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In
mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It has no generally ...
, the compact-open topology is a
topology s, which have only one surface and one edge, are a kind of object studied in topology. In mathematics, topology (from the Greek language, Greek words , and ) is concerned with the properties of a mathematical object, geometric object that are p ...
defined on the set of continuous maps between two
topological space In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
s. The compact-open topology is one of the commonly used topologies on function spaces, and is applied in homotopy theory and
functional analysis Image:Drum vibration mode12.gif, 200px, One of the possible modes of vibration of an idealized circular drum head. These modes are eigenfunctions of a linear operator on a function space, a common construction in functional analysis. Functional an ...
. It was introduced by Ralph Fox in 1945. If the codomain of the functions under consideration has a uniform structure or a metric structure then the compact-open topology is the "topology of
uniform convergenceIn the mathematical field of analysis, uniform convergence is a mode of Limit of a sequence, convergence of functions stronger than pointwise convergence. A sequence of Function (mathematics), functions (f_n) converges uniformly to a limiting func ...
on compact sets." That is to say, a
sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order theory, order matters. Like a Set (mathematics), set, it contains Element (mathematics), members (also called ''elements'', or ''terms''). ...

of functions converges in the compact-open topology precisely when it converges uniformly on every compact subset of the domain.

# Definition

Let and be two
topological space In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
s, and let denote the set of all continuous maps between and . Given a compact subset of and an open set, open subset of , let denote the set of all functions such that Then the collection of all such is a subbase for the compact-open topology on . (This collection does not always form a base (topology), base for a topology on .) When working in the category (mathematics), category of compactly generated spaces, it is common to modify this definition by restricting to the subbase formed from those that are the image of a compact set, compact Hausdorff space. Of course, if is compactly generated and Hausdorff, this definition coincides with the previous one. However, the modified definition is crucial if one wants the convenient category of Weak Hausdorff space, compactly generated weak Hausdorff spaces to be Cartesian closed category, Cartesian closed, among other useful properties. The confusion between this definition and the one above is caused by differing usage of the word compact set, compact.

# Properties

* If is a one-point space then one can identify with , and under this identification the compact-open topology agrees with the topology on . More generally, if is a discrete space, then can be identified with the cartesian product of copies of and the compact-open topology agrees with the product topology. * If is , , Hausdorff space, Hausdorff, regular space, regular, or tychonoff space, Tychonoff, then the compact-open topology has the corresponding separation axiom. * If is Hausdorff and is a subbase for , then the collection is a subbase for the compact-open topology on . * If is a metric space (or more generally, a uniform space), then the compact-open topology is equal to the topology of compact convergence. In other words, if is a metric space, then a sequence limit (mathematics), converges to in the compact-open topology if and only if for every compact subset of , converges uniformly to on . If is compact and is a uniform space, then the compact-open topology is equal to the topology of
uniform convergenceIn the mathematical field of analysis, uniform convergence is a mode of Limit of a sequence, convergence of functions stronger than pointwise convergence. A sequence of Function (mathematics), functions (f_n) converges uniformly to a limiting func ...
. * If and are topological spaces, with locally compact Hausdorff (or even just locally compact preregular space, preregular), then the function composition, composition map given by is continuous (here all the function spaces are given the compact-open topology and is given the product topology). *If is a locally compact Hausdorff (or preregular) space, then the evaluation map , defined by , is continuous. This can be seen as a special case of the above where is a one-point space. * If is compact, and is a metric space with metric (mathematics), metric , then the compact-open topology on is metrisable space, metrisable, and a metric for it is given by for in .

## Applications

The compact open topology can be used to topologize the following sets: * $\Omega\left(X,x_0\right) = \$, the loop space of $X$ at $x_0$, * $E\left(X, x_0, x_1\right) = \$, * $E\left(X, x_0\right) = \$. In addition, there is a Homotopy#Homotopy equivalence, homotopy equivalence between the spaces $C\left(\Sigma X, Y\right) \cong C\left(X, \Omega Y\right)$. These topological spaces, $C\left(X,Y\right)$ are useful in homotopy theory because it can be used to form a topological space and a model for the homotopy type of the ''set'' of homotopy classes of maps :$\pi\left(X,Y\right) = \.$ This is because $\pi\left(X,Y\right)$ is the set of path components in $C\left(X,Y\right)$, that is, there is an isomorphism of sets :$\pi\left(X,Y\right) \to C\left(I, C\left(X, Y\right)\right)/\sim$ where $\sim$ is the homotopy equivalence.

# Fréchet differentiable functions

Let and be two Banach spaces defined over the same field (mathematics), field, and let denote the set of all -continuously Fréchet derivative, Fréchet-differentiable functions from the open subset to . The compact-open topology is the initial topology induced by the seminorms :$p_\left(f\right) = \sup \left\$ where , for each compact subset .

# References

* * O.Ya. Viro, O.A. Ivanov, V.M. Kharlamov and N.Yu. Netsvetaev (2007
Textbook in Problems on Elementary Topology
*{{planetmath reference, id=3976, title=Compact-open topology *

Ronald Brown, 2006 General topology Topology of function spaces