In
mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, a filter on a set
is a
family
Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
of
subset
In mathematics, set ''A'' is a subset of a set ''B'' if all elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are unequal, then ''A'' is a proper subset of ...
s such that:
#
and
# if
and
, then
# If
, and
, then
A filter on a set may be thought of as representing a "collection of large subsets". Filters appear in
order
Order, ORDER or Orders may refer to:
* Categorization, the process in which ideas and objects are recognized, differentiated, and understood
* Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
,
model theory
In mathematical logic, model theory is the study of the relationship between theory (mathematical logic), formal theories (a collection of Sentence (mathematical logic), sentences in a formal language expressing statements about a Structure (math ...
,
set theory
Set theory is the branch of mathematical logic that studies sets, which can be informally described as collections of objects. Although objects of any kind can be collected into a set, set theory, as a branch of mathematics, is mostly concern ...
, but can also be found in
topology
In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing ...
, from which they originate. The dual notion of a filter is an
ideal
Ideal may refer to:
Philosophy
* Ideal (ethics), values that one actively pursues as goals
* Platonic ideal, a philosophical idea of trueness of form, associated with Plato
Mathematics
* Ideal (ring theory), special subsets of a ring considered ...
.
Filters were introduced by
Henri Cartan
Henri Paul Cartan (; 8 July 1904 – 13 August 2008) was a French mathematician who made substantial contributions to algebraic topology.
He was the son of the mathematician Élie Cartan, nephew of mathematician Anna Cartan, oldest brother of c ...
in
1937
Events
January
* January 1 – Anastasio Somoza García becomes President of Nicaragua.
* January 5 – Water levels begin to rise in the Ohio River in the United States, leading to the Ohio River flood of 1937, which continues into ...
and as described in the article dedicated to
filters in topology, they were subsequently used by
Nicolas Bourbaki
Nicolas Bourbaki () is the collective pseudonym of a group of mathematicians, predominantly French alumni of the École normale supérieure - PSL (ENS). Founded in 1934–1935, the Bourbaki group originally intended to prepare a new textbook i ...
in their book ''
Topologie Générale
Nicolas Bourbaki () is the collective pseudonym of a group of mathematicians, predominantly French alumni of the École normale supérieure - PSL (ENS). Founded in 1934–1935, the Bourbaki group originally intended to prepare a new textbook in ...
'' as an alternative to the related notion of a
net developed in
1922
Events
January
* January 7 – Dáil Éireann (Irish Republic), Dáil Éireann, the parliament of the Irish Republic, ratifies the Anglo-Irish Treaty by 64–57 votes.
* January 10 – Arthur Griffith is elected President of Dáil Éirean ...
by
E. H. Moore and
Herman L. Smith.
Order filters are generalizations of filters from sets to arbitrary
partially ordered set
In mathematics, especially order theory, a partially ordered set (also poset) formalizes and generalizes the intuitive concept of an ordering, sequencing, or arrangement of the elements of a set. A poset consists of a set together with a binary ...
s. Specifically, a filter on a set is just a proper order filter in the special case where the partially ordered set consists of the
power set
In mathematics, the power set (or powerset) of a set is the set of all subsets of , including the empty set and itself. In axiomatic set theory (as developed, for example, in the ZFC axioms), the existence of the power set of any set is post ...
ordered by
set inclusion
In mathematics, set ''A'' is a subset of a set ''B'' if all elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are unequal, then ''A'' is a proper subset of ...
.
Preliminaries, notation, and basic notions
In this article, upper case Roman letters like
denote sets (but not families unless indicated otherwise) and
will denote the
power set
In mathematics, the power set (or powerset) of a set is the set of all subsets of , including the empty set and itself. In axiomatic set theory (as developed, for example, in the ZFC axioms), the existence of the power set of any set is post ...
of
A subset of a power set is called (or simply, ) where it is if it is a subset of
Families of sets will be denoted by upper case calligraphy letters such as
Whenever these assumptions are needed, then it should be assumed that
is non–empty and that
etc. are families of sets over
The terms "prefilter" and "filter base" are synonyms and will be used interchangeably.
Warning about competing definitions and notation
There are unfortunately several terms in the theory of filters that are defined differently by different authors.
These include some of the most important terms such as "filter."
While different definitions of the same term usually have significant overlap, due to the very technical nature of filters (and point–set topology), these differences in definitions nevertheless often have important consequences.
When reading mathematical literature, it is recommended that readers check how the terminology related to filters is defined by the author.
For this reason, this article will clearly state all definitions as they are used.
