds to I_(c+1)A(F_n) in Kofinas (2026).


See also

Automorphism Group, Free Group, Lower Central Series

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References

Andreadakis, S. "On the Automorphisms of Free Groups and Free Nilpotent Groups." Proc. London Math. Soc. 15, 239-268, 1965. https://doi.org/10.1112/plms/s3-15.1.239.Bartholdi, L. "Automorphisms of Free Groups. I-Erratum." New York J. Math. 22, 1135-1137, 2016.Bartholdi, L. "Automorphisms of Free Groups. I." New York J. Math. 19, 395-421, 2013.Kofinas, C. E. "The Andreadakis Conjecture." 26 Oct 2026. https://arxiv.org/abs/2610.36302.

Cite this as:

Weisstein, Eric W. "Andreadakis Conjecture." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/AndreadakisConjecture.html

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