(joint work with Amir Abboud and Virginia Vassilevska Williams)
Measuring the importance of a node in a network is a major goal in the analysis of social networks, biological systems, transportation networks etc. Different centrality measures have been proposed to capture the notion of node importance. For example, the center of a graph is a node that minimizes the maximum distance to any other node (the latter distance is the radius of the graph). The median of a graph is a node that minimizes the sum of the distances to all other nodes. Informally, the betweenness centrality of a node w measures the fraction of shortest paths that have w as an intermediate node. Finally, the reach centrality of a node w
is the smallest distance r such that any s-t shortest path passing through w has either s or t in the ball of radius r around w.
The fastest known algorithms to compute the center and the median of a graph, and to compute the betweenness or reach centrality even of a single node take roughly cubic time in the number n of nodes in the input graph. It is open whether these problems admit truly subcubic algorithms, i.e. algorithms with running time $\tilde{O}(n^{3-\delta})$ for some constant $delta>0$.
We relate the complexity of the mentioned centrality problems to two classical problems for which no truly subcubic algorithm is known, namely All Pairs Shortest Paths (APSP) and Diameter. It is easy to see that Diameter can be solved using an algorithm for APSP with a small overhead. However, no reduction is known in the other direction, and it is entirely possible that Diameter is a truly easier problem than APSP.
We show that Radius, Median and Betweenness Centrality are equivalent under subcubic reductions to APSP, i.e. that a truly subcubic algorithm for any of these problems implies a truly subcubic algorithm for all of them. We then show that Reach Centrality is equivalent to Diameter under subcubic reductions. The same holds for the problem of approximating Betweenness Centrality within any constant factor. Thus the latter two centrality problems could potentially be solved in truly subcubic time, even if APSP required essentially cubic time. Indeed, our reductions already imply an algorithm for Reach Centrality in graphs with small integer weights that is faster than the best known algorithm for APSP in the same family of graphs. |