For any finite, undirected, non-bipartite, vertex-transitive graph, we establish an explicit lower bound for the smallest eigenvalue of its normalised adjacency operator, which depends on graph only through degree and vertex-Cheeger constant. We also prove analogous result a large class irregular graphs, obtained as spanning subgraphs graphs. Using Babai, obtain operator in terms diameter degree.

A graph is semi-symmetric if it is regular and edge transitive but not vertex transitive. The 3and 4-valent semi-symmetric graphs are wellstudied. Several papers describe infinite families of such graphs and their properties. 3-valent semi-symmetric graphs have been completely classified up to 768 vertices. The goal of this project is to extend this work to 5-valent semi-symmetric graphs. In th...

A construction is given for an infinite family {0n} of finite vertex-transitive non-Cayley graphs of fixed valency with the property that the order of the vertex-stabilizer in the smallest vertex-transitive group of automorphisms of 0n is a strictly increasing function of n. For each n the graph is 4-valent and arc-transitive, with automorphism group a symmetric group of large prime degree p> 2...

We find a natural construction of a large class of symmetric graphs from pointand block-transitive 1-designs. The graphs in this class can be characterized as G-symmetric graphs whose vertex sets admit a G-invariant partition B of block size at least 3 such that, for any two blocks B,C of B, either there is no edge between B and C , or there exists only one vertex in B not adjacent to any verte...

A classification is given of finite graphs that are vertex primitive and 2arc regular. The classification involves various new constructions of interesting 2-arc transitive graphs.

The relative fixity of a permutation group is the maximum proportion points fixed by non-trivial element group, and graph its automorphism viewed as on vertex-set graph. We prove in this paper that connected 2-arc-transitive graphs valence tends to 0 number vertices grows infinity. same result for class arc-transitive prime valence, more generally, any locally-L graphs, where L quasiprimitive g...