# Maths - Concrete Category - Topology

There are various categories associated with topology. The most fundamental is Top The category of topological spaces.

 It has: objects: topological spaces morphisms: continuous maps In the most general definition the points map like sets and there is an additional requirement that the preimage of an open set is an open set. That is the open sets map in the reverse direction.
 This is a category because the composition of two continuous maps is also a continuous map. The composition is f•g for points and g-1•f-1 for open sets. note: change of order for open sets.
 Also the Id is the Id for points and the Id for opensets.

The isomorphisms in this category correspond to homeomorphisms in topology.

Top preserves limits and co-limits. Simplicial sets are co-limits of Top.

Top is a category but it does not have exponential objects for all spaces and so it does not have some nice properties: it is not cartesian closed or a topos. If these properties are required then there may be variations of Top for the spaces required that have these properties. For example, nice topological spaces such as compactly generated topological spaces. Or see homotopy category at ncatlib.

See page here about categorical limits.

There is a category of the subset structure of open sets. There are also categories of fibres and sheaves.

## Top - Topological Spaces

Consider a category C as follows:

• Objects are spaces
• Morphisms are structure preserving morphisms between these spaces

For a non-category theory view of this structure see page here.

## Subset Structure of Open Sets as Category

Subsets give interesting structure to open sets. This allows us to define a category of topological spaces. We can also further elaborate this subset structure to get fibre bundles and sheaves.

 In this diagram the open set U is a subset of V. UV We can represent this as an arrow from V to U V->U This has the properties we expect from a category, for instance, the identity map: U->U (identity map) and composition (U->V)*(V ->W) = U->W (composition)

For a non-category theory view of this structure see page here.

## Bn - Category of Bundles

 A fibre bundle is a function f:(A->I) This is described on the following pages:
 We can make this a category where: objects are pairs (A,f) consisting of a stalk 'A' and a mapping of 'A' to 'I'. morphisms are functors between these where I does not change. This triangle must commute. So the elements (germs) of a stalk in 'A' must map to the same stalk in 'B'. This is a comma category as discussed on this page.

The Bn Category can lead on to the concept of a Topos.

## Simplical Sets

We have looked at these, mostly from a topology point of view, on the pages here:

The subject can also be approached from a purely combinatorial point of view.

Here we investigate how these structures can be viewed in a category theory way.

 Δ is a category with: objects - are ordered sets which represent simplicies. morphisms - are order preserving maps. The morphisms are inclusions Δop is a category with the same objects but morphisms are the face maps.

## Category of Presheaves

### Objects: Cop -> set

contraveriant functors X: C -> set

(written X: Cop -> set to indicate contraveriance)

### Morphisms:

are natural transformations N: X -> Y

For a non-category theory view of this structure see page here.