{{Short description|Mathematical map between topological spaces}} {{About|the concept in topology|the concept in convex analysis|proper convex function}}

In mathematics, a function between topological spaces is called '''proper''' if inverse images of compact subsets are compact.{{sfn|Lee|2012|p=610|loc=above Prop. A.53}} In algebraic geometry, the analogous concept is called a proper morphism.

==Definition==

There are several competing definitions of a "proper function". Some authors call a function <math>f : X \to Y</math> between two topological spaces '''{{em|proper}}''' if the preimage of every compact set in <math>Y</math> is compact in <math>X.</math> Other authors call a map <math>f</math> {{em|proper}} if it is continuous and '''{{em|closed with compact fibers}}'''; that is if it is a continuous closed map and the preimage of every point in <math>Y</math> is compact. The two definitions are equivalent if <math>Y</math> is locally compact and Hausdorff. {{Collapse top|title=Partial proof of equivalence}} Let <math>f : X \to Y</math> be a closed map, such that <math>f^{-1}(y)</math> is compact (in <math>X</math>) for all <math>y \in Y.</math> Let <math>K</math> be a compact subset of <math>Y.</math> It remains to show that <math>f^{-1}(K)</math> is compact.

Let <math>\left\{U_a : a \in A\right\}</math> be an open cover of <math>f^{-1}(K).</math> Then for all <math>k \in K</math> this is also an open cover of <math>f^{-1}(k).</math> Since the latter is assumed to be compact, it has a finite subcover. In other words, for every <math>k \in K,</math> there exists a finite subset <math>\gamma_k \subseteq A</math> such that <math>f^{-1}(k) \subseteq \cup_{a \in \gamma_k} U_{a}.</math> The set <math>X \setminus \cup_{a \in \gamma_k} U_{a}</math> is closed in <math>X</math> and its image under <math>f</math> is closed in <math>Y</math> because <math>f</math> is a closed map. Hence the set <math display=block>V_k = Y \setminus f\left(X \setminus \cup_{a \in \gamma_k} U_{a}\right)</math> is open in <math>Y.</math> It follows that <math>V_k</math> contains the point <math>k.</math> Now <math>K \subseteq \cup_{k \in K} V_k</math> and because <math>K</math> is assumed to be compact, there are finitely many points <math>k_1, \dots, k_s</math> such that <math>K \subseteq \cup_{i =1}^s V_{k_i}.</math> Furthermore, the set <math>\Gamma = \cup_{i=1}^s \gamma_{k_i}</math> is a finite union of finite sets, which makes <math>\Gamma</math> a finite set.

Now it follows that <math>f^{-1}(K) \subseteq f^{-1}\left( \cup_{i=1}^s V_{k_i} \right) \subseteq \cup_{a \in \Gamma} U_{a}</math> and we have found a finite subcover of <math>f^{-1}(K),</math> which completes the proof. {{Collapse bottom}}

If <math>X</math> is Hausdorff and <math>Y</math> is locally compact Hausdorff then proper is equivalent to '''{{em|universally closed}}'''. A map is universally closed if for any topological space <math>Z</math> the map <math>f \times \operatorname{id}_Z : X \times Z \to Y \times Z</math> is closed. In the case that <math>Y</math> is Hausdorff, this is equivalent to requiring that for any map <math>Z \to Y</math> the pullback <math>X \times_Y Z \to Z</math> be closed, as follows from the fact that <math>X \times_YZ</math> is a closed subspace of <math>X \times Z.</math>

An equivalent, possibly more intuitive definition when <math>X</math> and <math>Y</math> are metric spaces is as follows: we say an infinite sequence of points <math>\{p_i\}</math> in a topological space <math>X</math> '''{{em|escapes to infinity}}''' if, for every compact set <math>S \subseteq X</math> only finitely many points <math>p_i</math> are in <math>S.</math> Then a continuous map <math>f : X \to Y</math> is proper if and only if for every sequence of points <math>\left\{p_i\right\}</math> that escapes to infinity in <math>X,</math> the sequence <math>\left\{f\left(p_i\right)\right\}</math> escapes to infinity in <math>Y.</math>

==Properties==

* Every continuous map from a compact space to a Hausdorff space is both proper and closed. * Every surjective proper map is a compact covering map. ** A map <math>f : X \to Y</math> is called a '''{{em|compact covering}}''' if for every compact subset <math>K \subseteq Y</math> there exists some compact subset <math>C \subseteq X</math> such that <math>f(C) = K.</math> * A topological space is compact if and only if the map from that space to a single point is proper. * If <math>f : X \to Y</math> is a proper continuous map and <math>Y</math> is a compactly generated Hausdorff space (this includes Hausdorff spaces that are either first-countable or locally compact), then <math>f</math> is closed.<ref name=palais>{{cite journal|last=Palais|first=Richard S.|author-link=Richard Palais| title=When proper maps are closed|journal=Proceedings of the American Mathematical Society|year=1970|volume=24|issue=4|pages=835–836|doi=10.1090/s0002-9939-1970-0254818-x|doi-access=free|mr=0254818 }}</ref>

==Generalization==

It is possible to generalize the notion of proper maps of topological spaces to locales and topoi, see {{Harv|Johnstone|2002}}.

==See also==

* {{annotated link|Almost open map}} * {{annotated link|Open and closed maps}} * {{annotated link|Perfect map}} * {{annotated link|Topology glossary}}

==Citations==

{{reflist}}

==References== {{sfn whitelist |CITEREFLee2012}} {{refbegin}} * {{cite book | last1=Bourbaki | first1=Nicolas | author1-link = Nicolas Bourbaki | title=General topology. Chapters 5–10 | publisher=Springer-Verlag | location=Berlin, New York | series=Elements of Mathematics | isbn=978-3-540-64563-4 |mr=1726872 | year=1998}} * {{cite book |last=Johnstone |first=Peter |author-link=Peter Johnstone (mathematician)| title=Sketches of an elephant: a topos theory compendium |publisher=Oxford University Press |location=Oxford |year=2002 |isbn=0-19-851598-7 }}, esp. section C3.2 "Proper maps" * {{cite book |last=Brown |first=Ronald |author-link=Ronald Brown (mathematician)| title=Topology and groupoids |publisher=Booksurge |location= North Carolina |year=2006 |isbn=1-4196-2722-8 }}, esp. p.&nbsp;90 "Proper maps" and the Exercises to Section 3.6. * {{cite journal |last=Brown |first=Ronald |author-link=Ronald Brown (mathematician)| title=Sequentially proper maps and a sequential compactification| journal= Journal of the London Mathematical Society | series=Second series|volume=7 | issue=3 |year=1973|pages= 515-522 | doi=10.1112/jlms/s2-7.3.515}} * {{Lee Introduction to Smooth Manifolds|second}} {{refend}}

{{Topology}}

Category:Theory of continuous functions