Stefanov, Simeon. “Nonexistence of Orbital Morphisms Between Dynamical Systems on Spheres”. Ann. Sofia Univ. Fac. Math. and Inf. 83, no. 1 (December 12, 1992): 123–128. Accessed January 16, 2025. https://admin.uni-sofia.bg/index.php/fmi/article/view/492.