18 research outputs found
A new near octagon and the Suzuki tower
We construct and study a new near octagon of order which has its
full automorphism group isomorphic to the group and which
contains copies of the Hall-Janko near octagon as full subgeometries.
Using this near octagon and its substructures we give geometric constructions
of the -graph and the Suzuki graph, both of which are strongly
regular graphs contained in the Suzuki tower. As a subgeometry of this octagon
we have discovered another new near octagon, whose order is .Comment: 24 pages, revised version with added remarks and reference
Geometric contextuality from the Maclachlan-Martin Kleinian groups
There are contextual sets of multiple qubits whose commutation is
parametrized thanks to the coset geometry of a subgroup of
the two-generator free group . One defines
geometric contextuality from the discrepancy between the commutativity of
cosets on and that of quantum observables.It is shown in this
paper that Kleinian subgroups that are
non-compact, arithmetic, and generated by two elliptic isometries and
(the Martin-Maclachlan classification), are appropriate contextuality filters.
Standard contextual geometries such as some thin generalized polygons (starting
with Mermin's grid) belong to this frame. The Bianchi groups
, defined over the imaginary quadratic field
play a special role
The L₃(4) near octagon
In recent work we constructed two new near octagons, one related to the finite simple group and another one as a sub-near-octagon of the former. In the present paper, we give a direct construction of this sub-near-octagon using a split extension of the group . We derive several geometric properties of this near octagon, and determine its full automorphism group. We also prove that the near octagon is closely related to the second subconstituent of the distance-regular graph on 486 vertices discovered by Soicher (Eur J Combin 14:501-505, 1993)
Zoology of Atlas-groups: dessins d'enfants, finite geometries and quantum commutation
Every finite simple group P can be generated by two of its elements. Pairs of
generators for P are available in the Atlas of finite group representations as
(not neccessarily minimal) permutation representations P. It is unusual but
significant to recognize that a P is a Grothendieck's dessin d'enfant D and
that most standard graphs and finite geometries G-such as near polygons and
their generalizations-are stabilized by a D. In our paper, tripods P -- D -- G
of rank larger than two, corresponding to simple groups, are organized into
classes, e.g. symplectic, unitary, sporadic, etc (as in the Atlas). An
exhaustive search and characterization of non-trivial point-line configurations
defined from small index representations of simple groups is performed, with
the goal to recognize their quantum physical significance. All the defined
geometries G' s have a contextuality parameter close to its maximal value 1.Comment: 19 page
Geometric aspects of 2-walk-regular graphs
A -walk-regular graph is a graph for which the number of walks of given
length between two vertices depends only on the distance between these two
vertices, as long as this distance is at most . Such graphs generalize
distance-regular graphs and -arc-transitive graphs. In this paper, we will
focus on 1- and in particular 2-walk-regular graphs, and study analogues of
certain results that are important for distance regular graphs. We will
generalize Delsarte's clique bound to 1-walk-regular graphs, Godsil's
multiplicity bound and Terwilliger's analysis of the local structure to
2-walk-regular graphs. We will show that 2-walk-regular graphs have a much
richer combinatorial structure than 1-walk-regular graphs, for example by
proving that there are finitely many non-geometric 2-walk-regular graphs with
given smallest eigenvalue and given diameter (a geometric graph is the point
graph of a special partial linear space); a result that is analogous to a
result on distance-regular graphs. Such a result does not hold for
1-walk-regular graphs, as our construction methods will show