Kurzhanski, A.B. & Valyi, I. (1988). Set valued solutions to control problems and their approximations. In: Analysis and Optimization of Systems. Eds. Bensoussan, A. & Lions, J.L., pp. 775-785 Germany: Springer Berlin Heidelberg. ISBN 978-3-540-39161-6 10.1007/BFb0042263.