Given a set of n vertices suppose that between each of the (2) pairs of vertices there is (independently) an edge with probability p. Call the sets of vertices and edges a graph. Say that any set of three vertices constitutes a triangle if the graph contains all of the (2) edges connecting these vertices. Let W denote the number of triangles in the graph. Assuming that p is small, approximate P{W = k} by

(a) using the Poisson approximation.

(b) using the approximation in Section