My text says "the average number of vertices of the Voronoi cells is less than six". Then it creates the vertex "at infinity", connects the half-infinite edges to this vertex and shows the equation: $$(v + 1) - e + n = 2$$ where v = number of vertices (before creation of the one at infinity), e is the number of edges, and n is the number of point sites. It then shows the inequality: $$2e \geq 3(v + 1)$$
I've probably been staring at this too long, but how to show the average number of vertices of the Voronoi cells is less than six?