Notice that the value 55 is the sum of 30, the shortest (indeed, only) path
of length 1 from v1 to v2, and 25, the length of the edge between v2 and v3.
Thus a(2)
13 represents the length of the shortest path of two or fewer edges from v1
to v3. So A2 represents the lengths of all shortest paths with two or fewer edges
between any two vertices.