Inner and outer approximations of technology: a data envelopment analysis approach. - Rolf Färe; Sung Ko Li (Eur. J. Oper. Res., 1998)

A Unifying Framework for Farrell Profit Efficiency Measurement. - Rolf Färe; Xinju He; Sung Ko Li; Valentin Zelenyuk (Oper. Res., 2019)