Unfortunately, not all notation related to filters is well established and some notation varies greatly across the literature (for example, the notation for the set of all prefilters on a set) so in such cases this article uses whatever notation is most self describing or easily remembered.
The theory of filters and prefilters is well developed and has a plethora of definitions and notations, many of which are now unceremoniously listed to prevent this article from becoming prolix and to allow for the easy look up of notation and definitions.
Their important properties are described later.
Sets operations
The or in
of a
family of sets
In set theory and related branches of mathematics, a collection F of subsets of a given set S is called a family of subsets of S, or a family of sets over S. More generally, a collection of any sets whatsoever is called a family of sets, set fami ...
is
and similarly the of
is
Throughout,
is a map and
is a set.
Nets and their tails
A is a set
together with a
preorder
In mathematics, especially in order theory, a preorder or quasiorder is a binary relation that is reflexive and transitive. Preorders are more general than equivalence relations and (non-strict) partial orders, both of which are special cas ...
, which will be denoted by
(unless explicitly indicated otherwise), that makes
into an () ; this means that for all
there exists some
such that
For any indices
the notation
is defined to mean
while
is defined to mean that
holds but it is true that
(if
is
antisymmetric then this is equivalent to
).
A is a map from a non–empty directed set into
The notation
will be used to denote a net with domain
Warning about using strict comparison
If
is a net and
then it is possible for the set
which is called , to be empty (for example, this happens if
is an
upper bound
In mathematics, particularly in order theory, an upper bound or majorant of a subset of some preordered set is an element of that is greater than or equal to every element of .
Dually, a lower bound or minorant of is defined to be an eleme ...
of the
directed set
In mathematics, a directed set (or a directed preorder or a filtered set) is a nonempty set A together with a reflexive and transitive binary relation \,\leq\, (that is, a preorder), with the additional property that every pair of elements has ...
).
In this case, the family
would contain the empty set, which would prevent it from being a prefilter (defined later).
This is the (important) reason for defining
as
rather than
or even
and it is for this reason that in general, when dealing with the prefilter of tails of a net, the strict inequality
may not be used interchangeably with the inequality
Filters and prefilters
The following is a list of properties that a family
of sets may possess and they form the defining properties of filters, prefilters, and filter subbases. Whenever it is necessary, it should be assumed that
Many of the properties of
defined above and below, such as "proper" and "directed downward," do not depend on
so mentioning the set
is optional when using such terms. Definitions involving being "upward closed in
" such as that of "filter on
" do depend on
so the set
should be mentioned if it is not clear from context.
\text X containing
called the , and
is said to this filter. This filter is equal to the intersection of all filters on
that are supersets of
The –system generated by
denoted by
will be a prefilter and a subset of
Moreover, the filter generated by
is equal to the upward closure of
meaning
However,
if
is a prefilter (although
is always an upward closed filter base for
).
* A
–smallest (meaning smallest relative to
) filter containing a filter subbase
will exist only under certain circumstances. It exists, for example, if the filter subbase
happens to also be a prefilter. It also exists if the filter (or equivalently, the –system) generated by
is
principal, in which case
is the unique smallest prefilter containing
Otherwise, in general, a
–smallest filter containing
might not exist. For this reason, some authors may refer to the –system generated by
as However, if a
–smallest prefilter does exist (say it is denoted by
) then contrary to usual expectations, it is necessarily equal to "
the prefilter generated by " (that is,
is possible). And if the filter subbase
happens to also be a prefilter but not a -system then unfortunately, "
the prefilter generated by this prefilter" (meaning
) will not be
(that is,
is possible even when
is a prefilter), which is why this article will prefer the accurate and unambiguous terminology of "the
–system generated by
".
of a filter and that is a of if is a filter and where for filters,
* Importantly, the expression "is a filter of" is for filters the analog of "is a sequence of". So despite having the prefix "sub" in common, "is a filter of" is actually the of "is a sequence of." However, can also be written which is described by saying " is subordinate to " With this terminology, "is ordinate to" becomes for filters (and also for prefilters) the analog of "is a sequence of," which makes this one situation where using the term "subordinate" and symbol may be helpful.
There are no prefilters on
(nor are there any nets valued in
), which is why this article, like most authors, will automatically assume without comment that
whenever this assumption is needed.
Basic examples
Named examples
- The singleton set is called the or It is the unique filter on because it is a subset of every filter on ; however, it need not be a subset of every prefilter on
- The dual ideal is also called (despite not actually being a filter). It is the only dual ideal on that is not a filter on
- If is a topological space and then the
neighborhood filter In topology and related areas of mathematics, the neighbourhood system, complete system of neighbourhoods, or neighbourhood filter \mathcal(x) for a point x in a topological space is the collection of all neighbourhoods of x.
