verret
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Is there anything significant about GAP's SmallGroup(512,2045)?
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19 votes

It is the "free" group of nilpotency class at most $2$, exponent dividing $4$, on three generators. (In other words, every group in that class is a quotient of it.) According to https://etd....

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Are there infinitely many insipid numbers?
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17 votes

Almost all $n$ are insipid. In fact, the number of non-insipid numbers at most $n$ grows like $2n/\log n$. See the paper Cameron, Peter J.; Neumann, Peter M.; Teague, David N. On the degrees of ...

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Fantastic properties of Z/2Z
10 votes

It is the only finite group with exactly two conjugacy classes and it is the only non-trivial group with trivial automorphism group. EDIT: For the second one, it turns out you need the axiom of ...

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Subgroup ranks of the symmetric group
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9 votes

After some googling, one finds a few references. Most point to Cameron, Peter J.; Solomon, Ron; Turull, Alexandre, Chains of subgroups in symmetric groups. J. Algebra 127 (1989), no. 2, 340–352. ...

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Highly symmetric 6-regular graph with 20 vertices
9 votes

EDIT : Cleaned up answer, added more info. 20 is small enough that it is possible to find ALL the symmetric 6-valent graphs on 20 vertices. In fact, all the vertex-transitive graphs up to 32 ...

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Population of P people, where each person knows K others, how many people mutually know each other
8 votes

For $x\geq 2$, and $m$ odd, there is a regular digraph of order $P=(x-1)m$ and of out-valency $K=(x-2)m+\frac{m-1}{2}=\frac{(2x-3)m}{2}-\frac{1}{2}$ with no cliques of size $x$: start with the ...

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Characteristic subgroups of $p$-groups
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8 votes

Have a look at : S. P. Glasby, P. P. Pálfy, Csaba Schneider, p-Groups with a unique proper non-trivial characteristic subgroup, Journal of Algebra, Volume 348,Pages 85-109 http://www.sciencedirect....

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Are there only finitely many distinct cubic walk-regular graphs that are neither vertex-transitive nor distance-regular?
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8 votes

A graph is called semisymmetric if it is regular, edge-transitive but not vertex-transitive. Semisymmetric graphs are walk-regular hence they provide example of graphs that are regular and walk-...

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When Cayley graphs of the symmetric group wrt generating sets of transpositions are isomorphic?
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8 votes

I claim the answer to your question is yes. This is my first time posting on mathoverflow. I hope my latex goes ok. Given $\Gamma(S_n,A)$, build an auxiliary graph $X(\Gamma(S_n,A))$, with vertex set ...

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Lower frattini subgroup
7 votes

The smallest counterexample is the dicyclic group of order $36$: $G=C_9\rtimes C_4$, with the generator of $C_4$ acting by inversion on $C_9$. In this case, $\Phi_{-}(G)\cong C_6$, while $G/\Phi_{-}(...

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Condition(s) for the full autormophism group $\operatorname{Aut}(C(G, S))$ of the Cayley graph of $G$ to be isomorphic to $G$
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7 votes

First, some terminology: if $G\cong\mathrm{Aut}(\Gamma)$ then $\Gamma$ is often called a GRR (for graphical regular representation). This may help in looking for references. Determining whether a ...

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Regular graph of order 50, degree 7 and Automorphism group of order 288000. How to check if it is Cayley
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7 votes

A graph is Cayley if and only if its automorphism group has a regular subgroup. A computer algebra system such as Magma or GAP (I think) usually can determine if a permutation group of order 288000 ...

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Can $1\ne H\cap H^g\lhd H$ happen if $G$ is a primitive permutation group with stabiliser $H$?
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6 votes

An example was constructed by Pablo Spiga: https://arxiv.org/abs/2102.13614 "A generalization of Sims conjecture for finite primitive groups and two point stabilizers in primitive groups"

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Generator sets of a subgroup of $S_n$ with $O(n)$ total support - do they always exist ?
6 votes

The answer to Q2 is no. Let $H$ be an elementary abelian $2$-group, in its regular action, so $n=2^a$ for some $a$. Clearly, to generate $H$, we need $a$ elements with full support, so the total ...

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Cubic Cayley (undirected) graphs
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6 votes

Everything that is not obviously forbidden can happen. For example, a single group can have two Cayley graphs that are isomorphic (as graphs) but which are not of the same type. Such examples will ...

