arXiv subjects
Publications and source records attributed to Alice Devillers.
We show that a 3-spherical building in which each rank 2 residue is connected far away from a chamber, and each rank 3 residue is simply 2-connected far away from a chamber, admits a twinning (i.e., is one half of a twin building) as soon as it admits a codistance, i.e., a twinning with a single chamber.
A new infinite family of bipartite cubic 3-arc transitive graphs is constructed and studied. They provide the first known examples admitting a 2-arc transitive vertex-biquasiprimitive group of automorphisms for which the index two subgroup fixing each half of the bipartition is not quasiprimitive on either bipartite half.
We develop a theory for quotients of geometries and obtain sufficient conditions for the quotient of a geometry to be a geometry. These conditions are compared with earlier work on quotients, in particular by Pasini and Tits. We also explore geometric properties such as connectivity, firmness and transitivity conditions to determine when they are preserved under the quotienting operation. We show that the class of coset pregeometries, which contains all flag-transitive geometries, is closed under an appropriate quotienting operation.
The subdivision graph $S(Σ)$ of a graph $Σ$ is obtained from $Σ$ by `adding a vertex' in the middle of every edge of $\Si$. Various symmetry properties of $§(Σ)$ are studied. We prove that, for a connected graph $Σ$, $S(Σ)$ is locally $s$-arc transitive if and only if $Σ$ is $\lceil\frac{s+1}{2}\rceil$-arc transitive. The diameter of $S(Σ)$ is $2d+δ$, where $Σ$ has diameter $d$ and $0\leqslant δ\leqslant 2$, and local $s$-distance transitivity of $§(Σ)$ is defined for $1\leqslant s\leqslant 2d+δ$. In the general case where $s\leqslant 2d-1$ we prove that $S(Σ)$ is locally $s$-distance transitive if and only if $Σ$ is $\lceil\frac{s+1}{2}\rceil$-arc transitive. For the remaining values of $s$, namely $2d\leqslant s\leqslant 2d+δ$, we classify the graphs $Σ$ for which $S(Σ)$ is locally $s$-distance transitive in the cases, $s\leqslant 5$ and $s\geqslant 15+δ$. The cases $\max\{2d, 6\}\leqslant s\leqslant \min\{2d+δ, 14+δ\}$ remain open.
We give a unified approach to analysing, for each positive integer $s$, a class of finite connected graphs that contains all the distance transitive graphs as well as the locally $s$-arc transitive graphs of diameter at least $s$. A graph is in the class if it is connected and if, for each vertex $v$, the subgroup of automorphisms fixing $v$ acts transitively on the set of vertices at distance $i$ from $v$, for each $i$ from 1 to $s$. We prove that this class is closed under forming normal quotients. Several graphs in the class are designated as degenerate, and a nondegenerate graph in the class is called basic if all its nontrivial normal quotients are degenerate. We prove that, for $s\geq 2$, a nondegenerate, nonbasic graph in the class is either a complete multipartite graph, or a normal cover of a basic graph. We prove further that, apart from the complete bipartite graphs, each basic graph admits a faithful quasiprimitive action on each of its (1 or 2) vertex orbits, or a biquasiprimitive action. These results invite detailed additional analysis of the basic graphs using the theory of quasiprimitive permutation groups.
We prove that the complex of proper non-trivial non-degenerate subspaces of a finite-dimensional vector space endowed with a non-degenerate sesquilinear form is homotopy equivalent to a wedge of spheres. Additionally, we show that the same is true for a slight generalization, the so-called generalized Phan geometries of type A_n. These generalized Phan geometries occur as relative links of certain filtrations. Their sphericity implies finiteness properties of suitable arithmetic groups and allows for a revision of Phan's group-theoretical local recognition of suitable finite groups of Lie type with simply laced diagram.
A transitive decomposition of a graph is a partition of the edge set together with a group of automorphisms which transitively permutes the parts. In this paper we determine all transitive decompositions of the Johnson graphs such that the group preserving the partition is arc-transitive and acts primitively on the parts.