Definitions
Neighbou ...
at is a filter on By definition, a family is called a (resp. a ) at if and only if is a prefilter (resp. is a filter subbase) and the filter on that generates is equal to the neighborhood filter The subfamily of open neighborhoods is a filter base for Both prefilters also form a bases for topologies on with the topology generated being coarser than This example immediately generalizes from neighborhoods of points to neighborhoods of non–empty subsets
- is an
if for some sequence
- is an or a on if is a filter on generated by some elementary prefilter. The filter of tails generated by a sequence that is not eventually constant is necessarily an ultrafilter. Every principal filter on a countable set is sequential as is every cofinite filter on a countably infinite set. The intersection of finitely many sequential filters is again sequential.
- The set of all cofinite subsets of (meaning those sets whose complement in is finite) is proper if and only if is infinite (or equivalently, is infinite), in which case is a filter on known as the or the on If is finite then is equal to the dual ideal which is not a filter. If is infinite then the family of complements of singleton sets is a filter subbase that generates the Fréchet filter on As with any family of sets over that contains the kernel of the Fréchet filter on is the empty set:
- The intersection of all elements in any non–empty family is itself a filter on called the or of which is why it may be denoted by Said differently, Because every filter on has as a subset, this intersection is never empty. By definition, the infimum is the finest/largest (relative to ) filter contained as a subset of each member of
* If are filters then their infimum in is the filter If are prefilters then is a prefilter that is coarser (with respect to ) than both (that is, ); indeed, it is one of the finest such prefilters, meaning that if is a prefilter such that then necessarily More generally, if are non−empty families and if then and is a
greatest element
In mathematics, especially in order theory, the greatest element of a subset S of a partially ordered set (poset) is an element of S that is greater than every other element of S. The term least element is defined dually, that is, it is an elem ...
(with respect to ) of
- Let and let
The or of denoted by is the smallest (relative to ) dual ideal on containing every element of as a subset; that is, it is the smallest (relative to ) dual ideal on containing as a subset.
This dual ideal is where is the –system generated by
As with any non–empty family of sets, is contained in filter on if and only if it is a filter subbase, or equivalently, if and only if is a filter on in which case this family is the smallest (relative to ) filter on containing every element of as a subset and necessarily
- Let and let
The or of denoted by if it exists, is by definition the smallest (relative to ) filter on containing every element of as a subset.
If it exists then necessarily (as defined above) and will also be equal to the intersection of all filters on containing
This supremum of exists if and only if the dual ideal is a filter on
The least upper bound of a family of filters may fail to be a filter. Indeed, if contains at least 2 distinct elements then there exist filters for which there does exist a filter that contains both
If is not a filter subbase then the supremum of does not exist and the same is true of its supremum in but their supremum in the set of all dual ideals on will exist (it being the degenerate filter ).
* If are prefilters (resp. filters on ) then is a prefilter (resp. a filter) if and only if it is non–degenerate (or said differently, if and only if mesh), in which case it is coarsest prefilters (resp. coarsest filter) on (with respect to ) that is finer (with respect to ) than both this means that if is any prefilter (resp. any filter) such that then necessarily in which case it is denoted by
- Let be non−empty sets and for every let be a dual ideal on If is any dual ideal on then is a dual ideal on called or .
- The
club filter In mathematics, particularly in set theory, if \kappa is a regular uncountable cardinal then \operatorname(\kappa), the filter of all sets containing a club subset of \kappa, is a \kappa-complete filter closed under diagonal intersection call ...
of a regular uncountable
In mathematics, an uncountable set (or uncountably infinite set) is an infinite set that contains too many elements to be countable. The uncountability of a set is closely related to its cardinal number: a set is uncountable if its cardinal num ...
cardinal
Cardinal or The Cardinal may refer to:
Animals
* Cardinal (bird) or Cardinalidae, a family of North and South American birds
**'' Cardinalis'', genus of cardinal in the family Cardinalidae
**'' Cardinalis cardinalis'', or northern cardinal, t ...
is the filter of all sets containing a club subset of It is a -complete filter closed under diagonal intersection.
