A filter on a set is a set of of subsets of X which has the following properties:
() Every subset of X which contains a set of belongs to .
() Every finite intersection of sets of belongs to .
Subsets in are called neighbourhoods or -big.
A mapping of underlying sets of filters is continuous iff the preimage of a neighbourhood is necessarily a neighbourhood. Note that is a topology on , but this notion of continuity is stronger: it means continuous with dense image.
Let α denote the category of filters and the continuous mappings of underlying sets.
It is convenient to talk about a filter on a set as a filter structure on a set and think of it as a space with a notion of smallness, in analogy to a topological structure on a set and thinking of it as a (topological) space with a notion of nearness of points.
A filter on a set gives a precise meaning to phrases βproperty holds for all points small enoughββ and βholds for almost all elements of ββ, namely that
, in the same way that the topology (topological structure) on a set gives a precise meaning to the phrase βproperty holds for all points sufficiently near a given point ββ, namely is a neighbourhood of .
Thus, for example, in the neighbourhood filter of a point in a topological space, a point is small iff is near : we think of near as an approximation to up to a small error.
A number of elementary notions can be formulated using the category of simplicial objects of the category of filters.
In a topological space , a filter on converges to a point of if every neighbourhood of belongs to , i.e. the map from to the neighbourhood filter of is continuous. The filter on the empty set is the initial object; the proper filter on a single point is the terminal object.
The concepts of continuous function and uniformly continuous function may be defined quite nicely in terms of the continuity of maps of filters. A map of topological spaces is continuous iff it induces continuous maps of neighbourhood filters. A map of uniform spaces is continuous iff it induces a continuous map of the uniform structures (filters on ).
In a metric space , a filter on is Cauchy if it has elements of arbitrarily small diameter, i.e. the map is continuous with respect to the uniformity (filter) on . Then a sequence is a Cauchy sequence iff its eventuality filter is Cauchy, i.e. it induces a continuous map of certain filters. This gives a diagram chasing meaning to the concept of completion of a metric space.
Last revised on May 19, 2022 at 02:17:49. See the history of this page for a list of all contributions to it.