◀ ▲ ▶Branches / Topology / Definition: Open, Closed, Clopen
Definition: Open, Closed, Clopen
Let $(X,\mathcal{O})$ be a topological space. Every element of the topology $\mathcal O$ is called open. A subset $U\subseteq X$ is called closed, if its set complement $U^C$ is open, i.e. an element of the topology $\mathcal O.$
A subset of $X$, which is both, open and closed, is called clopen.
Notes
 By definition of topology, it is clear that clopen sets exist. Depending on the specific collection of subsets of $X$ in the topology $\mathcal O$, there might be subsets of $X$ that are both, closed (since their complement is open) and open (since they are themselves elements of $\mathcal O$).
 Moreover, there might be subsets of $X$ which are neither open (since they are not elements of the topology $\mathcal O$), nor closed (since their complement is neither an element of this topology).
Mentioned in:
Axioms: 1
Definitions: 2 3 4 5 6 7 8 9
Examples: 10
Parts: 11
Proofs: 12 13 14 15 16 17 18 19 20 21
Propositions: 22 23 24 25 26 27
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References
Bibliography
 Forster Otto: "Analysis 2, Differentialrechnung im \(\mathbb R^n\), Gewöhnliche Differentialgleichungen", Vieweg Studium, 1984
 Steen, L.A.;Seebach J.A.Jr.: "Counterexamples in Topology", Dover Publications, Inc, 1970
 Jänich, Klaus: "Topologie", Springer, 2001, 7th Edition