Gottschalk (1973). Gottschalk, Walter (1973), "Some general dynamical notions", Recent Advances in Topological Dynamics (Proc. Conf. Topological Dynamics, Yale Univ., New Haven, Conn., 1972; in honor of Gustav Arnold Hedlund), Lecture Notes in Math., vol. 318, Berlin, New York: Springer-Verlag, pp. 120–125, doi:10.1007/BFb0061728, ISBN978-3-540-06187-8, MR0407821.
Gottschalk (1973). Gottschalk, Walter (1973), "Some general dynamical notions", Recent Advances in Topological Dynamics (Proc. Conf. Topological Dynamics, Yale Univ., New Haven, Conn., 1972; in honor of Gustav Arnold Hedlund), Lecture Notes in Math., vol. 318, Berlin, New York: Springer-Verlag, pp. 120–125, doi:10.1007/BFb0061728, ISBN978-3-540-06187-8, MR0407821.