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Let $G$ be a subgroup of the permutation group $S_n$, and let $H$ be a normal subgroup of $G$ such that the quotient group $G/H$ is abelian. What is the best known upper estimate for the cardinality $|G/H|$?

If $p_1, p_2, \ldots, p_m$ are the first $m$ prime numbers and $n=p_1+\cdots+p_m$, let $G$ be the group generated by $m$ independent cycles of orders $p_1, \ldots, p_m$. Then $G$ itself is abelian, and $|G|=p_1\cdots p_m$. The Prime Number Theorem implies that $\log|G|\sim(n\log n)^{1/2}$, so in general one cannot expect anything better than $\log|G/H|\le (n\log n)^{1/2}(1+o(1))$. Is this true? If this is not true, or unknown to be true, what is the best known estimate?

In the special case of transitive $G$, what is the best known estimate? Here one cannot expect anything better than $|G/H|\le n$.

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  • $\begingroup$ Oh, I see. So my construction with consecutive primes is no longer relevant. Still, the question persists: can one show anything better than the trivial estimate $|G/H|\le n!$ ? $\endgroup$
    – Yuri Bilu
    Commented Jun 5, 2023 at 8:37
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    $\begingroup$ Are you asking for an upper estimate not depending on $G$ (in which case, in other words, you're asking about the maximum cardinal of abelian subquotients of $S_n$). $\endgroup$
    – YCor
    Commented Jun 5, 2023 at 13:22
  • $\begingroup$ Peter, many thanks! This is exactly what I am looking for! $\endgroup$
    – Yuri Bilu
    Commented Jun 6, 2023 at 14:09

1 Answer 1

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In Finite permutation groups with large abelian quotients Kovacs and Praeger show (among other things) the following: If $G$ is a (not necessarily transitive) permutation group of degree $n$ and if the derived subgroup $G'$ of $G$ is a proper subgroup of $G$, then $\lvert G/G'\rvert\le p^{n/p}$ for some prime divisor $p$ of $\lvert G/G'\rvert$. In particular, we always have $\lvert G/H\rvert\le 3^{n/3}$.

The proof does not require the classification of the finite simple groups.

Remark: In On abelian quotients of primitive groups, Aschbacher and Guralnick prove the sharper bound $\lvert G/H\rvert\le n$ if $G$ is primitive (and also reprove the Kovacs-Prager result from above).

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