Optimal performance region of energy selective electron cooling devices consisting of three reservoirs Susu Qiu, Zemin Ding, Lingen Chen, Fankai Meng and Fengrui Sun Eur. Phys. J. Plus, 134 6 (2019) 273 Published online: 20 June 2019 DOI: 10.1140/epjp/i2019-12600-8