Other examples
- Let and let which makes a prefilter and a filter subbase that is not closed under finite intersections. Because is a prefilter, the smallest prefilter containing is The –system generated by is In particular, the smallest prefilter containing the filter subbase is equal to the set of all finite intersections of sets in The filter on generated by is All three of the –system generates, and are examples of fixed, principal, ultra prefilters that are principal at the point is also an ultrafilter on
- Let be a topological space, and define where is necessarily finer than If is non–empty (resp. non–degenerate, a filter subbase, a prefilter, closed under finite unions) then the same is true of If is a filter on then is a prefilter but not necessarily a filter on although is a filter on equivalent to
- The set of all dense open subsets of a (non–empty) topological space is a proper –system and so also a prefilter. If the space is a
Baire space
In mathematics, a topological space X is said to be a Baire space if countable unions of closed sets with empty interior also have empty interior.
According to the Baire category theorem, compact Hausdorff spaces and complete metric spaces are e ...
, then the set of all countable intersections of dense open subsets is a –system and a prefilter that is finer than If (with ) then the set of all such that has finite Lebesgue measure
In measure theory, a branch of mathematics, the Lebesgue measure, named after French mathematician Henri Lebesgue, is the standard way of assigning a measure to subsets of ''n''-dimensional Euclidean space. For ''n'' = 1, 2, or 3, it coincides wi ...
is a proper –system and free prefilter that is also a proper subset
In mathematics, set ''A'' is a subset of a set ''B'' if all elements of ''A'' are also elements of ''B''; ''B'' is then a superset of ''A''. It is possible for ''A'' and ''B'' to be equal; if they are unequal, then ''A'' is a proper subset of ...
of The prefilters and are equivalent and so generate the same filter on
The prefilter is properly contained in, and not equivalent to, the prefilter consisting of all dense subsets of Since is a Baire space
In mathematics, a topological space X is said to be a Baire space if countable unions of closed sets with empty interior also have empty interior.
According to the Baire category theorem, compact Hausdorff spaces and complete metric spaces are e ...
, every countable intersection of sets in is dense in (and also comeagre and non–meager) so the set of all countable intersections of elements of is a prefilter and –system; it is also finer than, and not equivalent to,
- ''A filter subbase with no smallest prefilter containing it'': In general, if a filter subbase is not a –system then an intersection of sets from will usually require a description involving variables that cannot be reduced down to only two (consider, for instance when ). This example illustrates an atypical class of a filter subbases where all sets in both and its generated –system can be described as sets of the form so that in particular, no more than two variables (specifically, ) are needed to describe the generated –system.
For all let
where always holds so no generality is lost by adding the assumption
For all real if is non-negative then
[More generally, for any real numbers satisfying where ]
For every set of positive reals, let[If This property and the fact that is nonempty and proper if and only if actually allows for the construction of even more examples of prefilters, because if is any prefilter (resp. filter subbase, –system) then so is ]
Let and suppose is not a singleton set. Then is a filter subbase but not a prefilter and is the –system it generates, so that is the unique smallest filter in containing However, is a filter on (nor is it a prefilter because it is not directed downward, although it is a filter subbase) and is a proper subset of the filter
If are non−empty intervals then the filter subbases generate the same filter on if and only if
If is a prefilter satisfying [It may be shown that if is any family such that then is a prefilter if and only if for all real there exist real such that ] then for any the family is also a prefilter satisfying This shows that there cannot exist a minimal/ least (with respect to ) prefilter that both contains and is a subset of the –system generated by This remains true even if the requirement that the prefilter be a subset of is removed; that is, (in sharp contrast to filters) there does exist a minimal/least (with respect to ) filter containing the filter subbase
Ultrafilters
There are many other characterizations of "ultrafilter" and "ultra prefilter," which are listed in the article on
ultrafilter
In the mathematical field of order theory, an ultrafilter on a given partially ordered set (or "poset") P is a certain subset of P, namely a maximal filter on P; that is, a proper filter on P that cannot be enlarged to a bigger proper filter o ...
s. Important properties of ultrafilters are also described in that article.
B there exists some set
such that
* This characterization of "
is ultra" does not depend on the set
so mentioning the set
is optional when using the term "ultra."
For set (not necessarily even a subset of ) there exists some set such that
* If satisfies this condition then so does superset For example, if is any singleton set
In mathematics, a singleton, also known as a unit set or one-point set, is a set with exactly one element. For example, the set \ is a singleton whose single element is 0.
Properties
Within the framework of Zermelo–Fraenkel set theory, the ...
then is ultra and consequently, any non–degenerate superset of (such as its upward closure) is also ultra.
if it is a prefilter that is also ultra. Equivalently, it is a filter subbase that is ultra. A prefilter is ultra if and only if it satisfies any of the following equivalent conditions:
- is in with respect to which means that
-
* Although this statement is identical to that given below for ultrafilters, here is merely assumed to be a prefilter; it need not be a filter.
