2002, Andrew G. Bronevich, Alexander N. Karkishchenko, “The structure of fuzzy measure families induced by upper and lower probabilities”, in Carlo Bertoluzza, María Á. Gil, Dan A. Ralescu, editors, Statistical Modeling, Analysis and Management of Fuzzy Data, Springer, page 165:
Hence the event will be a certain event too, i.e. .
2018, Athanasios Christou Micheas, Theory of Stochastic Objects, Taylor & Francis (CRC Press / Chapman & Hall), unnumbered page: