Bose-Einstein condensation in random directed networks.

We consider the phenomenon of Bose-Einstein condensation in a random growing directed network. The network grows by the addition of vertices and edges. At each time step the network gains a vertex with probability p and an edge with probability 1-p. The new vertex has a fitness (a,b) a,b>0, with probability f(a,b). A vertex with fitness (a,b), with in-degree i and out-degree j, gains a new incoming edge with rate a(i+1) and an outgoing edge with rate b(j+1). The Bose-Einstein condensation occurs as a function of fitness distribution f(a,b).