View source for Exponential law (in topology)

The idea for a topology on spaces of functions goes back to the metric <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e1300601.png" /> on functions from a [[Compact space|compact space]] <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e1300602.png" /> to a [[Metric space|metric space]] <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e1300603.png" />. It was found desirable to extend this to the case when <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e1300604.png" /> is only locally compact (cf. also [[Locally compact space|Locally compact space]]).

To this end, R.H. Fox introduced the compact-open topology on the set of continuous functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e1300605.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e1300606.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e1300607.png" /> are topological spaces (cf. also [[Compact-open topology|Compact-open topology]]; [[Topological space|Topological space]]). This has a sub-base of sets <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e1300608.png" /> for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e1300609.png" /> compact in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006010.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006011.png" /> open in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006012.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006013.png" /> is the set of continuous functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006014.png" /> such that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006015.png" />. Fox also began the investigation of the relation of this to the  "exponential law" .

The exponential law for sets uses the set <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006016.png" /> of functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006017.png" /> and states that for any sets <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006018.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006019.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006020.png" /> there is a natural bijection <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006021.png" />, given by <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006022.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006023.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006024.png" />. This law is an expression of the standard idea that a function of two variables can be thought of as a variable function of one variable.

Fox sought a similar result when <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006025.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006026.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006027.png" /> are topological spaces and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006028.png" /> is replaced by <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006029.png" />, the set of continuous functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006030.png" />. This required finding an appropriate topology on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006031.png" />. Unfortunately, it was found that this worked well only for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006032.png" /> locally compact, in the sense of having a neighbourhood base of compact sets, and that the appropriate topology was the [[Compact-open topology|compact-open topology]]. A careful analysis of topologies on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006033.png" /> in relation to the exponential law was given by R. Arens and J. Dugundji.

The restriction to locally compact spaces for the validity of the exponential law was awkward for topology. It was suggested by E. Spanier in [[#References|[a13]]] that the situation could be remedied by using  "quasi-topological spaces" , which specify for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006034.png" /> a set of mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006035.png" /> for all compact Hausdorff spaces, satisfying appropriate axioms (cf. also [[Hausdorff space|Hausdorff space]]). This suggestion was subsequently felt to be vitiated by the fact that a two-point set had a class of quasi-topological structures (see the discussion and references in [[#References|[a12]]]).

R. Brown in [[#References|[a8]]] found that the exponential law was satisfied in the category of Hausdorff <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006037.png" />-spaces (cf. [[Space of mappings, topological|Space of mappings, topological]]) and continuous mappings. In [[#References|[a2]]] it was suggested that this category  "may be adequate and convenient for all purposes of topology" . The exposition in [[#References|[a3]]] suggested the equivalent category of Hausdorff spaces and mappings continuous on compact subsets. It also explained the failure of the exponential law for all spaces, by giving a law of the form <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006038.png" /> for a new product topology <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006039.png" />.

The theme of  "convenient categories"  was also taken up in the expository paper [[#References|[a14]]], again using Hausdorff <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006040.png" />-spaces, but called  "compactly-generated spaces" .

It was known about that time (1967) that the Hausdorff condition could be removed by taking compactly generated to mean  "having the final topology with respect to all mappings of compact Hausdorff spaces into the space" . In [[#References|[a7]]] this is generalized to the case of certain classes <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006041.png" /> of compact Hausdorff spaces, considering the set <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006042.png" /> of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006043.png" />-continuous mappings between spaces, and giving this set a topology with a sub-base of sets <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006044.png" /> for all open sets in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006045.png" /> and all  "test"  mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006046.png" /> for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006047.png" />.

An important extension of results on the exponential law involves spaces of partial maps on closed subsets. A useful trick here is the representability of such partial mappings, an idea which comes from [[Topos|topos]] theory: The set <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006048.png" /> of partial mappings with closed domain is bijective with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006049.png" /> where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006050.png" />, where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006051.png" />, and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006052.png" /> is closed in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006053.png" /> if and only if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006054.png" /> is closed in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006055.png" /> or <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006056.png" /> — thus <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006057.png" /> is open but not closed in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006058.png" />.

Using this device one can obtain an exponential law in the slice category <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006059.png" /> of compactly generated spaces over the compactly generated space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006060.png" /> provided <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006061.png" /> is a <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006062.png" />-space (cf. [[Separation axiom|Separation axiom]]). If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006063.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006064.png" /> are spaces over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006065.png" />, then the space of functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006066.png" /> has as fibre over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006067.png" /> the space of mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006068.png" />. The topology is the join (in the given convenient category) of the topology on partial mappings with closed domain and that which makes <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006069.png" /> continuous. A consequence of the fibred exponential law is that a mapping <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006070.png" /> over <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006071.png" /> corresponds exactly to a section of the mapping <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006072.png" />. This law has been extended to more general situations in [[#References|[a6]]]. These laws are very useful tools in [[Algebraic topology|algebraic topology]].

