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We determine all finite sets of equiangular lines spanning finite-dimensional complex unitary spaces for which the action on the lines of the set-stabiliser in the unitary group is 2-transitive with a regular normal subgroup.
A group is $\frac 32$-generated if every non-trivial element is part of a generating pair. In 2019, Donoven and Harper showed that many Thompson groups are $\frac 32$-generated and posed five questions. The first of these is whether there exists a 2-generated group with every proper quotient cyclic that is not $\frac 32$-generated. This is a natural question given the significant work in proving that no finite group has this property, but we show that there is such an infinite group. The groups we consider are a family of finite index subgroups $G_1,\, G_2,\, \ldots$ of the Houghton group $\operatorname {FSym}(\mathbb {Z})\rtimes \mathbb {Z}$. We then show that $G_1$ and $G_2$ are $\frac 32$-generated and investigate the related notion of spread for these groups. We are able to show that they have finite spread at least 2. These are, therefore, the first infinite groups to be shown to have finite positive spread, and the first to be shown to have spread at least 2 (other than $\mathbb {Z}$ and the Tarski monsters, which have infinite spread). As a consequence, for each $k\in \{2,\, 3,\, \ldots \}$, we also have that $G_{2k}$ is index $k$ in $G_2$ but $G_2$ is $\frac 32$-generated whereas $G_{2k}$ is not.
If G is permutation group acting on a finite set
$\Omega $
, then this action induces a natural action of G on the power set
$\mathscr{P}(\Omega )$
. The number
$s(G)$
of orbits in this action is an important parameter that has been used in bounding numbers of conjugacy classes in finite groups. In this context,
$\inf ({\log _2 s(G)}/{\log _2 |G|})$
plays a role, but the precise value of this constant was unknown. We determine it where G runs over all permutation groups not containing any
${{\textrm {A}}}_l, l> 4$
, as a composition factor.
A noncomplete graph is $2$-distance-transitive if, for $i \in \{1,2\}$ and for any two vertex pairs $(u_1,v_1)$ and $(u_2,v_2)$ with the same distance i in the graph, there exists an element of the graph automorphism group that maps $(u_1,v_1)$ to $(u_2,v_2)$. This paper determines the family of $2$-distance-transitive Cayley graphs over dihedral groups, and it is shown that if the girth of such a graph is not $4$, then either it is a known $2$-arc-transitive graph or it is isomorphic to one of the following two graphs: $ {\mathrm {K}}_{x[y]}$, where $x\geq 3,y\geq 2$, and $G(2,p,({p-1})/{4})$, where p is a prime and $p \equiv 1 \ (\operatorname {mod}\, 8)$. Then, as an application of the above result, a complete classification is achieved of the family of $2$-geodesic-transitive Cayley graphs for dihedral groups.
Let $G$ be a primitive permutation group of degree $n$ with nonabelian socle, and let $k(G)$ be the number of conjugacy classes of $G$. We prove that either $k(G)< n/2$ and $k(G)=o(n)$ as $n\rightarrow \infty$, or $G$ belongs to explicit families of examples.
A classical theorem of Frucht states that any finite group appears as the automorphism group of a finite graph. In the quantum setting, the problem is to understand the structure of the compact quantum groups which can appear as quantum automorphism groups of finite graphs. We discuss here this question, notably with a number of negative results.
We show that, given a compact minimal system
$(X,g)$
and an element h of the topological full group
$\tau [g]$
of g, the infinite orbits of h admit a locally constant orientation with respect to the orbits of g. We use this to obtain a clopen partition of
$(X,G)$
into minimal and periodic parts, where G is any virtually polycyclic subgroup of
$\tau [g]$
. We also use the orientation of orbits to give a refinement of the index map and to describe the role in
$\tau [g]$
of the submonoid generated by the induced transformations of g. Finally, we consider the problem, given a homeomorphism h of the Cantor space X, of determining whether or not there exists a minimal homeomorphism g of X such that
$h \in \tau [g]$
.
A permutation group G on a set A is
${\kappa }$
-homogeneous iff for all
$X,Y\in \bigl [ {A} \bigr ]^ {\kappa } $
with
$|A\setminus X|=|A\setminus Y|=|A|$
there is a
$g\in G$
with
$g[X]=Y$
. G is
${\kappa }$
-transitive iff for any injective function f with
$\operatorname {dom}(f)\cup \operatorname {ran}(f)\in \bigl [ {A} \bigr ]^ {\le {\kappa }} $
and
$|A\setminus \operatorname {dom}(f)|=|A\setminus \operatorname {ran}(f)|=|A|$
there is a
$g\in G$
with
$f\subset g$
.
Giving a partial answer to a question of P. M. Neumann [6] we show that there is an
${\omega }$
-homogeneous but not
${\omega }$
-transitive permutation group on a cardinal
${\lambda }$
provided
(i)
${\lambda }<{\omega }_{\omega }$
, or
(ii)
$2^{\omega }<{\lambda }$
, and
${\mu }^{\omega }={\mu }^+$
and
$\Box _{\mu }$
hold for each
${\mu }\le {\lambda }$
with
${\omega }=\operatorname {cf}({\mu })<{{\mu }}$
, or
(iii) our model was obtained by adding
$(2^{\omega })^+$
many Cohen generic reals to some ground model.
For
${\kappa }>{\omega }$
we give a method to construct large
${\kappa }$
-homogeneous, but not
${\kappa }$
-transitive permutation groups. Using this method we show that there exist
${\kappa }^+$
-homogeneous, but not
${\kappa }^+$
-transitive permutation groups on
${\kappa }^{+n}$
for each infinite cardinal
${\kappa }$
and natural number
$n\ge 1$
provided
$V=L$
.
