13
$\begingroup$

Let $G$ be a finite group, and $n$ a positive integer. It is not hard to check that the following are equivalent:

  1. For every $g\in G\setminus\{1\}$ there is a subgroup $H\leq G$ with $|G/H|\leq n$ and $g\notin H$.
  2. There is a finite $G$-set $X$ with faithful action such that each orbit has size at most $n$.
  3. There is an injective homomorphism $G\to\Sigma_n^r$ for some $r\geq 0$ (where $\Sigma_n$ is the symmetric group).

I'll say that the embedding rank of $G$ (written $\operatorname{erank}(G)$) is the least $n$ such that these conditions hold. (Is there a more established name for this?) Basic facts about embedding rank include the following.

  1. If $H\leq G$, it is clear that $\operatorname{erank}(H)\leq\operatorname{erank}(G)$.
  2. If $G$ is abelian then we can write it as a product of cyclic groups of prime power order, and then $\operatorname{erank}(G)$ is just the maximum of those orders.
  3. Using the same ideas: if $N\lhd G$ and $G/N$ is abelian then $\operatorname{erank}(G/N)\leq\operatorname{erank}(G)$.

My main question: if $N\lhd G$ but $G/N$ is not abelian, is it still true that $\operatorname{erank}(G/N)\leq\operatorname{erank}(G)$? I can prove this if $\operatorname{erank}(G)\leq 3$, but the proof uses many special considerations that are unlikely to generalise much further. It is easy to produce examples where $G$ acts faithfully on a set $X$ but the induced action of $G/N$ on $X/N$ is no longer faithful, so there is no obvious argument based on that construction or anything equivalent to it.

(UPDATE: Assertion based on incorrect computer calculations removed)

$\endgroup$
2
  • 2
    $\begingroup$ I hope I haven't misunderstood your question, because according to my answer below $G=D_8 \times D_8$ is a counterexample of order $64$. $\endgroup$
    – Derek Holt
    Commented Aug 12 at 21:53
  • 1
    $\begingroup$ @DerekHolt Indeed, there was a stupid typo in my code. The minimal counterexample if $G=\langle a,b|a^4=b^4=aba^{-1}b=1\rangle$ of order $16$ and rank $4$: the quotient by $\langle a^2b^2\rangle$ is $Q_8$ with rank $8$. $\endgroup$ Commented Aug 13 at 7:59

1 Answer 1

15
$\begingroup$

The answer is no.

There is a well known family (due to Peter Neumann) of groups $G \le {\rm Sym}(n)$ with normal subgroups $N$ such that $G/N$ does not embed into ${\rm Sym}(n)$, and the same examples are counterexamples for your question.

Let $G$ be a direct product of $k$ copies of the dihedral group of order $8$, and let $N$ be a subgroup of order $2^{k-1}$ of $Z(G)$ such that $G/N$ is an extraspecial group of order $2^{1+2k}$.

Then ${\rm erank}(G) = 4$ (clearly), but ${\rm erank}(G/N) = 2^{1+k}$.

To see this note that, since $Z(G/N)$ of order $2$ is the only minimal normal subgroup of $G/N$, any faithful action of $G/N$ must have an orbit with faithful action. The largest core-free subgroup of $G/N$ is an elementary abelian subgroup of order $2^k$, so the index of this subgroup is the ${\rm erank}$ of $G/N$.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.