{{Short description|Topological space made of a set of spines joined at a point}} thumb|upright=1.2|A hedgehog space with a large but finite number of spines In mathematics, a '''hedgehog space''' is a topological space consisting of a set of spines joined at a point.
For any cardinal number <math>\kappa</math>, the <math>\kappa</math>-hedgehog space is formed by taking the disjoint union of <math>\kappa</math> real unit intervals identified at the origin (though its topology is not the quotient topology, but that defined by the metric below). Each unit interval is referred to as one of the hedgehog's ''spines.'' A <math>\kappa</math>-hedgehog space is sometimes called a '''hedgehog space of spininess <math>\kappa</math>'''.
The hedgehog space is a metric space, when endowed with the '''hedgehog metric''' <math>d(x,y)=\left| x - y \right|</math> if <math>x</math> and <math>y</math> lie in the same spine, and by <math>d(x,y)=\left|x\right| + \left|y\right|</math> if <math>x</math> and <math>y</math> lie in different spines. Although their disjoint union makes the origins of the intervals distinct, the metric makes them equivalent by assigning them 0 distance.
Hedgehog spaces are examples of real trees.<ref name="carlisle">{{cite conference |first=Sylvia |last=Carlisle |title=Model Theory of Real Trees |conference=Graduate Student Conference in Logic |place=University of Illinois, Chicago, IL |year=2007}}</ref>
==Paris metric== <!-- This section is linked from French railroad metric --> The metric on the plane in which the distance between any two points is their Euclidean distance when the two points belong to a ray through the origin, and is otherwise the sum of the distances of the two points from the origin, is sometimes called the '''Paris metric'''<ref name="carlisle"/> because navigation in this metric resembles that in the radial street plan of Paris: for almost all pairs of points, the shortest path passes through the center. The Paris metric, restricted to the unit disk, is a hedgehog space where ''K'' is the cardinality of the continuum.
==Kowalsky's theorem== Kowalsky's theorem, named after Hans-Joachim Kowalsky,<ref>{{cite book |first=H.J. |last=Kowalsky |title=Topologische Räume |trans-title=Topological Spaces |lang=de |publisher=Birkhäuser |place=Basel-Stuttgart |year=1961}}</ref><ref>{{cite journal |first=M.A. |last=Swardson |title=A short proof of Kowalsky's hedgehog theorem |journal=Proceedings of the American Mathematical Society |volume=75 |year=1979 |issue=1 |page=188 |doi=10.1090/s0002-9939-1979-0529240-7 |doi-access=free}}</ref> states that any metrizable space of weight <math>\kappa</math> can be represented as a topological subspace of the product of countably many {{nowrap|<math>\kappa</math>-hedgehog}} spaces.
== See also == * Comb space * Long line (topology) * Rose (topology)
==References== {{reflist|25em}}
==Other sources== *{{cite book |first1=A.V. |last1=Arkhangelskii |author-link=Alexander Arhangelskii |first2=L.S. |last2=Pontryagin |author-link2=Lev Pontryagin |title=General Topology |volume=I |year=1990 |publisher=Springer-Verlag |place=Berlin, DE |isbn=3-540-18178-4}} *{{cite book |first1=L.A. |last1=Steen |first2=J.A. Jr. |last2=Seebach |title=Counter-Examples in Topology |year=1970 |publisher=Holt, Rinehart, and Winston}} *{{cite arXiv |last1=Torres |first1=Igor |title=A tale of three hedgehogs |date=2017 |class=math.GN |eprint=1711.08656 }}
Category:General topology Category:Topological spaces Category:Trees (topology)