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Concept
Net (mathematics)
Summary
In mathematics, more specifically in general topology and related branches, a net or Moore–Smith sequence is a generalization of the notion of a sequence. In essence, a sequence is a function whose domain is the natural numbers. The codomain of this function is usually some topological space.
The motivation for generalizing the notion of a sequence is that, in the context of topology, sequences do not fully encode all information about functions between topological spaces. In particular, the following two conditions are, in general, not equivalent for a map between topological spaces and :
The map is continuous in the topological sense;
Given any point in and any sequence in converging to the composition of with this sequence converges to (continuous in the sequential sense).
While condition 1 always guarantees condition 2, the converse is not necessarily true if the topological spaces are not both first-countable. In particular, the two conditions are equivalent for metric spaces. The spaces for which the converse holds are the sequential spaces.
The concept of a net, first introduced by E. H. Moore and Herman L. Smith in 1922, is to generalize the notion of a sequence so that the above conditions (with "sequence" being replaced by "net" in condition 2) are in fact equivalent for all maps of topological spaces. In particular, rather than being defined on a countable linearly ordered set, a net is defined on an arbitrary directed set. This allows for theorems similar to the assertion that the conditions 1 and 2 above are equivalent to hold in the context of topological spaces that do not necessarily have a countable or linearly ordered neighbourhood basis around a point. Therefore, while sequences do not encode sufficient information about functions between topological spaces, nets do, because collections of open sets in topological spaces are much like directed sets in behavior. The term "net" was coined by John L. Kelley.
Nets are one of the many tools used in topology to generalize certain concepts that may not be general enough in the context of metric spaces.
Official source
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