Key Takeaway: This thesis presents characterizations of geometries for finite sporadic simple groups (FSSGs) without assumptions on automorphism groups, contributing to the theory of graphs with prescribed neighbourhoods.

Abstract

Geometric approach to classical and exceptional groups of Lie type has been quite successful and has led to the deveopment of the concept of buildings and polar spaces. The latter have been characterized by simple systems of axioms with a combinatorial-geometric flavour. Similar to buildings geometries can be associated with finite sporadic simple groups (FSSGs). However, most of the known characterizations of such geometries for FSSGs require additional assumptions of a group-theoretic nature. One aim of this thesis is to present characterizations of geometries for FSSGs J_2, Suz, McL, Co_3, Fi(22), Fi(23), Fi(24) and He, which are in the same spirit as the characterizations of buildings and polar spaces mentioned above, in particular without any assumption on the automorphism groups of the geometries. A by-product of these results for J_2, Suz and He is a proof that certain presentations for those groups are faithful. Most of this work may be viewed as a contribution to the theory of graphs with prescribed neighbourhood. The result on graphs of (+)-points of GF(3)-orthogonal spaces, which is also used for characterization of geometries for Fi(22), Fi(23) and Fi(24), may be considered as a generalization of a well-known theorem on locally co-triangular graphs. Hyperovals of polar spaces are natural generalizations of hyperovals of projective planes of even order and play an important role in investigations of extensions of polar spaces. As a by-product new extended generalized quadrangles were found as hyperovals of the polar spaces Q_5^+(4) and H_5(4).