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Topological spaces are structures which allow one to formalize concepts such as convergence, connectedness and continuity. They appear in all branches of modern mathematics and can be seen as a central unifying notion. The branch of mathematics which studies topological spaces in their own right is called topology.
Formally, a topological space is a set X together with a set T of subsets of X (i.e., T is a subset of the power set of X) satisfying:
The set T is also called a topology on X. The sets in T are referred to as open sets, and their complements in X are called closed sets. Roughly speaking, open sets are thought of as neighborhoods of points; two points are "close together" if there are many open sets that contain both of them.
A function between topological spaces is said to be continuous if the inverse image of every open set is open. This is an attempt to capture the intuition that points which are "close together" get mapped to points which are "close together".

Topological spaces can be broadly classified according to their degree of connectedness, their size, their degree of compactness and the degree of separation of their points. A great many terms are used in topology to achieve these distinctions. These terms and definitions are collected together in the Topology Glossary.
There are many other equivalent ways to define a topological space. Instead of defining open sets, it is possible to define first the closed sets, with the properties that the intersection of arbitrarily many closed sets is closed, the union of a finite number of closed sets is closed, and X and the empty set are closed. Open sets are then defined as the complements of closed sets. Another method is to define the topology by means of the closure operator. The closure operator is a function from the power set of X to itself which satisfies the following axioms (called the Kuratowski closure axioms): the closure operator is idempotent, every set is a subset of its closure, the closure of the empty set is empty, and the closure of the union of two sets is the union of their closures. Closed sets are then the fixed points of this operator. A topology on X is also completely determined if for every net in X the set of its limits is specified.
Metric spaces were defined and investigated by Fréchet in 1906, Hausdorff spaces by Felix Hausdorff in 1914 and the current concept of topological space was described by Kuratowski in 1922.
It is almost universally true that all "large" algebraic objects carry a natural topology which is compatible with the algebraic operations. In order to study these objects, one typically has to take the topology into account. This leads to concepts such as topological groups, topological vector spaces and topological rings.
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