@inproceedings{Groetschel1982, author = {Martin Gr{\"o}tschel}, title = {Approaches to Hard Combinatorial Optimization Problems}, booktitle = {Modern Applied Mathematics, Optimization and Operations Research, Lect. Summer Sch., Bonn 1979}, editor = {Bernhard Korte}, pages = {437 -- 515}, year = {1982}, }