Campiti, Michele. “Approximation of Continuous Set-Valued Functions in Fréchet Spaces I”. Anal. Numér. Théor. Approx. 20, no. 1 (August 1, 1991): 15–23. Accessed September 3, 2025. https://www.ictp.acad.ro/jnaat/journal/article/view/1991-vol20-nos1-2-art3.