 Net (mathematics)

 This article is about nets in topological spaces and not about εnets in other fields.
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 with domain the natural numbers, and in the context of topology, the range of this function is usually any topological space. However, in the context of topology, sequences do not fully encode all information about a function between topological spaces. In particular, the following two conditions are not equivalent in general for a map ƒ between topological spaces X and Y:
 The map ƒ is continuous
 Given any point x in X, and any sequence in X converging to x, the composition of ƒ with this sequence converges to ƒ(x)
It is true however, that condition 1 implies condition 2, in the context of all spaces. The difficulty encountered when attempting to prove that condition 2 implies condition 1 lies in the fact that topological spaces are, in general, not firstcountable. If the firstcountability axiom were imposed on the topological spaces in question, the two above conditions would be equivalent. In particular, the two conditions are equivalent for metric spaces.
The purpose of the concept of a net, first introduced by E. H. Moore and H. L. Smith in 1922,^{[1]} is to generalize the notion of a sequence so as to confirm the equivalence of the conditions (with "sequence" being replaced by "net" in condition 2). In particular, rather than being defined on a countable linearly ordered set, a net is defined on an arbitrary directed set. In particular, this allows theorems similar to that asserting the equivalence of condition 1 and condition 2, to hold in the context of topological spaces which 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 of the fact that collections of open sets in topological spaces are much like directed sets in behaviour. The term "net" was coined by Kelley.^{[2]}^{[3]}
Nets are one of the many tools used in topology to generalize certain concepts that may only be general enough in the context of metric spaces. A related notion, that of the filter, was developed in 1937 by Henri Cartan.
Contents
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.
Examples of nets
Every nonempty totally ordered set is directed. Therefore every function on such a set is a net. In particular, the natural numbers with the usual order form such a set, and a sequence is a function on the natural numbers, so 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 led 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 (or residually 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 the example net given above on the neighborhood system of a point x does indeed converge to x according to this definition.
Given a base for the topology, in order to prove convergence of a net it is necessary and sufficient to prove that there exists some point x, such that (x_{α}) is eventually in all members of the base containing this putative limit.
Examples of limits of nets
 Limit of a sequence and limit of a function: see below.
 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.
Supplementary definitions
If φ is a net on X based on directed set D and A is a subset of X, then φ is frequently in (or cofinally in) A if for every α in D there exists some β ≥ α, β in D, so that φ(β) is in A.
A point x in X is said to be an accumulation point or cluster point of a net if (and only if) for every neighborhood U of x, the net is frequently in U.
A net φ on set X is called universal, or an ultranet if for every subset A of X, either φ is eventually in A or φ is eventually in XA.
Examples
Sequence in a topological space:
A sequence (a_{1}, a_{2}, ...) in a topological space V can be considered a net in V defined on N.
The net is eventually in a subset Y of V if there exists an N in N such that for every n ≥ N, the point a_{n} is in Y.
We have lim_{x → c} a_{n} = L if and only if for every neighborhood Y of L, the net is eventually in Y.
The net is frequently in a subset Y of V if and only if for every N in N there exists some n ≥ N such that a_{n} is in Y, that is, if and only if infinitely many elements of the sequence are in Y. Thus a point y in V is a cluster point of the net if and only if every neighborhood Y of y contains infinitely many elements of the sequence.
Function from a metric space to a topological space:
Consider a function from a metric space M to a topological space V, and a point c of M. We direct the set M\{c} reversely according to distance from c, that is, the relation is "has at least the same distance to c as", so that "large enough" with respect to the relation means "close enough to c". The function ƒ is a net in V defined on M\{c}.
The net ƒ is eventually in a subset Y of V if there exists an a in M\{c} such that for every x in M\{c} with d(x,c) ≤ d(a,c), the point f(x) is in Y.
We have lim_{x → c} ƒ(x) = L if and only if for every neighborhood Y of L, ƒ is eventually in Y.
The net ƒ is frequently in a subset Y of V if and only if for every a in M\{c} there exists some x in M\{c} with d(x,c) ≤ d(a,c) such that f(x) is in Y.
A point y in V is a cluster point of the net ƒ if and only if for every neighborhood Y of y, the net is frequently in Y.
Function from a wellordered set to a topological space:
Consider a wellordered set [0, c] with limit point c, and a function ƒ from [0, c) to a topological space V. This function is a net on [0, c).
It is eventually in a subset Y of V if there exists an a in [0, c) such that for every x ≥ a, the point f(x) is in Y.
We have lim_{x → c} ƒ(x) = L if and only if for every neighborhood Y of L, ƒ is eventually in Y.
The net ƒ is frequently in a subset Y of V if and only if for every a in [0, c) there exists some x in [a, c) such that f(x) is in Y.
A point y in V is a cluster point of the net ƒ if and only if for every neighborhood Y of y, the net is frequently in Y.
The first example is a special case of this with c = ω.
See also ordinalindexed sequence.
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. The following set of theorems and lemmas help cement that similarity:
 A function ƒ: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 ƒ(x_{α}) = ƒ(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 firstcountable.
 In general, a net in a space X can have more than one limit, but if X is a Hausdorff space, the limit of a net, if it exists, is unique. Conversely, if X is not Hausdorff, then there exists a net on X with two distinct limits. Thus the uniqueness of the limit is equivalent to the Hausdorff condition on the space, and indeed this may be taken as the definition. Note that this result depends on the directedness condition; a set indexed by a general preorder or partial order may have distinct limit points even in a Hausdorff space.
 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.
 A subset A of X is closed if and only if, whenever (x_{α}) is a net with elements in A and limit x, then x is in A.
 The set of cluster points of a net is equal to the set of limits of its convergent subnets.

