Improved Approximation Algorithms for Optimization Problems in Graphs with Superlogarithmic Treewidth

In this paper we present two novel generic schemes for approximation algorithms for optimization NP-hard graph problems constrained to partial k-trees. Our first scheme yields deterministic polynomialtime algorithms achieving typically an approximation factor of k/ log n, where k = polylog(n). The second scheme yields randomized polynomialtime algorithms achieving an approximation factor of k/ log n for k = Ω(log n). Both our approximation methods lead to the best known approximation guarantees for some basic optimization problems. In particular, we obtain best known polynomial-time approximation guarantees for the classical maximum independent set problem in partial trees.