- is ultra (and thus an ultrafilter).
- is equivalent (with respect to ) to some ultrafilter.
* A filter subbase that is ultra is necessarily a prefilter. A filter subbase is ultra if and only if it is a maximal filter subbase with respect to (as above).
if it is a filter on that is ultra. Equivalently, an ultrafilter on is a filter that satisfies any of the following equivalent conditions:
- is generated by an ultra prefilter.
- For any
- This condition can be restated as: is partitioned by and its dual
* The sets are disjoint whenever is a prefilter.
- is an ideal.
- For any if then
- For any if then (a filter with this property is called a ).
* This property extends to any finite union of two or more sets.
- For any if then
- is a filter on ; meaning that if is a filter on such that then necessarily (this equality may be replaced by ).
* If is upward closed then So this characterization of ultrafilters as maximal filters can be restated as:
* Because subordination is for filters the analog of "is a subnet/subsequence of" (specifically, "subnet" should mean " AA–subnet," which is defined below), this characterization of an ultrafilter as being a "maximally subordinate filter" suggests that an ultrafilter can be interpreted as being analogous to some sort of "maximally deep net" (which could, for instance, mean that "when viewed only from " in some sense, it is indistinguishable from its subnets, as is the case with any net valued in a singleton set for example),
[For instance, one sense in which a net could be interpreted as being "maximally deep" is if all important properties related to (such as convergence for example) of any subnet is completely determined by in all topologies on In this case and its subnet become effectively indistinguishable (at least topologically) if one's information about them is limited to only that which can be described in solely in terms of and directly related sets (such as its subsets).] which is an idea that is actually made rigorous by ultranets. The ultrafilter lemma
In the mathematical field of set theory, an ultrafilter is a ''maximal proper filter'': it is a filter U on a given non-empty set X which is a certain type of non-empty family of subsets of X, that is not equal to the power set \wp(X) of X (suc ...
is then the statement that every filter ("net") has some subordinate filter ("subnet") that is "maximally subordinate" ("maximally deep").
Any non–degenerate family that has a singleton set as an element is ultra, in which case it will then be an ultra prefilter if and only if it also has the finite intersection property.
The trivial filter
is ultra if and only if
is a singleton set.
The ultrafilter lemma
The following important theorem is due to
Alfred Tarski
Alfred Tarski (, born Alfred Teitelbaum;School of Mathematics and Statistics, University of St Andrews ''School of Mathematics and Statistics, University of St Andrews''. January 14, 1901 – October 26, 1983) was a Polish-American logician a ...
(1930).
A consequence of the ultrafilter lemma is that every filter is equal to the intersection of all ultrafilters containing it.
[Let be a filter on that is not an ultrafilter. If is such that has the finite intersection property (because if ) so that by the ultrafilter lemma, there exists some ultrafilter such that (so in particular, ). Intersecting all such proves that ]
Assuming the axioms of
Zermelo–Fraenkel (ZF), the ultrafilter lemma follows from the
Axiom of choice
In mathematics, the axiom of choice, or AC, is an axiom of set theory equivalent to the statement that ''a Cartesian product of a collection of non-empty sets is non-empty''. Informally put, the axiom of choice says that given any collection ...
(in particular from
Zorn's lemma) but is strictly weaker than it. The ultrafilter lemma implies the Axiom of choice for finite sets. If dealing with
Hausdorff spaces, then most basic results (as encountered in introductory courses) in Topology (such as
Tychonoff's theorem for compact Hausdorff spaces and the
Alexander subbase theorem
In topology, a subbase (or subbasis, prebase, prebasis) for a topological space X with topology T is a subcollection B of T that generates T, in the sense that T is the smallest topology containing B. A slightly different definition is used by so ...
) and in
functional analysis
Functional analysis is a branch of mathematical analysis, the core of which is formed by the study of vector spaces endowed with some kind of limit-related structure (e.g. inner product, norm, topology, etc.) and the linear functions defi ...
(such as the
Hahn–Banach theorem
The Hahn–Banach theorem is a central tool in functional analysis.
It allows the extension of bounded linear functionals defined on a subspace of some vector space to the whole space, and it also shows that there are "enough" continuous linear f ...
) can be proven using only the ultrafilter lemma; the full strength of the axiom of choice might not be needed.
Kernels
The kernel is useful in classifying properties of prefilters and other families of sets.
