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He is known for his highly cited pioneering research in the hardness of approximation;[SG76][3]
for his sub-linear and best possible approximation algorithm (unless P = NP) based on the farthest-first traversal for the metric k-center problem[G85][3] (k-tMM clustering);
and for introducing the open-shop scheduling problem as well as algorithms for its solution that have found numerous applications in several research areas as well as for his research on flow shop scheduling, and job shop schedulingalgorithms.[GS76][GS78][4]
He is the editor of the Handbook on Approximation Algorithms and Metaheuristics first edition[G07], second edition[G18] and he is co-editor of Volume 1 (Computer Science and Software Engineering) of the Computing Handbook Set.[CH]