A dual device yields a topology on spaces of mappings with open domain [[#References|[a1]]], but this has not yet (2000) been much exploited. This is surprising, since the solutions of many standard problems, such as differential equations, are often partial functions with variable open domain.

In [[#References|[a11]]] the category of sequential spaces is embedded into a topos.

Approaches based on other kinds of set-open topologies and on graph topologies, with the aim of such applications, can be found in, for example, [[#References|[a5]]], [[#References|[a9]]], which point to a substantial literature in this area. However, [[#References|[a15]]] uses categorical concepts and constructions to give a fairly comprehensive theory of differentiation in fairly general linear spaces of arbitrary dimension.

====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  R. Brown,   A.M. Abd-Allah,   "A compact-open topology on partial maps with open domain"  ''J. London Math. Soc.'' , '''2''' :  21  (1980)  pp. 480–486</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  R. Brown,   "Ten topologies for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e130/e130060/e13006073.png" />"  ''Quart. J. Math.'' , '''2''' :  14  (1963)  pp. 303–319</TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top">  R. Brown,   "Function spaces and product topologies"  ''Quart. J. Math.'' , '''2''' :  15  (1964)  pp. 238–250</TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  R. Brown,   "Topology: a geometric account of general topology, homotopy types, and the fundamental groupoid" , Ellis Horwood  (1988)</TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top">  P. Brandi,   R. Ceppitelli,   "A new graph topology. Connections with the compact open topology"  ''Applic. Anal.'' , '''53'''  (1994)  pp. 185–196</TD></TR><TR><TD valign="top">[a6]</TD> <TD valign="top">  P.I. Booth,   P.R. Heath,   R. Piccinini,   "Fibre preserving maps and functions spaces" , ''Algebraic Topology, Proc. Vancouver, 1977'' , ''Lecture Notes in Mathematics'' , '''673''' , Springer  (1978)  pp. 158–167</TD></TR><TR><TD valign="top">[a7]</TD> <TD valign="top">  P.I. Booth,   A. Tillotson,   "Monoidal closed, cartesian closed and convenient categories of topological spaces"  ''Pacific J. Math.'' , '''88'''  (1980)  pp. 35–53</TD></TR><TR><TD valign="top">[a8]</TD> <TD valign="top">  R. Brown,   "Some problems of algebraic topology: a study of function spaces, function complexes and FD-complexes"  ''DPhil Thesis, Oxford''  (1961)</TD></TR><TR><TD valign="top">[a9]</TD> <TD valign="top">  A. Concilio,   S. Naimpally,   "Proximal set-open topologies"  ''Acta Math. Acad. Sci. Hungar.'' , '''88'''  (2000)  pp. 227–237</TD></TR><TR><TD valign="top">[a10]</TD> <TD valign="top">  H. Herrlich,   "Topological improvements of categories of structured sets"  ''Topol. Appl.'' , '''27'''  (1987)  pp. 145–155</TD></TR><TR><TD valign="top">[a11]</TD> <TD valign="top">  P. Johnstone,   "On a topological topos"  ''Proc. London Math. Soc.'' , '''3''' :  38  (1979)  pp. 237–271</TD></TR><TR><TD valign="top">[a12]</TD> <TD valign="top">  K.C. Min,   Y.S. Kim,   J.W. Park,   "Fibrewise exponential laws in a quasitopos"  ''Cah. Topol. Géom. Diff. Cat.'' , '''40'''  (1999)  pp. 242–260</TD></TR><TR><TD valign="top">[a13]</TD> <TD valign="top">  E.H. Spanier,   "Quasi-topologies"  ''Duke Math. J.'' , '''30'''  (1963)  pp. 1–14</TD></TR><TR><TD valign="top">[a14]</TD> <TD valign="top">  N. Steenrod,   "A convenient category of topological spaces"  ''Michigan Math. J.'' , '''14'''  (1967)  pp. 133–152</TD></TR><TR><TD valign="top">[a15]</TD> <TD valign="top">  A. Kriegl,   P.W. Michor,   "The convenient setting of global analysis" , ''Math. Surveys and Monographs'' , '''53''' , Amer. Math. Soc.  (1997)</TD></TR></table>