Campiti, M. “Approximation of Continuous Set-Valued Functions in Fréchet Spaces I”. Anal. Numér. Théor. Approx., vol. 20, no. 1, Aug. 1991, pp. 15-23, https://ictp.acad.ro/jnaat/journal/article/view/1991-vol20-nos1-2-art3.