# Portal:Topology

## Topology

**Topology** (Greek *topos*, "place," and *logos*, "study") is a branch of mathematics that is an extension of geometry. Topology begins with a consideration of the nature of space, investigating both its fine structure and its global structure. Topology builds on set theory, considering both sets of points and families of sets.

The word *topology* is used both for the area of study and for a family of sets with certain properties described below that are used to define a topological space. Of particular importance in the study of topology are functions or maps that are *homeomorphisms*. Informally, these functions can be thought of as those that stretch space without tearing it apart or sticking distinct parts together.

When the discipline was first properly founded, toward the end of the 19th century, it was called *geometria situs* (Latin *geometry of place*) and *analysis situs* (Latin *analysis of place*). From around 1925 to 1975 it was an important growth area within mathematics.

Topology is a large branch of mathematics that includes many **subfields**. The most basic division within topology is **point-set topology**, which investigates such concepts as compactness, connectedness, and countability; **algebraic topology**, which investigates such concepts as homotopy and homology; and **geometric topology**, which studies manifolds and their embeddings, including knot theory.

## Selected article

The **homotopy groups of spheres** describe the different ways spheres of various dimensions can be wrapped around each other. They are studied as part of algebraic topology. The topic can be hard to understand because the most interesting and surprising results involve spheres in higher dimensions. These are defined as follows: an *n*-dimensional sphere, ** n-sphere**, consists of all the points in a space of

*n+1*dimensions that are a fixed distance from a center point. This definition is a generalization of the familiar circle (1-sphere) and sphere (2-sphere).

A homotopy from a circle around a sphere down to a single point. |

The goal of algebraic topology is to categorize or classify topological spaces. Homotopy groups were invented in the late 19th century as a tool for such classification, in effect using the set of mappings from an *n*-sphere in to a space as a way to probe the structure of that space. An obvious question was how this new tool would work on *n*-spheres themselves. No general solution to this question has been found to date, but many homotopy groups of spheres have been computed and the results are surprisingly rich and complicated. The study of the homotopy groups of spheres has led to the development of many powerful tools used in algebraic topology.

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## Selected picture

It is often suggested that a topologist cannot tell the difference between a coffee cup and a doughnut. This is because these objects when thought of as topological spaces are homeomorphic. The above picture depicts a continuous deformation of a coffee cup into a doughnut such that at each stage the object is homeomorphic to the original.

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## Did you know?

- ...that it is possible to turn a sphere inside out without tears or creases? (The sphere is allowed to pass through itself).
- ...that the Klein bottle gives a two-fold covering space of itself?
- ...that outstanding mathematician Grigori Perelman was offered a Fields Medal in 2006, in part for his proof of the Poincaré conjecture, which he declined?
- ...that you cannot knot strings in 4-dimensions? You can, however, knot 2-dimensional surfaces like spheres.

## Categories

*Topics in Topology*

Main articles | Key concepts | Algebraic topology | Geometric topology |
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