Counting Paths between Vertices We can use the adjacency matrix of a graph to find the number of paths between two vertices in the graph. Theorem: Let G be a graph with adjacency matrix A with respect to the ordering v1, ..., V, of vertices (with directed or undirected edges, multiple edges and loops allowed). The number of different paths of length r from v; where r >0 is a positive integer, equals the (i,j)th entry of A". to Vi, Proof by mathematical induction: Basis Step: By definition of the adjacency matrix, the number of paths from v; to v, of length 1 is the (i,j)th entry of A. Inductive Step: For the inductive hypothesis, we assume that that the (i,j)th entry of A' is the number of different paths of length r from v; to vj. Because A+1=AA, the (i,j)th entry of Ar+1 equals bija1; +b2a2; ++ binani where bik is the (i,k)th entry of A'. By the inductive hypothesis, bk is the number of paths of length r from v; to Vk. A path of length + 1 from v; to v; is made up of a path of length r from v; to some vk, and an edge from V to vj. By the product rule for counting, the number of such paths is the product of the number of paths of length r from v; to vk (i.e., bik) and the number of edges from from v to v, (i.e, ak). The sum over all possible intermediate vertices vk is ba1; + biza2; +.... + binani

My professor went over the proof on this slide and I don't really understand it. Can you explain it in detail step by step?