SERIE A | MATHEMATIK The local density of triangle-free graphs

How dense can every induced subgraph of bnc vertices (0 < 1) of a triangle-free graph of order n be? Tools will be developed to estimate the local density of graphs, based on the spectrum of the graph and on a fractional viewpoint. These tools are used to refute a conjecture of Erd} os et.al. about the local density of triangle-free graphs for a certain range of , by estimating the local density of the Higman-Sims graph. Moreover, the local density will be related to a long-standing conjecture of Erd} os, saying that every triangle-free graph can be made bipartite by the omission of at most n 2 =25 edges. Finally, a conjecture about the spectrum of regular triangle-free graphs is raised, which can be seen as a common generalization of the two previous questions.