Nazih Abderrazzak Gadhi Stephan Dempe Mohammed El Idrissi New optimality conditions for bilevel programs by using an exact separation principle. 1381-1392 2020 14 Optim. Lett. 6 https://doi.org/10.1007/s11590-019-01443-8 db/journals/ol/ol14.html#GadhiDI20