Research

Publications

Thesises

• Lower Bounds and Quality Guarantees for Online-Dispatching, 2004, PDF [663 kB]
• Aspects of Online Routing and Scheduling,2007, PDF [796 kB]

Journals

• F. Bollwein, S. Westphal, A branch & bound algorithm to determine optimal bivariate splits for oblique decision tree induction, Applied Intelligence, 1-21, 2021, https://link.springer.com/article/10.1007/s10489-021-02281-x
• F. Bollwein, M. Dahmen, S. Westphal, A branch & bound algorithm to determine optimal cross-splits for decision tree induction, Annals of Mathematics and Artificial Intelligence 88 (4), 291-311, 2020, https://link.springer.com/article/10.1007/s10472-019-09684-0
• S.O. Krumke, C. Thielen, P. Weinschenk, S. Westphal, Full implementation of social choice functions in dominant strategies, International Journal of Game Theory 48 (1), 337-361, 2019, https://link.springer.com/article/10.1007/s00182-018-0654-6
• M. Bender, C. Thielen, S. Westphal, Online interval scheduling with a bounded number of failures, Journal of Scheduling, 2017, Pages 1-15, http://link.springer.com/article/10.1007%2Fs10951-016-0506-9
• M. Bender, S. Westphal, A combined approximation for the traveling tournament problem and the traveling umpire problem, Journal of Quantitative Analysis in Sports, September 2016, Volume 12, Issue 3, Pages 139-149 , https://doi.org/10.1515/jqas-2015-0111
• M. Goerigk, S. Westphal, A combined local search and integer programming approach to the traveling tournament problem, Annals of Operations Research 239, Volume 1, 2016, Pages 343-354, http://link.springer.com/article/10.1007/s10479-014-1586-6
• C. Thielen, M. Tiedemann, S. Westphal, The online knapsack problem with incremental capacity, Mathematical Methods of Operations Research, April 2016, Volume 83, Issue 2, Pages 207-242, http://dx.doi.org/10.1007/s00186-015-0526-9
• M. Bender, C. Thielen, S. Westphal, Packing Items Into Several Bins Facilitates Approximating the Separable Assignment Problem, Information Processing Letters, June–August 2015, Volume 115, Issues 6-8, Pages 570–575 http://dx.doi.org/10.1016/j.ipl.2015.02.001
• S. Westphal, Scheduling the German Basketball League, Interfaces, Oct. 2014, http://dx.doi.org/10.1287/inte.2014.0764
• J. Ide, M. Tiedemann, S. Westphal, F. Haiduk, An Application of Deterministic and Robust Optimization in the Wood Cutting Industry, 4OR, 2014, http://link.springer.com/article/10.1007%2Fs10288-014-0265-4

• M. Goerigk, S. Westphal, A combined local search and integer programming approach to the traveling tournament problem, Annals of Operations Research, April 2014, Pages 1-12, http://link.springer.com/article/10.1007%2Fs10479-014-1586-6#
• M. Bender, S. Westphal, An Optimal Randomized Online Algorithm for the k-Canadian Traveller Problem on Node-Disjoint Paths, Journal of Combinatorial Optimization, June 2013, Pages 1-10, http://link.springer.com/article/10.1007%2Fs10878-013-9634-8
• C. Thielen, S. Westphal, Complexity and Approximability of the Maximum Flow Problem with Minimum Quantities, Networks, September 2013, Volume 62, Issue 2, Pages 125–131, http://onlinelibrary.wiley.com/doi/10.1002/net.21502/abstract
• C. Thielen, S. Westphal, Approximation Algorithms for TTP(2), Mathematical Methods of Operations Research, August 2012, Volume 76, Issue 1, Pages 1-20, http://www.springerlink.com/content/c18642r305670161/
• S. Westphal, K. Noparlik, A 5.875-Approximation for the Traveling Tournament Problem, Annals of Operations Research, 2012, http://www.springerlink.com/content/l640933463l66288/
• S.O. Krumke, C. Thielen, S. Westphal, Interval Scheduling on Related Machines, Computers and Operations Research, Volume 38, Issue 12, December 2011, Pages 1836-1844, http://dx.doi.org/10.1016/j.cor.2011.03.001
• C. Thielen, S. Westphal, Complexity of the Traveling Tournament Problem, Theoretical Computer Science, Volume 412, Issues 4-5, 4 February 2011, Pages 345-351, PDF [178 kB], http://dx.doi.org/10.1016/j.tcs.2010.10.001
• S.O. Krumke, A. Taudes, S. Westphal, Online Scheduling of Weighted Equal-Length Jobs with Hard Deadlines on Parallel Machines, Computers and Operations Research, Volume 38, Issue 8, 8 January 2011, Pages 1103-1108, PDF [307 kB], http://dx.doi.org/10.1016/j.cor.2010.10.012
• I. Fink, S. O. Krumke, S. Westphal, New Lower Bounds for Online k-Server Routing Problems, Information Processing Letters, Volume 109, Issue 11, 16 May 2009, Pages 563-567, PDF [138 kB], http://dx.doi.org/10.1016/j.ipl.2009.01.024
• S. O. Krumke, R. van Stee, A. Schwahn, S. Westphal, A monotone approximation algorithm for scheduling with precedence constraints, Operations Research Letters, Volume 36, Issue 2, March 2008, Pages 247-249, PDF [92 kB], http://dx.doi.org/10.1016/j.orl.2007.05.003
• S. Westphal, A note on the k-Canadian Traveller Problem, Information Processing Letters, Volume 106, Issue 3, 30 April 2008, Pages 87-89, PDF [76 kB], http://dx.doi.org/10.1016/j.ipl.2007.10.004
• S. O. Krumke, S. Saliba, T. Vredeveld, S. Westphal, Approximation Algorithms for a Vehicle Routing Problem, Mathematical Methods of Operations Research , 2008, Volume 68, Number 2, Pages 333-359, Springer, PDF [245 kB], http://www.springerlink.com/content/x785546u27115781/
• S. Westphal, S. O. Krumke, Pruning in Column Generation for Service Vehicle Dispatching, Annals of Operations Research, 2008, Volume 159, Number 1, Pages 355-371, PDF [220 kB], http://www.springerlink.com/content/5101634641580x47/

