Knaster–Kuratowski fan

The Knaster-Kuratowski fan, or "Cantor's teepee"

In topology, a branch of mathematics, the Knaster–Kuratowski fan (named after Polish mathematicians Bronisław Knaster and Kazimierz Kuratowski) is a specific connected topological space with the property that the removal of a single point makes it totally disconnected. It is also known as Cantor's leaky tent or Cantor's teepee (named after Georg Cantor), depending on the presence or absence of the apex.

Let $C$ be the Cantor set, let $p$ be the point $(\tfrac{1}{2}, \tfrac{1}{2})\in\mathbb R^2$, and let $L(c)$, for $c \in C$, denote the line segment connecting $(c,0)$ to $p$. If $c \in C$ is an endpoint of an interval deleted in the Cantor set, let $X_{c} = \{ (x,y) \in L(c) : y \in \mathbb{Q} \}$; for all other points in $C$ let $X_{c} = \{ (x,y) \in L(c) : y \notin \mathbb{Q} \}$; the Knaster–Kuratowski fan is defined as $\bigcup_{c \in C} X_{c}$ equipped with the subspace topology inherited from the standard topology on $\mathbb{R}^2$.

The fan itself is connected, but becomes totally disconnected upon the removal of $p$.

References

• Knaster, B.; Kuratowski, C. (1921), "Sur les ensembles connexes" (PDF), Fundamenta Mathematicae 2 (1): 206–255
• 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 507446

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