In mathematics the term net has at least two meanings. See the glossary of Riemannian and metric geometry for its meaning for metric spaces.

This article is about its meaning in topology, where the concept of a net is a generalization of that of a sequence, intended to unify the various notions of limit and generalize them to arbitrary topological spaces. Limits of nets accomplish for all topological spaces what limits of sequences accomplish for first-countable spaces such as metric spaces. Nets were first introduced by E. H. Moore and H. L. Smith in 1922.

Definition

If X is a topological space, a net in X is a function from some directed set A to X.

If A is a directed set, we often write a net from A to X in the form (x_α), which expresses the fact that the element α in A is mapped to the element x_α in X. We usually use ≥ to denote the binary relation given on A.

Examples

Since the natural numbers with the usual order form a directed set and a sequence is a function on the natural numbers, every sequence is a net.

Another important example is as follows. Given a point x in a topological space, let N_x denote the set of all neighbourhoods containing x. Then N_x is a directed set, where the direction is given by reverse inclusion, so that S ≥ T if and only if S is contained in T. For S in N_x, let x_S be a point in S. Then x_S is a net. As S increases with respect to ≥, the points x_S in the net are constrained to lie in decreasing neighbourhoods of x, so intuitively speaking, we are lead to the idea that x_S must tend towards x in some sense. We can make this limiting concept precise.

Limits of nets

If (x_α) is a net from a directed set A into X, and if Y is a subset of X, then we say that (x_α) is eventually in Y if there exists an α in A so that for every β in A with β ≥ α, the point x_β lies in Y.

If (x_α) is a net in the topological space X, and x is an element of X, we say that the net converges towards x or has limit x and write

lim x_α = x

if and only if

for every neighborhood U of x, (x_α) is eventually in U.

Intuitively, this means that the values x_α come and stay as close as we want to x for large enough α.

Note that example net given above on the neighbourhood system of a point x does indeed converge to x according to this definition.

Examples of limits of nets

• Limits of sequences.

• Limits of functions of a real variable: lim_{x → c} f(x). Here we direct the set R\{c} according to distance from c.

• Limits of nets of Riemann sums, in the definition of the Riemann integral. In this example, the directed set is the set of partitions of the interval of integration, partially ordered by inclusion. A similar thing is done in the definition of the Riemann-Stieltjes integral.

Supplementary definitions

If D and E are directed sets, and h is a function from D to E, then h is called cofinal if for every e in E there is a d in D so that if q is in D and q ≥ d then h(q) ≥ e.

If D and E are directed sets, h is a cofinal function from D to E, and φ is a net on set X based on E, then φoh is called a subnet of φ. All subnets are of this form, by definition.

If φ is a net on X based on directed set D and A is a subset of X, then φ is frequently in A if for every α in D there is a β in D, β ≥ α so that φ(β) is in A.

A net φ on set X is called universal if for every subset A of X, either φ is eventually in A or φ is eventually in X-A.

Properties

Virtually all concepts of topology can be rephrased in the language of nets and limits. This may be useful to guide the intuition since the notion of limit of a net is very similar to that of limit of a sequence, which is widely used in the theory of metric spaces.

A function f : X → Y between topological spaces is continuous at the point x if and only if for every net (x_α) with

lim x_α = x

we have

lim f(x_α) = f(x).

Note that this theorem is in general not true if we replace "net" by "sequence". We have to allow for more directed sets than just the natural numbers if X is not first-countable.

In general, a net in a space X can have more that one limit, but if X is a Hausdorff space, the limit of a net, if it exists, is unique.

If U is a subset of X, then x is in the closure of U if and only if there exists a net (x_α) with limit x and such that x_α is in U for all α. In particular, U is closed if and only if, whenever (x_α) is a net with elements in U and limit x, then x is in U.

A net has a limit if and only if all of its subnets have limits. In that case, every limit of the net is also a limit of every subnet.

A space X is compact if and only if every net (x_α) in X has a subnet with a limit in X. This can be seen as a generalization of the Bolzano-Weierstrass theorem and Heine-Borel theorem.

In a metric space or uniform space, one can speak of Cauchy nets in much the same way as Cauchy sequences. The concept even generalises to Cauchy spaces.

See also

The theory of filters also provides a definition of convergence in general topological spaces.

Reference

E. H. Moore and H. L. Smith (1922). A General Theory of Limits. American Journal of Mathematics 44 (2), 102–121.