Edge-regular graphs and uniform shared neighborhood structures

Abstract

The definition of edge-regularity in graphs is a relaxation of the definition of strong regularity, so strongly regular graphs are edge-regular and, not surprisingly, the family of edge-regular graphs is much larger and more diverse than that of the strongly regular. A shared neighborhood structure (SNS) in a graph is a subgraph induced by the intersection of the open neighbor sets of two adjacent vertices. If a SNS is the same for all adjacent vertices in an edge-regular graph, call the SNS a uniform shared neighborhood structure (USNS). USNS-forbidden graphs (graphs which cannot be a USNS of an edge-regular graph) and USNS in graph products of edge-regular graphs are examined. Additionally, a few methods of constructing new graphs from old are of use. One of these is the unary ``graph shadow'' operation. Here, this operation is generalized, and then generalized again, and conditions are given under which application of the new operations to edge-regular graphs result in edge-regular graphs. Also, some attention to strongly regular graphs is given.