Graph bisection algorithms

In this thesis, we describe a polynomial time algorithm that, for every input graph, either outputs the minimum bisection of the graph or halts without output. More importantly, we show that the algorithm chooses the former course with high probability for many natural classes of graphs. In particular, for every fixed d > 3, all sufficiently large n and all b = o(ni-1/d2lJ), the algorithm finds the minimum bisection for almost all d-regular labelled simple graphs with 2n nodes and bisection width b. For example, the algorithm succeeds for almost all 5-regular graphs with 2n nodes and bisection width o(n2/3). The algorithm differs from other graph bisection heuristics (as well as from many heuristics for other NP-complete problems) in several respects. Most notably: (i) the algorithm provides exactly the minimum bisection for almost all input graphs with the specified form, instead of only an approximation of the minimum bisection, (ii) whenever the algorithm produces a bisection, it is guaranteed to be optimal (i.e., the algorithm also produces a proof that the bisection it outputs is an optimal bisection), (iii) the algorithm works well both theoretically and experimentally, (iv) the algorithm employs global methods such as network flow instead of local operations such as 2-changes, and (v) the algorithm works well for graphs with small bisections (as opposed to graphs with large bisections, for which arbitrary bisections are nearly optimal). We also show that with high probability the greedy algorithm will not be able to find the optimal bisection for almost every random regular graph with given bisection width. In the last part of the thesis we describe a new algorithm which is found to perform well in practice, but we have no analysis for it. Finally, we describe a heuristic that when combined with other well-known algorithms such as Kernighan-Lin seems to improve the performance of these algorithms for small degree graphs. Thesis Supervisor : F. Thomson Leighton Title : Associate Professor of Applied Mathematics