B
If
then for any point
Properties of kernels
If
then
and this set is also equal to the kernel of the –system that is generated by
In particular, if
is a filter subbase then the kernels of all of the following sets are equal:
:(1)
(2) the –system generated by
and (3) the filter generated by
If
is a map then
and
If
then
while if
and
are equivalent then
Equivalent families have equal kernels. Two principal families are equivalent if and only if their kernels are equal; that is, if
and
are principal then they are equivalent if and only if
Classifying families by their kernels
If
is a principal filter on
then
and
where
is also the smallest prefilter that generates
Family of examples: For any non–empty
the family
is free but it is a filter subbase if and only if no finite union of the form
covers
in which case the filter that it generates will also be free. In particular,
is a filter subbase if
is countable (for example,
the primes), a
meager set
In the mathematical field of general topology, a meagre set (also called a meager set or a set of first category) is a subset of a topological space that is small or negligible in a precise sense detailed below. A set that is not meagre is calle ...
in
a set of finite measure, or a bounded subset of
If
is a singleton set then
is a subbase for the Fréchet filter on
For every filter
there exists a unique pair of dual ideals
such that
is free,
is principal, and
and
do not mesh (that is,
). The dual ideal
is called of
while
is called where at least one of these dual ideals is filter. If
is principal then
otherwise,
and
is a free (non–degenerate) filter.
Finite prefilters and finite sets
If a filter subbase
is finite then it is fixed (that is, not free);
this is because
is a finite intersection and the filter subbase
has the finite intersection property.
A finite prefilter is necessarily principal, although it does not have to be closed under finite intersections.
If
is finite then all of the conclusions above hold for any
In particular, on a finite set
there are no free filter subbases (and so no free prefilters), all prefilters are principal, and all filters on
are principal filters generated by their (non–empty) kernels.
The trivial filter
is always a finite filter on
and if
is infinite then it is the only finite filter because a non–trivial finite filter on a set
is possible if and only if
is finite.
However, on any infinite set there are non–trivial filter subbases and prefilters that are finite (although they cannot be filters).
If
is a singleton set then the trivial filter
is the only proper subset of
and moreover, this set
is a principal ultra prefilter and any superset
(where
) with the finite intersection property will also be a principal ultra prefilter (even if
is infinite).
Characterizing fixed ultra prefilters
If a family of sets
is fixed (that is,
) then
is ultra if and only if some element of
is a singleton set, in which case
will necessarily be a prefilter. Every principal prefilter is fixed, so a principal prefilter
is ultra if and only if
is a singleton set.
Every filter on
that is principal at a single point is an ultrafilter, and if in addition
is finite, then there are no ultrafilters on
other than these.
The next theorem shows that every ultrafilter falls into one of two categories: either it is free or else it is a principal filter generated by a single point.
Finer/coarser, subordination, and meshing
The preorder
that is defined below is of fundamental importance for the use of prefilters (and filters) in topology. For instance, this preorder is used to define the prefilter equivalent of "subsequence", where "
" can be interpreted as "
is a subsequence of
" (so "subordinate to" is the prefilter equivalent of "subsequence of"). It is also used to define prefilter convergence in a topological space.
The definition of
meshes with
which is closely related to the preorder
is used in Topology to define
cluster point
In mathematics, a limit point, accumulation point, or cluster point of a set S in a topological space X is a point x that can be "approximated" by points of S in the sense that every neighbourhood of x with respect to the topology on X also contai ...
s.
Two families of sets
and are , indicated by writing
if
If
do not mesh then they are . If
then
are said to if
mesh, or equivalently, if the of
which is the family
does not contain the empty set, where the trace is also called the of
''Example'': If
is a
subsequence
In mathematics, a subsequence of a given sequence is a sequence that can be derived from the given sequence by deleting some or no elements without changing the order of the remaining elements. For example, the sequence \langle A,B,D \rangle is a ...
of
then
is subordinate to
in symbols:
and also
Stated in plain English, the prefilter of tails of a subsequence is always subordinate to that of the original sequence.
To see this, let
be arbitrary (or equivalently, let
be arbitrary) and it remains to show that this set contains some
For the set
to contain
it is sufficient to have
Since
are strictly increasing integers, there exists
such that
and so
holds, as desired.
Consequently,
The left hand side will be a subset of the right hand side if (for instance) every point of
is unique (that is, when
is injective) and
is the even-indexed subsequence
because under these conditions, every tail
(for every
) of the subsequence will belong to the right hand side filter but not to the left hand side filter.
For another example, if
is any family then
always holds and furthermore,
Assume that
are families of sets that satisfy
Then
and
and also
If in addition to
is a filter base and
then
is a filter subbase and also
mesh.
