# Topology

Topology, in mathematics, is both a structure used to capture the notions of continuity, connectedness and convergence, and the name of the branch of mathematics which studies these.

### Topology, the structure

Formally, a topology for a set X is defined as a set T of subsets of X (i.e., T is a subset of the power set of X) satisfying:

1. The union of any collection of sets in T is also in T.
2. The intersection of any pair of sets in T is also in T.
3. X and the empty set are in T.

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.

There are many other equivalent ways to define a topology. Instead of defining open sets, it is possible to define first the closed sets, with the properties that the intersection of 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 set together with a topology for the set is called a topological space. A function between topological spaces is said to be continuous if the inverse image of every open set is open.

A great many terms are used in topology. Some of these terms have been collected together in the Topology Glossary, and the rest of this article assumes that the reader is familiar with them.

#### Examples of topological spaces

• Any set with the discrete topology (i.e., every set is open, which has the effect that no two points are "close" to each other).
• Any set with the trivial topology (i.e., only the empty set and the whole set are open, which has the effect of "lumping all points together").
• Any infinite set with the cofinite topology (i.e., the open sets are the empty set and the sets whose complement is finite). This is the smallest T1 topology on the set.
• The real numbers R: the open sets are unions of (possibly infinitely many) open intervals. This is in many ways the most basic topological space and the one that guides most of our human intuition.
• The complex numbers C: the open sets are unions of open discs.
• A subset of a topological space. The open sets are the intersections of the open sets of the larger space with the subset. This is also called a subspace.
• Any metric space turns into a topological space if we define a set to be open if it is a (possibly infinite) union of open balls.

In particular we have:

### Topology, the field of mathematics

Topological spaces show up naturally in analysis, algebra and geometry. This has made topology one of the great unifying ideas of mathematics. Point-set topology (or general topology) defines and studies some very useful properties of spaces and maps. Algebraic topology is a powerful tool to study topological spaces, and the maps between them. It associates "discrete" more computable invariants to maps and spaces, often in a functorial way (see category theory). Ideas from algebraic topology have had strong influence on algebra and algebraic geometry.

#### Some useful theorems

• Every closed interval is compact. (Heine-Borel Theorem)
• Every closed interval is connected.
• A metric space is Hausdorff, also normal and paracompact.
• If X is a complete metric space or locally compact Hausdorff, then the complement of every countable union of nowhere dense sets is dense. (Baire category theorem)
• In a normal space, every continuous real-valued function defined on a closed subspace can be extended to a continuous map defined on the whole space. (Tietze extension theorem)
• The image of a connected space is connected.
• The (arbitrary) product of compact spaces is compact. (Tychonoff's Theorem)
• A compact subspace of a Hausdorff space is closed.
• Every sequence of points on a compact metric space has a convergent subsequence.
• On a paracompact Hausdorff space every open covering admits a partition of unity subordinate to the cover.

#### Sketchy outline of the deeper theory

• (Co)Fibre sequences --> Puppe sequence, computations
• homotopy groups of spheres
• obstruction theory
• K-theory --> KO, algebraic K-theory
• stable homotopy
• Brown representability
• (Co)bordism
• signatures
• BP and Morava K-theory,
• equivariant jazz
• Surgery obstructions
• H-spaces, infinite loop spaces, A_infty rings
• homotopy theory of schemes
• Intersection Cohomology

#### Generalizations

Occasionly, one needs to use the tools of topology but a "set of points" is not available. In pointless topology one considers instead the lattice of open sets as the basic notion of the theory, while Grothendieck topologies are certain structures defined on arbitrary categories which allow the definition of sheaves on those categories, and with that the definition of quite general cohomology theories.