Properties of almost all graphs and complexes

There are many results in the literature asserting that almost all or almost no graphs have some property. Our object is to develop a general logical theorem that will imply almost all of these results as corollaries. To this end, we propose the first-order theory of almost all graphs by presenting Axiom n which states that for each sequence of 2n distinct vertices in a graph (u₁, …, u_n, v₁, …, v_n), there exists another vertex w adjacent to each u₁ and not adjacent to any v_i. A simple counting argument proves that for each n, almost all graphs satisfy Axiom n. It is then shown that any sentence that can be stated in terms of these axioms is true in almost all graphs or in almost none. This has several immediate consequences, most of which have already been proved separately including: (1) For any graph H, almost all graphs have an induced subgraph isomorphic to H. (2) Almost no graphs are planar, or chordal, or line graphs. (3) Almost all grpahs are connected wiht diameter 2. It is also pointed out that these considerations extend to digraphs and to simplicial complexes.