 This is fine when dealing with large, dense graphs, since in this scenario 
the number of subgraphs and the number of graph homomorphisms from a fixed graph are asymptotically equal.Given two graphs formula_128 and formula_59, the 
homomorphism density formula_138 of formula_128 in formula_59 is defined to be the number of graph homomorphisms from formula_128 to formula_59 .In other words, formula_143 is the probability a randomly chosen map from the vertices of formula_128 to the vertices of formula_59 sends adjacent vertices in formula_128 to adjacent vertices in formula_59.