# Lebesgue covering dimension

Lebesgue covering dimension This article includes a list of general references, mas faltam citações em linha correspondentes suficientes. Ajude a melhorar este artigo introduzindo citações mais precisas. (abril 2018) (Saiba como e quando remover esta mensagem de modelo) Na matemática, the Lebesgue covering dimension or topological dimension of a topological space is one of several different ways of defining the dimension of the space in a topologically invariant way.[1][2] Conteúdo 1 Informal discussion 2 Formal definition 3 Exemplos 4 Propriedades 5 Veja também 6 Notas 7 Referências 8 Leitura adicional 8.1 Historical 8.2 Modern 9 External links Informal discussion For ordinary Euclidean spaces, the Lebesgue covering dimension is just the ordinary Euclidean dimension: zero for points, one for lines, two for planes, e assim por diante. No entanto, not all topological spaces have this kind of "obvious" dimension, and so a precise definition is needed in such cases. The definition proceeds by examining what happens when the space is covered by open sets.

No geral, a topological space X can be covered by open sets, in that one can find a collection of open sets such that X lies inside of their union. The covering dimension is the smallest number n such that for every cover, there is a refinement in which every point in X lies in the intersection of no more than n + 1 covering sets. This is the gist of the formal definition below. The goal of the definition is to provide a number (an integer) that describes the space, and does not change as the space is continuously deformed; isso é, a number that is invariant under homeomorphisms.

The general idea is illustrated in the diagrams below, which show a cover and refinements of a circle and a square.

Refinement of the cover of a circle The left diagram shows a refinement (à esquerda) of a cover (on the right) of a circular line (Preto). Notice how in the refinement no point on the line is contained in more than two sets. Note also how the sets link to each other to form a "chain".

Refinement of the cover of a square The bottom left is a refinement of a cover (topo) of a planar shape (dark) so that all points in the shape are contained in at most three sets. The bottom right is an attempt to refine the cover so that no point would be contained in more than two sets. This fails in the intersection of set borders. Desta forma, a planar shape isn't "webby" or cannot be covered with "chains", but is in a sense thicker; ou seja, its topological dimension must be higher than one. Formal definition Henri Lebesgue used closed "bricks" to study covering dimension in 1921.[3] The first formal definition of covering dimension was given by Eduard Čech, based on an earlier result of Henri Lebesgue.[4] A modern definition is as follows. An open cover of a topological space X is a family of open sets Uα such that their union is the whole space, ∪ Uα = X. The order or ply of an open cover {estilo de exibição {mathfrak {UMA}}} = {} is the smallest number m (if it exists) for which each point of the space belongs to at most m open sets in the cover: in other words Uα1 ∩ ⋅⋅⋅ ∩ Uαm+1 = {displaystyle emptyset } for α1, ..., αm+1 distinct. A refinement of an open cover {estilo de exibição {mathfrak {UMA}}} = {} is another open cover {estilo de exibição {mathfrak {B}}} = {}, such that each Vβ is contained in some Uα. The covering dimension of a topological space X is defined to be the minimum value of n, such that every open cover {estilo de exibição {mathfrak {UMA}}} of X has an open refinement {estilo de exibição {mathfrak {B}}} with order n + 1. Desta forma, if n is finite, VΒ1 ∩ ⋅⋅⋅ ∩ Vβn+2 = {displaystyle emptyset } for β1, ..., βn+2 distinct. If no such minimal n exists, the space is said to have infinite covering dimension.

As a special case, a topological space is zero-dimensional with respect to the covering dimension if every open cover of the space has a refinement consisting of disjoint open sets so that any point in the space is contained in exactly one open set of this refinement.

It is often convenient to say that the covering dimension of the empty set is −1.

Examples Any given open cover of the unit circle will have a refinement consisting of a collection of open arcs. The circle has dimension one, by this definition, because any such cover can be further refined to the stage where a given point x of the circle is contained in at most two open arcs. Aquilo é, whatever collection of arcs we begin with, some can be discarded or shrunk, such that the remainder still covers the circle but with simple overlaps.

De forma similar, any open cover of the unit disk in the two-dimensional plane can be refined so that any point of the disk is contained in no more than three open sets, while two are in general not sufficient. The covering dimension of the disk is thus two.

De forma geral, the n-dimensional Euclidean space {estilo de exibição mathbb {E} ^{n}} has covering dimension n.