Heidari, H., R. Ramezani, and A. Jamali. “Efficiency Analysis of Multi-Stage Systems in the Presence of Undesirable Outputs”. American Scientific Research Journal for Engineering, Technology, and Sciences, vol. 18, no. 1, Apr. 2016, pp. 206-18, https://asrjetsjournal.org/index.php/American_Scientific_Journal/article/view/1320.