Chapters in Books

• S. Westphal, S. O. Krumke, R. van Stee, Online Job Admission, Fundamental Problems in Computing, Part II, pages 435-454, S. S. Ravi and S. K. Shukla (eds.), Springer, 2009, PDF [192 kB], http://www.springerlink.com/content/mxm91153r9241384/
• G. Kendall, S. Westphal, Sports scheduling: Minimizing travel for English football supporters, Automated Scheduling: Real World Case Studies, pages 61-90, Neil Urquhart (editor), Springer, 2013, http://link.springer.com/chapter/10.1007/978-3-642-39304-4_3

Refereed Conference Proceedings

• F. Merz, C. Schwindt, S. Westphal, J. Zimmermann A multi-round auction for staff to job assignment under myopic best response dynamics, IEEE International Conference on Industrial Engineering and Engineering Management, 2020, https://ieeexplore.ieee.org/document/9309851
• J. Zillies, S. Westphal, D. Thakur, V. Kumar, G. Pappas, D. Scheidt A column generation approach for optimized routing and coordination of a UAV fleet, Proceedings of the 2016 IEEE International Symposium on Safety, Security, and Rescue Robotics (SSRR), October 2016, https://doi.org/10.1109/SSRR.2016.7784326
• M. Goerigk, R. Hoshino, K. Kawarabayashi, S. Westphal, Solving the Traveling Tournament Problem by Packing Three-Vertex Paths, Proceedings of the 28th AAAI Conference on Artificial Intelligence (AAAI 2014), PDF [424 kB], Quebec City, Quebec, July 2014
• M. Bender, S. Westphal, Maximum Generalized Assignment with Convex Costs, Proceedings of 3rd International Symposium on Combinatorial Optimization (ISCO 2014), LNCS, Volume 8596, Pages 75-86, Springer, http://link.springer.com/chapter/10.1007%2F978-3-319-09174-7_7
• M. Bender, C. Thielen, S. Westphal, A Constant Factor Approximation for the Generalized Assignment Problem with Minimum Quantities and Unit Size Items, Proceedings of Mathematical Foundations of Computer Science 2013 (MFCS 2013), LNCS, Volume 8087, Pages 135-145, Springer, http://link.springer.com/chapter/10.1007/978-3-642-40313-2_14
• S.O. Krumke, C. Thielen, S. Westphal, Interval Scheduling on Related Machines: Complexity and Online Algorithms, Proceedings of the 10th Workshop on Models and Algorithms for Planning and Scheduling Problems (MAPSP 2011)
• J. Frasch, S.O. Krumke, S. Westphal, MIP Formulations for Flowshop Scheduling with Limited Buffers, Proceedings of the 1st International ICST Conference on Theory and Practice of Algorithms in (Computer) Systems (TAPAS 2011), http://www.springerlink.com/content/r47x5v007139np3t/
• C. Thielen, S. Westphal, Approximating the Traveling Tournament Problem with Maximum Tour Length 2, Proceedings of the 21st International Symposium on Algorithms and Computation (ISAAC 2010), Part II, Otfried Cheong, Kyung-Yong Chwa, and Kunsoo Park (eds.), LNCS, Volume 6507 (2010), Pages 303-314, Springer, http://www.springerlink.com/content/p5727233h4586589/
• S. Westphal, K. Noparlik, A 5.875-Approximation for the Traveling Tournament Problem, Proceedings of the 8th International Conference on the Practice and Theory of Automated Timetabling (PATAT 2010)
• C. Thielen, S. Westphal, A Combinatorial Algorithm for Strong Implementation of Social Choice Functions, Proceedings of the 3rd International Workshop on Computational Social Choice (COMSOC 2010)
• K. Bauer, T. Fischer, S. O. Krumke, K. Gerhardt, S. Westphal and P. Merz, Improved Construction Heuristics and Iterated Local Search for the Routing and Wavelength Assignment Problem, Evolutionary Computation in Combinatorial Optimization Published Lecture Notes in Computer Science, 2008, Volume 4972, Pages 158-169, Springer, PDF [190 kB], http://www.springerlink.com/content/hq58j26000648t7l/
• S. Westphal, S. O. Krumke, Rob van Stee, Competitive Algorithms for Cottage Rental Journal Electronic Notes in Discrete Mathematics, Volume 25, 1 August 2006, Pages 187-188
• S. Saliba, S. O. Krumke, S. Westphal, Online-Optimization of Large-Scale Vehicle Dispatching Problems Journal Electronic Notes in Discrete Mathematics, Volume 25, 1 August 2006, Pages 145-146
• S. Westphal, S. O. Krumke, Improved Pruning in Column Generation of a Vehicle Routing Problem, Proceedings of the 2nd Multidisciplinary International Conference on Scheduling: Theory and Applications,  2005, 705-722

Research Areas and Application

• Combinatorial Optimization
• Mixed Integer Programming
• Online Optimization
• Sports Scheduling
• Vehicle Routing
• Scheduling