@article{gopalswamy_uzsoy_2021, title={Conic programming models for production planning with clearing functions: Formulations and duality}, volume={292}, ISSN={0377-2217}, url={http://dx.doi.org/10.1016/j.ejor.2020.11.039}, DOI={10.1016/j.ejor.2020.11.039}, abstractNote={Concave clearing functions that model the expected throughput of a production resource as a function of its planned workload have yielded promising results when used in production planning models. The most common of these models take the form of linear programs (LPs) obtained by piecewise linearization of the clearing function constraints, leading to large formulations and inaccurate estimates of dual prices for resources. We show that several clearing function forms considered in the literature to date can be reformulated as conic programs (CPs), for which efficient solution methods and an elegant duality theory analogous to that for linear programming exist. We derive expressions for the optimal values of the dual variables and present computational experiments showing that the dual prices obtained from the CP formulation are more accurate than those obtained from the piecewise linearized LP models. In addition, the CP solution outperforms the LP solutions with respect to nervousness.}, number={3}, journal={European Journal of Operational Research}, publisher={Elsevier BV}, author={Gopalswamy, Karthick and Uzsoy, Reha}, year={2021}, month={Aug}, pages={953–966} }
 @article{manda_gopalswamy_shashaani_uzsoy_2020, title={A SIMULATION OPTIMIZATION APPROACH FOR MANAGING PRODUCT TRANSITIONS IN MULTISTAGE PRODUCTION LINES}, ISSN={["0891-7736"]}, url={http://dx.doi.org/10.1109/wsc48552.2020.9384036}, DOI={10.1109/WSC48552.2020.9384036}, abstractNote={We explore the problem of managing releases into a multistage production system transitioning from producing a mature product in high volume to a new one whose production process is initially unreliable but improves as experience is accumulated. We use simulation optimization to develop solutions and examine the impact of learning at a single machine on the rest of the system. This work lays the foundation for studying product transitions using realistic fab scale simulation models.}, journal={2020 WINTER SIMULATION CONFERENCE (WSC)}, publisher={IEEE}, author={Manda, Atchyuta Bharadwaj and Gopalswamy, Karthick and Shashaani, Sara and Uzsoy, Reha}, year={2020}, pages={1730–1741} }
 @article{gopalswamy_uzsoy_2019, title={A data-driven iterative refinement approach for estimating clearing functions from simulation models of production systems}, volume={57}, ISSN={["1366-588X"]}, DOI={10.1080/00207543.2018.1557351}, abstractNote={Clearing functions that describe the expected output of a production resource as a function of its expected workload have yielded promising production planning models. However, there is as yet no fully satisfactory approach to estimating clearing functions from data. We identify several issues that arise in estimating clearing functions such as sampling issues, systematic underestimation and model misspecification. We address the model misspecification problem by introducing a generalised functional form, and the sampling issues via iterative refinement of initial parameter estimates. The iterative refinement approach yields improved performance for planning models at higher levels of utilisation, and the generalised functional form results in significantly better production plans both alone and when combined with the iterative refinement approach. The IR approach also obtains solutions of similar quality to the much more computationally demanding simulation optimisation approaches used in previous work.}, number={19}, journal={INTERNATIONAL JOURNAL OF PRODUCTION RESEARCH}, author={Gopalswamy, Karthick and Uzsoy, Reha}, year={2019}, month={Oct}, pages={6013–6030} }
 @article{gopalswamy_fathi_uzsoy_2019, title={Valid inequalities for concave piecewise linear regression}, volume={47}, ISSN={["1872-7468"]}, DOI={10.1016/j.orl.2018.12.004}, abstractNote={We consider the problem of fitting a concave piecewise linear function to multivariate data using the Least Absolute Deviation objective. We propose new valid inequalities for the problem using the properties of concave functions. Results with univariate data show that the proposed valid inequalities improve the root relaxation lower bound, permitting significant improvements in solution time.}, number={1}, journal={OPERATIONS RESEARCH LETTERS}, author={Gopalswamy, Karthick and Fathi, Yahya and Uzsoy, Reha}, year={2019}, month={Jan}, pages={52–58} }