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arXiv · 2611.00943

The Exact Central-Index Profile of Groups with Bounded Noncommutativity

Guillaume Lecomte
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Abstract

For a finite group $G$, let $ω(G)$ be the largest size of a pairwise noncommuting set and put $z(G)=[G:\mathrm Z(G)]$. We determine the maximum possible central index under $ω(G)\le k$ for every integer $k\ge1$, giving an exact answer to Neumann's central-index question and sharpening Pyber's exponential upper bound. The maximum is $1$ for $k=1,2$, $(k-1)^2$ for $3\le k\le6$, $81$ for $k=7,8$, and $4^{\lfloor(k-1)/2\rfloor}$ for $k\ge9$. In the last range, with $r=\lfloor(k-1)/2\rfloor$, equality holds exactly when $G'\cong C_2$ and $G/\mathrm Z(G)\cong\mathbb F_2^{2r}$. We also obtain the sharp universal bound $z(G)\le(81/64)2^{ω(G)-1}$. All maximizers are classified up to isoclinism. The results extend to arbitrary groups with finite clique number through finite isoclinic representatives.

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Guillaume Lecomte. 2026-10-31. The Exact Central-Index Profile of Groups with Bounded Noncommutativity. https://arxiv.org/abs/2611.00943

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