There are five homeomorphism classes of countable Toronto spaces, namely: the discrete topology, the indiscrete topology, the cofinite topology and the upper and lower topologies on the natural numbers. The only countable Hausdorff Toronto space is the discrete space.
The Toronto space problem asks for an uncountable Toronto Hausdorff space that is not discrete.
- Bonnet, Robert (1993), "On superatomic Boolean algebras", Finite and infinite combinatorics in sets and logic (Banff, AB, 1991), NATO Adv. Sci. Inst. Ser. C Math. Phys. Sci., 411, Dordrecht: Kluwer Acad. Publ., pp. 31–62, MR 1261195.
- van Mill, J.; Reed, George M. (1990), Open problems in topology, Volume 1, North-Holland, p. 15, ISBN 9780444887689.
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