Exact Realization of Edge and Induced Three-Vertex Subgraph Counts
The study of three-vertex subgraph densities has developed along two complementary lines: exact finite realization in large graphs of a given order, and asymptotic approximate realization by sequences of graphs. We bridge these viewpoints for the edge density and the four induced three-vertex subgraph densities. By analyzing local modifications of finite samples from graphons, we show that, for any nonempty subset of these densities, every asymptotically feasible target vector lying a fixed positive distance from the boundary of the corresponding feasible region is realized exactly by a graph of every sufficiently large order, subject only to the natural arithmetic constraints. Thus, in the interior of these feasible regions, asymptotic feasibility implies exact finite realizability up to the unavoidable arithmetic obstructions.