Graph Local Overlap Measure: Resource Allocation Index

The Resource Allocation (RA) index is

$$ \mathbf S_{\text{RA}}[v_1,v_2] = \sum_{u\in\mathcal N(u) \cap \mathcal N(v)} \frac{1}{d_u}, $$

where $d_u$ is the node degree of node $u$ and $\mathcal N(u)$ is the neighbor nodes of $u$.