Kirr, Eduard, and Adrian Petrușel. “Continuous Dependence and Fixed Points for Some Multivalued Operators”. Rev. Anal. Numér. Théor. Approx., vol. 26, no. 1, Aug. 1997, pp. 99-101, https://ictp.acad.ro/jnaat/journal/article/view/1997-vol26-nos1-2-art14.