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?

I'm also concerned about other similarity indices like salton index. Salton index 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. But the question, what degree to use for directed networks?

![enter image description here][1]

$k_x$ and $k_y$ are the degrees of $x$ and $y$. 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?

  [1]: https://i.sstatic.net/89qi9.png