Incremental Upper Bound for the Maximum Clique Problem

The maximum clique problem (MaxClique for short) consists of searching for a maximum complete subgraph in a graph. A branch-and-bound (BnB) MaxClique algorithm computes an upper bound of the number of vertices of a maximum clique at every search tree node, to prune the subtree rooted at the node. Existing upper bounds are usually computed from scratch at every search tree node. In this paper, we define an incremental upper bound, called IncUB, which is derived efficiently from previous searches instead of from scratch. Then, we describe a newBnB MaxClique algorithm, called IncMC2, which uses graph coloring and MaxSAT reasoning to filter out the vertices that do not need to be branched on, and uses IncUB to prune the remaining branches. Our experimental results show that IncMC2 is significantly faster than algorithms such as BBMC and IncMaxCLQ. Finally, we carry out experiments to provide evidence that the performance of IncMC2 is due to IncUB.