Simple on-line algorithms for the maximum disjoint paths problem

摘要

In this paper we study the problem of finding disjoint paths in graphs. Whereas for specific graphs many (almost) matching upper and lower bounds are known for the competitiveness of on-line path selection algorithms, much less is known about how well on-line algorithms can perform in the general setting. In several papers the expansion has been used to measure the performance of off-line and on-line algorithms in this field. We study a class of simple deterministic on-line algorithms and show that they achieve a competitive ratio that is asymptotically equal to the best possible competitive ratio that can be achieved by any deterministic on-line algorithm. For this we use a parameter caled routing number which allows more precise results than the expansion. Interestingly, our upper bound on the competitive ratio is even better than the best approximation ratio known for off-line algorithms. Furthermore, we show that a refined variant of the routing number allows to construct on-line algorithms with a competitive ratio that is for many graphs significantly below the best possible upper bound for deterministic on-line algorithms if only the routing number or expansion of a graph is known. We also show that our algorithms can be transformed into efficient algorithms for the related unsplittable flow problem.