Fiacca, A., Nikodem, K. and Papalini, F. (1994) “Hull-concave set-valued functions”,
Annales Mathematicae Silesianae
, 8, pp. 211–216. Available at: https://journals.us.edu.pl/index.php/AMSIL/article/view/14236 (Accessed: 17 April 2025).