# Multilevel Refinement for Combinatorial Optimisation Problems

**C. Walshaw**

### Abstract:

We consider the multilevel paradigm and its potential to aid the solution of combinatorial optimisation problems.
The multilevel paradigm is a simple one, which at its most basic involves recursive coarsening to create a hierarchy of approximations to the original problem.
An initial solution is found (sometimes for the original problem, sometimes at the coarsest level) and then iteratively refined at each level.
As a general solution strategy the multilevel procedure has been in use for many years and has been applied to many problem areas (most notably multigrid).
However, with the exception of the graph partitioning problem, multilevel techniques have not been widely applied to combinatorial optimisation problems.
In this paper we address the issue of multilevel refinement for such problems and, with the aid of examples and results in graph partitioning, graph colouring and the travelling salesman problem, make a case for its use as a meta-heuristic.
The results provide compelling evidence that, although the multilevel framework cannot be considered as a panacea for combinatorial problems, it can provide a useful addition to the combinatorial optimisation toolkit.
We also give a possible explanation for the underlying process and extract some generic guidelines for its future use on other combinatorial problems.

**Keywords:** Multilevel Refinement; Combinatorial Optimisation; Meta-heuristic; Graph Partitioning;

Travelling Salesman; Graph Colouring.

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Fri Aug 13 13:39:32 BST 2004