On the Longest Paths and the Diameter in Random Apollonian Networks

We consider the following iterative construction of a random planar triangulation. Start with a triangle embedded in the plane. In each step, choose a bounded face uniformly at random, add a vertex inside that face and join it to the vertices of the face. After n − 3 steps, we obtain a random triangulated plane graph with n vertices, which is called a Random Apollonian Network (RAN). We show that asymptotically almost surely (a.a.s.) every path in a RAN has length o ( n ) , refuting a conjecture of Frieze and Tsourakakis. We also show that a RAN always has a path of length ( 2 n − 5 ) log 2 / log 3 , and that the expected length of its longest path is Ω ( n 0.88 ) . Finally, we prove that a.a.s. the diameter of a RAN is asymptotic to c log n , where c ≈ 1.668 is the solution of an explicit equation.