Proof Let X be a topological space, A a directed set, be a net in X, and . It is easily seen that if y is a limit of a subnet of , then y is a cluster point of .
Conversely, assume that y is a cluster point of . Let B be the set of pairs (U,α) where U is an open neighborhood of y in X and is such that . The map mapping (U,α) to α is then cofinal. Moreover, giving B the product order (the neighborhoods of y are ordered by inclusion) makes it a directed set, and the net defined by y_{β} = x_{h(β)} converges to y.
 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.

Proof First, suppose that X is compact. We will need the following observation (see Finite intersection property). Let I be any set and be a collection of closed subsets of X such that for each finite . Then as well. Otherwise, would be an open cover for X with no finite subcover contrary to the compactness of X. Let A be a directed set and be a net in X. For every define
The collection has the property that every finite subcollection has nonempty intersection. Thus, by the remark above, we have that
and this is precisely the set of cluster points of . By the above property, it is equal to the set of limits of convergent subnets of . Thus has a convergent subnet.
Conversely, suppose that every net in X has a convergent subnet. For the sake of contradiction, let be an open cover of X with no finite subcover. Consider . Observe that D is a directed set under inclusion and for each , there exists an such that for all . Consider the net . This net cannot have a convergent subnet, because for each there exists such that U_{c} is a neighbourhood of x; however, for all , we have that . This is a contradiction and completes the proof.
 A net in the product space has a limit if and only if each projection has a limit. Symbolically, if (x_{α}) is a net in the product X = π_{i}X_{i}, then it converges to x if and only if for each i. Armed with this observation and the above characterization of compactness in terms on nets, one can give a slick proof of Tychonoff's theorem.
 If ƒ:X→ Y and (x_{α}) is an ultranet on X, then (ƒ(x_{α})) is an ultranet on Y.
Related ideas
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.
The theory of filters also provides a definition of convergence in general topological spaces.
Limit superior
Limit superior and limit inferior of a net of real numbers can be defined in a similar manner as for sequences.^{[4]}^{[5]}^{[6]} Some authors work even with more general structures, like complete lattices.^{[7]}
For a net we put
Limit superior of a net of real numbers has many properties analogous to the case of sequences, e.g.
where equality holds whenever one of the nets is convergent.
References
 ^ Moore, E. H.; Smith, H. L. (1922). "A General Theory of Limits". American Journal of Mathematics 44 (2): 102–121. doi:10.2307/2370388. JSTOR 2370388
 ^ (Sundström 2010, p. 16n)
 ^ Megginson, p.143
 ^ AliprantisBorder, p.32
 ^ Megginson, p.217, p.221, Exercises 2.532.55
 ^ Beer, p.2
 ^ Schechter, Sections 7.437.47
 Kelley, John L. (1991). General Topology. Springer. ISBN 3540901256.
 Wilard, Stephen (2004). General Topology. Dover Publications. ISBN 0486434796.
 Sundström, Manya Raman (2010). "A pedagogical history of compactness". arXiv:1006.4131v1 [math.HO].
 Aliprantis, Charalambos D.; Border, Kim C. (2006). Infinite dimensional analysis: A hitchhiker's guide (Third ed.). Berlin: Springer. pp. xxii+703 pp.. ISBN 9783540326960, 3540326960. MR2378491.
 Beer, Gerald (1993). Topologies on closed and closed convex sets. Mathematics and its Applications 268. Dordrecht: Kluwer Academic Publishers Group. pp. xii+340. ISBN 0792325311. MR1269778.
 Megginson, Robert E. (1998). An Introduction to Banach Space Theory. Graduate Texts in Mathematics. 193. New York: Springer. ISBN 0387984313.
 Schechter, Eric (1997). Handbook of Analysis and its Foundations. San Diego: Academic Press. ISBN 126227608.
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