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Is there a permutation group satisfying the following property?
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5 votes

Let $\Omega=\{1,2,3,4,5,6,7,8,9\}$ and let $G\leq\mathrm{Sym}(\Omega)$ be the group generated by the following permutations: $(1, 2, 9)$ $(4, 5)(7, 8)$ $(1, 4, 7)(2, 5, 8)(3, 6, 9)$ $(3, 6)(4, 7)(5, ...

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Is there a 2 transitive finite group with transitive normal subgroup having a cyclic quotient other than $A_n$ and $S_n$?
5 votes

The most obvious family of examples is $AGL(1,q)$ for $q$ a prime power. As spin said in the comments, finite $2$-transitive groups are classified. They are all almost simple or of affine type (...

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Finite groups with cyclic self-normalizing subgroup
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5 votes

$\mathrm{P\Sigma L}(2,8)$ is a counterexample. All its cyclic groups of order $6$ are self-normalising. (They fall into two conjugacy classes.) (This the automorphism group of $\mathrm{PSL}(2,8)$, it ...

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Characterization of finite groups generated by one additional element of prime order for every element of prime order
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5 votes

$\mathrm{P\Sigma L}(2,8)$ is a counter-example. A corrected version of the statement is that, under the hypothesis, either a) $G\cong Z_p$ for some prime $p$, b) $G\cong Z_p\times Z_q$ for some ...

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Max order for which connected Cayley Graphs are known to be Hamiltonian
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5 votes

According to : http://arxiv.org/pdf/1009.5795v3.pdf this is known for all n up to 120 except 72, 96, 108 and 120.

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What are all the transitive extensions of cyclic groups?
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4 votes

Updated: In the finite case (as in the reference), the only examples are $\mathrm{AGL}(1,q)$ for $q$ a prime power. This follows, for example, by "Lucchini, Mainardis, Stellmacher, Transitive ...

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transitive subgroups of $S_n$
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4 votes

Since nothing else came up, I'm posting this as an answer. For $n$ a power of a prime $p$, the answer is any group of exponent $p$, acting regularly. Indeed, if $n$ is a power of $p$, then a Sylow $...

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Kernel of a double cover of group as stem extension
4 votes

In SmallGroup(16,3), the derived subgroup has order 2, is central, and its generator is not a square. (See groupprops.subwiki.org/wiki/SmallGroup(16,3)#Subgroups)

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Graphs with prescribed Automorphism groups
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4 votes

For $k=0$, take a complete graph and for $k=1$, a star. For $k\geq 2$, take a path with $k$ vertices, and a complete graph with $n-k$ vertices, then join one end of the path with all the vertices in ...

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A question on the number of involutions in a 2-group
3 votes

Miller, G.A. Groups of order $g$ containing $g/2-1$ involutions. Tôhuku math. J. 17, (1920) 88-102 (https://www.jstage.jst.go.jp/article/tmj1911/17/0/17_0_88/_pdf ) is on exactly this question. The ...

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the number of minimal generating subsets of a group
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3 votes

1.No, this is a hard question in general. It could maybe be done for special classes of groups, say nilpotent groups. 2.The only (finitely-generated) groups which have a unique minimal generating ...

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Is there a subset of $\Sigma_n$ s.t. each pair of elements is once in each pair of positions?
3 votes

You are asking for a sharply $2$-transitive set of degree $n$. These objects are closely linked to projective planes of order $n$. In particular, they exist if $n$ is a prime power, and no example is ...

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Vertex-primitive graphs with two vertices having almost the same neighbourhood
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3 votes

Pablo Spiga found a proof a few weeks ago. Together, we then proved a slightly more general result which is now on the arxiv: http://arxiv.org/abs/1501.05046 It is more general in two ways: it deals ...

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Minimum word length for an unusual set of generators of the symmetric group
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3 votes

This problem is known as ''pancake sorting'' or ''sorting by prefix reversal''. Imagine you have a stack of pancakes numbered $1,2,3,\ldots$ starting from the top. Then $t_k$ corresponds to taking the ...

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simple graphs of degree 16 with a semiregular normal subgroup isomorphic to the quaternion group $Q_8$
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3 votes

Here is an example. Adjacency matrix: $ \left[ \begin{array}{cccccccccccccccc} 0&1&1&0&1&1&1&0&0&0&0&0&0&0&0&0\\1&0&0&1&1&...

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