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- Doratha E. Drake Vinkemeier, Stefan Hougardy
- Inf. Process. Lett.
- 2003

- Thomas Emden-Weinert, Stefan Hougardy, Bernd Kreuter
- Combinatorics, Probability & Computing
- 1998

For any integer k, we prove the existence of a uniquely k-colourable graph of girth at least g on at most k 12(g+1) vertices whose maximal degree is at most 5k 13. From this we deduce that, unless NP=RP, no polynomial time algorithm for k-Colourability on graphs G of girth g(G) log jGj 13 log k and maximum degree (G) 6k 13 can exist. We also study several… (More)

- Stefan Hougardy, Hans Jürgen Prömel
- SODA
- 1999

We present a general iterative framework for improving the performance ratio of Steiner tree approximation algorithms. By applying this framework to one specific algorithm we obtain a new polynomial time approximation algorithm for the Steiner tree problem in graphs that achieves a performance ratio of 1.598 after 11 iterations. This beats the so far best… (More)

- Doratha E. Drake Vinkemeier, Stefan Hougardy
- ACM Trans. Algorithms
- 2005

Approximation algorithms have so far mainly been studied for problems that are not known to have polynomial time algorithms for solving them exactly. Here we propose an approximation algorithm for the weighted matching problem in graphs which can be solved in polynomial time. The weighted matching problem is to find a matching in an edge weighted graph that… (More)

- Doratha E. Drake Vinkemeier, Stefan Hougardy
- Inf. Process. Lett.
- 2004

The terminal Steiner tree problem is a special version of the Steiner tree problem, where a Steiner minimum tree has to be found in which all terminals are leaves. We prove that no polynomial time approximation algorithm for the terminal Steiner tree problem can achieve an approximation ratio less than (1− o(1)) lnn unless NP has slightly superpolynomial… (More)

- Doratha E. Drake Vinkemeier, Stefan Hougardy
- RANDOM-APPROX
- 2003

The weighted matching problem is to find a matching in a weighted graph that has maximum weight. The fastest known algorithm for this problem has running time O(nm + n log n). Many real world problems require graphs of such large size that this running time is too costly. We present a linear time approximation algorithm for the weighted matching problem… (More)

The Steiner tree problem asks for a shortest subgraph connecting a given set of terminals in a graph. It is known to be APX-complete, which means that no polynomial time approximation scheme can exist for this problem, unless P=NP. Currently, the best approximation algorithm for the Steiner tree problem has a performance ratio of + , , whereas the… (More)

- Stefan Hougardy
- Comput. Geom.
- 2011

We prove that every set of squares with total area 1 can be packed into a rectangle of area at most 2867/2048 = 1.399. . . . This improves on the previous best bound of 1.53. Also, our proof yields a linear time algorithm for finding such a packing.

- Stefan Hougardy, Doratha E. Drake Vinkemeier
- Inf. Process. Lett.
- 2006

We present an NC approximation algorithm for the weighted matching problem in graphs with an approximation ratio of (1 − ). This improves the previously best approximation ratio of ( 1 2 − ) of an NC algorithm for this problem.