In mathematics, the compact complement topology is a topology defined on the set R {\displaystyle \scriptstyle \mathbb {R} } of real numbers, defined by declaring a subset X ⊆ R {\displaystyle \scriptstyle X\subseteq \mathbb {R} } open if and only if it is either empty or its complement R ∖ X {\displaystyle \scriptstyle \mathbb {R} \setminus X} is compact in the standard Euclidean topology on R {\displaystyle \scriptstyle \mathbb {R} } .
References Steen, Lynn Arthur; Seebach, J. Arthur Jr. (1995) [1978], Counterexamples in Topology (Dover reprint of 1978 ed.), Berlin, New York: Springer-Verlag, ISBN 978-0-486-68735-3, MR 0507446