[To prove that mesh, let Because (resp. because ), there exists some where by assumption so If is a filter subbase and if then taking implies that If then there are such that and now This shows that is a filter subbase. ]
More generally, if both
and if the intersection of any two elements of
is non–empty, then
mesh.
Every filter subbase is coarser than both the –system that it generates and the filter that it generates.
If
are families such that
the family
is ultra, and
then
is necessarily ultra. It follows that any family that is equivalent to an ultra family will necessarily ultra. In particular, if
is a prefilter then either both
and the filter
it generates are ultra or neither one is ultra.
If a filter subbase is ultra then it is necessarily a prefilter, in which case the filter that it generates will also be ultra. A filter subbase
that is not a prefilter cannot be ultra; but it is nevertheless still possible for the prefilter and filter generated by
to be ultra. If
is upward closed in
then
Relational properties of subordination
The relation
is
reflexive and
transitive, which makes it into a
preorder
In mathematics, especially in order theory, a preorder or quasiorder is a binary relation that is reflexive and transitive. Preorders are more general than equivalence relations and (non-strict) partial orders, both of which are special cas ...
on
The relation
is
antisymmetric but if
has more than one point then it is
symmetric.
:
For any
So the set
has more than one point if and only if the relation
is
symmetric.
:
If
but while the converse does not hold in general, it does hold if
is upward closed (such as if
is a filter).
Two filters are equivalent if and only if they are equal, which makes the restriction of
to
antisymmetric.
But in general,
is
antisymmetric on
nor on
; that is,
does necessarily imply
; not even if both
are prefilters. For instance, if
is a prefilter but not a filter then
Equivalent families of sets
The preorder
induces its canonical
equivalence relation
In mathematics, an equivalence relation is a binary relation that is reflexive, symmetric and transitive. The equipollence relation between line segments in geometry is a common example of an equivalence relation.
Each equivalence relatio ...
on
where for all
is to
if any of the following equivalent conditions hold:
- The upward closures of are equal.
Two upward closed (in
) subsets of
are equivalent if and only if they are equal.
If
then necessarily
and
is equivalent to
Every
equivalence class
In mathematics, when the elements of some set S have a notion of equivalence (formalized as an equivalence relation), then one may naturally split the set S into equivalence classes. These equivalence classes are constructed so that elements a ...
other than
contains a unique representative (that is, element of the equivalence class) that is upward closed in
Properties preserved between equivalent families
Let
be arbitrary and let
be any family of sets. If
are equivalent (which implies that
) then for each of the statements/properties listed below, either it is true of
or else it is false of
:
- Not empty
- Proper (that is, is not an element)
* Moreover, any two degenerate families are necessarily equivalent.
- Filter subbase
- Prefilter
* In which case generate the same filter on (that is, their upward closures in are equal).
- Free
- Principal
- Ultra
- Is equal to the trivial filter
* In words, this means that the only subset of that is equivalent to the trivial filter the trivial filter. In general, this conclusion of equality does not extend to non−trivial filters (one exception is when both families are filters).
- Meshes with
- Is finer than
- Is coarser than
- Is equivalent to
Missing from the above list is the word "filter" because this property is preserved by equivalence.
However, if
are filters on
then they are equivalent if and only if they are equal; this characterization does extend to prefilters.
Equivalence of prefilters and filter subbases
If
is a prefilter on
then the following families are always equivalent to each other:
- ;
- the –system generated by ;
- the filter on generated by ;
and moreover, these three families all generate the same filter on
(that is, the upward closures in
of these families are equal).
In particular, every prefilter is equivalent to the filter that it generates.
By transitivity, two prefilters are equivalent if and only if they generate the same filter.
[This is because if are prefilters on then ]
Every prefilter is equivalent to exactly one filter on
which is the filter that it generates (that is, the prefilter's upward closure).
Said differently, every equivalence class of prefilters contains exactly one representative that is a filter.
In this way, filters can be considered as just being distinguished elements of these equivalence classes of prefilters.
A filter subbase that is also a prefilter can be equivalent to the prefilter (or filter) that it generates.
In contrast, every prefilter is equivalent to the filter that it generates.
This is why prefilters can, by and large, be used interchangeably with the filters that they generate while filter subbases cannot.
Every filter is both a
–system and a
ring of sets
In mathematics, there are two different notions of a ring of sets, both referring to certain Family of sets, families of sets.
In order theory, a nonempty family of sets \mathcal is called a ring (of sets) if it is closure (mathematics), closed u ...
.
