Ernst W. Mayr, Hans Jürgen Prömel, Angelika Steger (Eds.):
Lectures on Proof Verification and Approximation Algorithms. (the book grow out of a Dagstuhl Seminar, April 21-25, 1997).
Lecture Notes in Computer Science 1367 Springer 1998, ISBN 3-540-64201-3 
Thomas Jansen:
Introduction to the Theory of Complexity and Approximation Algorithms.
5-28
Katja Wolf:
Dense Instances of Hard Optimization Problems.
299-312
Richard Mayr,
Annette Schelten:
Polynomial Time Approximation Schemes for Geometric Optimization Problems in Euclidean Metric Spaces.
313-324
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