Preferential Attachment and salton similarity in directed networks

[gucko]

Recently I'm learning about complex networks and I'm interested in similarity functions that give how similar two nodes are in a network/graph. I found couple similarity functions/indices that were described in the sense of undirected networks. However my concern is with directed networks. For example Preferential Attachment similarity between two nodes in an undirected graph is the degree of the first node multiplied by the degree of the second node. But what about directed graphs? Which degree should one use? In-degree or out-degree?

 

Another index is Salton index which is defined as the number of common neighbors between two nodes divided by the square root of the multiplication of the degrees of the nodes.

 

 

k_x and k_y are the degrees of x and y in undirected network sense .The numerator is just the number of common neighbors between the two nodes. As you can see the same issue here, which degree to use in case of directed graphs?

Edited March 28, 2014 by gucko

[gucko]

Nobody?