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Approximation algorithms via contraction decomposition

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Abstract

We prove that the edges of every graph of bounded (Euler) genus can be partitioned into any prescribed number k of pieces such that contracting any piece results in a graph of bounded treewidth (where the bound depends on k). This decomposition result parallels an analogous, simpler result for edge deletions instead of contractions, obtained in [4,20, 10, 17], and it generalizes a similar result for “compression” (a variant of contraction) in planar graphs [29]. Our decomposition result is a powerful tool for obtaining PTASs for contraction-closed problems (whose optimal solution only improves under contraction), a much more general class than minor-closed problems. We prove that any contraction-closed problem satisfying just a few simple conditions has a PTAS in bounded-genus graphs. In particular, our framework yields PTASs for the weighted Traveling Salesman Problem and for minimum-weight c-edge-connected submultigraph on bounded-genus graphs, improving and generalizing previous algorithms of [24, 1, 29, 25, 8, 5]. We also highlight the only main difficulty in extending our results to general H-minor-free graphs.

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Correspondence to Erik D. Demaine.

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A preliminary version of this paper appeared in Proceedings of the 18th Annual ACM-SIAM Symposium on Discrete Algorithms, January 2007.

on leave from: Department of Mathematics University of Ljubljana 1000 Ljubljana Slovenia

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Demaine, E.D., Hajiaghayi, M. & Mohar, B. Approximation algorithms via contraction decomposition. Combinatorica 30, 533–552 (2010). https://doi.org/10.1007/s00493-010-2341-5

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