1. Perkins, D.: The Eureka Effect. The Art and Logic of Breakthrough Thinking. Norton, New York, London (2001)
2. Ivanov, V.K.: Incorrect Problems in Topological Spaces. Siberian Math. J. 10, 785–791, (Novosibirsk, Russia, 1969)
3. Kelley, J.L.: General Topology, Springer, New York, XIV, 298 p. (1975)