A graph is edge-primitive if its automorphism group acts primitively on the edge set, and
$2$
-arc-transitive if its automorphism group acts transitively on the set of
$2$
-arcs. In this paper, we present a classification for those edge-primitive graphs that are
$2$
-arc-transitive and have soluble edge-stabilizers.
Let G be a permutation group on a set
$\Omega $
of size t. We say that
$\Lambda \subseteq \Omega $
is an independent set if its pointwise stabilizer is not equal to the pointwise stabilizer of any proper subset of
$\Lambda $
. We define the height of G to be the maximum size of an independent set, and we denote this quantity
$\textrm{H}(G)$
. In this paper, we study
$\textrm{H}(G)$
for the case when G is primitive. Our main result asserts that either
$\textrm{H}(G)< 9\log t$
or else G is in a particular well-studied family (the primitive large–base groups). An immediate corollary of this result is a characterization of primitive permutation groups with large relational complexity, the latter quantity being a statistic introduced by Cherlin in his study of the model theory of permutation groups. We also study
$\textrm{I}(G)$
, the maximum length of an irredundant base of G, in which case we prove that if G is primitive, then either
$\textrm{I}(G)<7\log t$
or else, again, G is in a particular family (which includes the primitive large–base groups as well as some others).
We construct a sofic approximation of
${\mathbb F}_2\times {\mathbb F}_2$
that is not essentially a ‘branched cover’ of a sofic approximation by homomorphisms. This answers a question of L. Bowen.
Let $G$ be a finite permutation group of degree $n$ and let ${\rm ifix}(G)$ be the involution fixity of $G$, which is the maximum number of fixed points of an involution. In this paper, we study the involution fixity of almost simple primitive groups whose socle $T$ is an alternating or sporadic group; our main result classifies the groups of this form with ${\rm ifix}(T) \leqslant n^{4/9}$. This builds on earlier work of Burness and Thomas, who studied the case where $T$ is an exceptional group of Lie type, and it strengthens the bound ${\rm ifix}(T) > n^{1/6}$ (with prescribed exceptions), which was proved by Liebeck and Shalev in 2015. A similar result for classical groups will be established in a sequel.
Generalising previous results on classical braid groups by Artin and Lin, we determine the values of m, n ∈ $\mathbb N$ for which there exists a surjection between the n- and m-string braid groups of an orientable surface without boundary. This result is essentially based on specific properties of their lower central series, and the proof is completely combinatorial. We provide similar but partial results in the case of orientable surfaces with boundary components and of non-orientable surfaces without boundary. We give also several results about the classification of different representations of surface braid groups in symmetric groups.
Vertex-primitive self-complementary graphs were proved to be affine or in product action by Guralnick et al. [‘On orbital partitions and exceptionality of primitive permutation groups’, Trans. Amer. Math. Soc.356 (2004), 4857–4872]. The product action type is known in some sense. In this paper, we provide a generic construction for the affine case and several families of new self-complementary Cayley graphs are constructed.
A graph
$\Gamma $
is called
$(G, s)$
-arc-transitive if
$G \le \text{Aut} (\Gamma )$
is transitive on the set of vertices of
$\Gamma $
and the set of s-arcs of
$\Gamma $
, where for an integer
$s \ge 1$
an s-arc of
$\Gamma $
is a sequence of
$s+1$
vertices
$(v_0,v_1,\ldots ,v_s)$
of
$\Gamma $
such that
$v_{i-1}$
and
$v_i$
are adjacent for
$1 \le i \le s$
and
$v_{i-1}\ne v_{i+1}$
for
$1 \le i \le s-1$
. A graph
$\Gamma $
is called 2-transitive if it is
$(\text{Aut} (\Gamma ), 2)$
-arc-transitive but not
$(\text{Aut} (\Gamma ), 3)$
-arc-transitive. A Cayley graph
$\Gamma $
of a group G is called normal if G is normal in
$\text{Aut} (\Gamma )$
and nonnormal otherwise. Fang et al. [‘On edge transitive Cayley graphs of valency four’, European J. Combin.25 (2004), 1103–1116] proved that if
$\Gamma $
is a tetravalent 2-transitive Cayley graph of a finite simple group G, then either
$\Gamma $
is normal or G is one of the groups
$\text{PSL}_2(11)$
,
${\rm M} _{11}$
,
$\text{M} _{23}$
and
$A_{11}$
. However, it was unknown whether
$\Gamma $
is normal when G is one of these four groups. We answer this question by proving that among these four groups only
$\text{M} _{11}$
produces connected tetravalent 2-transitive nonnormal Cayley graphs. We prove further that there are exactly two such graphs which are nonisomorphic and both are determined in the paper. As a consequence, the automorphism group of any connected tetravalent 2-transitive Cayley graph of any finite simple group is determined.
In this paper, we study finite semiprimitive permutation groups, that is, groups in which each normal subgroup is transitive or semiregular. These groups have recently been investigated in terms of their abstract structure, in a similar way to the O'Nan–Scott Theorem for primitive groups. Our goal here is to explore aspects of such groups which may be useful in place of precise structural information. We give bounds on the order, base size, minimal degree, fixed point ratio, and chief length of an arbitrary finite semiprimitive group in terms of its degree. To establish these bounds, we study the structure of a finite semiprimitive group that induces the alternating or symmetric group on the set of orbits of an intransitive minimal normal subgroup.