Fiacca, Antonella, Kazimierz Nikodem, and Francesca Papalini. “Hull-Concave Set-Valued Functions”. Annales Mathematicae Silesianae 8 (September 30, 1994): 211-216. Accessed November 21, 2024. https://journals.us.edu.pl/index.php/AMSIL/article/view/14236.