Topology

Topology is the study or science of places. It derives it's name from the greek words τόπος meaning place and λόγος meaning study.

Topology is a term used in architecture to describe spatial effects which can not be described by topography, i.e., social, economical, spatial or phenomenological interactions.

Topology is that branch of mathematics concerned with the study of topological spaces. (The term topology is also used for a system of open sets used to define topological spaces, but this article will focus on the branch of mathematics. Wiring and computer network topologies are discussed in Network topology.)

Topological spaces show up naturally in mathematical analysis, abstract algebra and geometry. This has made topology one of the great unifying ideas of mathematics. General topology, or point-set topology, defines and studies some very useful properties of spaces and maps, such as connectedness, compactness and continuity. 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. Ideas from algebraic topology have had strong influence on algebra and algebraic geometry.

Please refer to the Topology glossary for the definitions of terms used throughout topology.

Elementary introduction

The motivating insight behind topology is that some geometric problems depend not on the exact shape of the objects involved, but rather on the "way they are connected together". One of the first papers in topology was the demonstration, by Leonhard Euler, that it was impossible to find a route through the town of Königsberg (now Kaliningrad) that would cross each of its seven bridges exactly once. This result did not depend on the lengths of the bridges, nor on their distance from one another, but only on connectivity properties of which bridges are connected to which islands or riverbanks. This problem, the seven bridges of Königsberg, is now a famous problem in introductory mathematics.

Similarly, the hairy ball theorem[?] of algebraic topology says that "you can't comb the hair on a ball smooth". This fact is immediately convincing to most people, even though they might not recognize the more formal statement of the theorem: There is no nonvanishing continuous tangent vector field on the sphere. As with the bridges of Königsberg, the result doesn't depend on the exact shape of the sphere. It applies to pear shapes and in fact any kind of blob, as long as it has no holes.

In order to deal with these problems that don't rely on the exact shape of the objects, one must be clear about just what properties these problems do rely on. From this need arises the notion of topological equivalence. The impossibility of crossing each bridge just once applies to any arrangement of bridges topologically equivalent to those in Königsberg, and the hairy ball theorem applies to any space topologically equivalent to a sphere. Formally, two spaces are topologically equivalent if there is a homeomorphism between them. In that case the spaces are said to be homeomorphic, and they are considered to be essentially the same for the purposes of topology.

Formally, a homeomorphism is defined as a continuous bijection with a continuous inverse, which is not terribly evocative even when you know what the words in the definition mean. A more informal criterion gives a better visual sense: Two spaces are topologically equivalent if one can be deformed into the other without cutting it apart or gluing pieces of it together. The traditional joke is that the topologist can't tell the coffee cup she is drinking out of from the donut she is eating, since a sufficiently pliable donut could be reshaped to the form of a coffee cup by creating a dimple and progressively enlarging it, while shrinking the hole into a handle.

One simple introductory exercise is to classify the letters of the English alphabet[?] according to topological equivalence. To be simple, let's assume that the lines of the letters have nonzero width. Then in most fonts, we have the class {a,b,d,e,g,o,p,q} of letters with a hole, the class {c,f,h,k,l,m,n,r,s,t,u,v,w,x,y,z} of letters without a hole, and the class {i,j} of letters consisting of two pieces. For a more complicated exercise, assume that the lines have zero width; you can get several different classifications depending on which font you use.

Some useful theorems

• Every closed interval in R of finite length is compact. More is true: In Rⁿ, a set is compact iff it is closed and bounded. (See Heine-Borel theorem.)
• Every continuous image of a compact space is compact.
• Tychonoff's theorem: The (arbitrary) product of compact spaces is compact.
• A compact subspace of a Hausdorff space is closed.
• Every sequence of points in a compact metric space has a convergent subsequence.
• Every interval in R is connected.
• The continuous image of a connected space is connected.
• A metric space is Hausdorff, also normal and paracompact.
• The metrization theorems provide necessary and sufficient conditions for a topology to come from a metric.
• The Tietze extension 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.
• The Baire category theorem: If X is a complete metric space or a locally compact Hausdorff space, then the interior of every union of countably many nowhere dense sets is empty.
• On a paracompact Hausdorff space every open cover admits a partition of unity subordinate to the cover.
• Every path-connected, locally path-connected and semi-locally simply connected[?] space has a universal cover.

Some useful notions from algebraic topology

• Homology and cohomology: Betti numbers[?], Euler characteristic.
• Nice applications: Brouwer Fixed Point Theorem, Borsuk-Ulam Theorem
• Homotopy groups (including the fundamental group.)
• Chern classes[?], Stiefel Whitney classes[?], Pontrjagin classes[?]

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
• Surgery obstructions
• H-spaces[?], infinite loop spaces, A_∞ rings
• Homotopy theory of affine schemes[?]
• Intersection cohomology

Generalizations

Occasionally, 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.

Related articles

• Topological space
• Network topology
• Link topology
• Topology of the universe
• Covering map

External link

• ODP category (http://dmoz.org/Science/Math/Topology/)