Extremal Regular Graphs: Independent Sets and Graph Homomorphisms

This survey concerns regular graphs that are extremal with respect to the number of independent sets and, more generally, graph homomorphisms. More precisely, in the family of of d-regular graphs, which graph G maximizes/minimizes the quantity i(G)1/v(G), the number of independent sets in G normalized exponentially by the size of G? What if i (G) is replaced by some other graph parameter? We review existing techniques, highlight some exciting recent developments, and discuss open problems and conjectures for future research.