On graphs with well-distributed edge density

In this paper, we introduce a class of graphs which we call average hereditary graphs. Most graphs that occur in the usual graph theory applications belong to this class of graphs. Many popular types of graphs fall under this class, such as regular graphs, trees and other popular classes of graphs. We prove a new upper bound for the chromatic number of a graph in terms of its maximum average degree and show that this bound is an improvement on previous bounds. From this, we show a relationship between the average degree and the chromatic number of an average hereditary graph. This class of graphs is explored further by proving some interesting properties regarding the class of average hereditary graphs. An equivalent condition is provided for a graph to be average hereditary, through which we show that we can decide if a given graph is average hereditary in polynomial time. We then provide a construction for average hereditary graphs, using which an average hereditary graph can be recursively constructed. We also show that this class of graphs is closed under a binary operation, from this another construction is obtained for average hereditary graphs, and we see some interesting algebraic properties this class of graphs has. We then explore the effect on the complexity of graph 3-coloring problem when the input is restricted to average hereditary graphs.