Examples of determining equivalence/non–equivalence
Examples: Let
and let
be the set
of integers (or the set
). Define the sets
All three sets are filter subbases but none are filters on
and only
is prefilter (in fact,
is even free and closed under finite intersections). The set
is fixed while
is free (unless
). They satisfy
but no two of these families are equivalent; moreover, no two of the filters generated by these three filter subbases are equivalent/equal. This conclusion can be reached by showing that the –systems that they generate are not equivalent. Unlike with
every set in the –system generated by
contains
as a subset,
[The –system generated by (resp. by ) is a prefilter whose elements are finite unions of open (resp. closed) intervals having endpoints in with two of these intervals being of the forms (resp. ) where ; in the case of it is possible for one or more of these closed intervals to be singleton sets (that is, degenerate closed intervals).] which is what prevents their generated –systems (and hence their generated filters) from being equivalent. If
was instead
then all three families would be free and although the sets
would remain equivalent to each other, their generated –systems would be equivalent and consequently, they would generate the same filter on
; however, this common filter would still be strictly coarser than the filter generated by
Set theoretic properties and constructions
Trace and meshing
If
is a prefilter (resp. filter) on
then the trace of
which is the family
is a prefilter (resp. a filter) if and only if
mesh (that is,
), in which case the trace of
is said to be .
If
is ultra and if
mesh then the trace
is ultra.
If
is an ultrafilter on
then the trace of
is a filter on
if and only if
For example, suppose that
is a filter on
is such that
Then
mesh and
generates a filter on
that is strictly finer than
When prefilters mesh
Given non–empty families
the family
satisfies
and
If
is proper (resp. a prefilter, a filter subbase) then this is also true of both
In order to make any meaningful deductions about
from
needs to be proper (that is,
which is the motivation for the definition of "mesh".
In this case,
is a prefilter (resp. filter subbase) if and only if this is true of both
Said differently, if
are prefilters then they mesh if and only if
is a prefilter.
Generalizing gives a well known characterization of "mesh" entirely in terms of subordination (that is,
):
Two prefilters (resp. filter subbases)
mesh if and only if there exists a prefilter (resp. filter subbase)
such that
and
If the least upper bound of two filters
exists in
then this least upper bound is equal to
Images and preimages under functions
Throughout,
will be maps between non–empty sets.
Images of prefilters
Let
Many of the properties that
may have are preserved under images of maps; notable exceptions include being upward closed, being closed under finite intersections, and being a filter, which are not necessarily preserved.
Explicitly, if one of the following properties is true of
then it will necessarily also be true of
(although possibly not on the codomain
unless
is surjective):
- Filter properties: ultra, ultrafilter, filter, prefilter, filter subbase, dual ideal, upward closed, proper/non–degenerate.
- Ideal properties: ideal, closed under finite unions, downward closed, directed upward.
Moreover, if
is a prefilter then so are both
The image under a map
of an ultra set
is again ultra and if
is an ultra prefilter then so is
If
is a filter then
is a filter on the range
but it is a filter on the codomain
if and only if
is surjective.
Otherwise it is just a prefilter on
and its upward closure must be taken in
to obtain a filter.
The upward closure of
is
where if
is upward closed in
(that is, a filter) then this simplifies to:
If
then taking
to be the inclusion map
shows that any prefilter (resp. ultra prefilter, filter subbase) on
is also a prefilter (resp. ultra prefilter, filter subbase) on
Preimages of prefilters
Let
Under the assumption that
is Surjective function, surjective:
is a prefilter (resp. filter subbase, –system, closed under finite unions, proper) if and only if this is true of
However, if
is an ultrafilter on
then even if
is surjective (which would make
a prefilter), it is nevertheless still possible for the prefilter
to be neither ultra nor a filter on
(see this
[For an example of how this failure can happen, consider the case where there exists some such that both and its complement in contains at least two distinct points.] footnote for an example).
If
is not surjective then denote the trace of
by
where in this case particular case the trace satisfies:
and consequently also:
This last equality and the fact that the trace
is a family of sets over
means that to draw conclusions about
the trace
can be used in place of
and the
can be used in place of
For example:
is a prefilter (resp. filter subbase, –system, proper) if and only if this is true of
In this way, the case where
is not (necessarily) surjective can be reduced down to the case of a surjective function (which is a case that was described at the start of this subsection).
Even if
is an ultrafilter on
if
is not surjective then it is nevertheless possible that
which would make
degenerate as well. The next characterization shows that degeneracy is the only obstacle. If
is a prefilter then the following are equivalent:
- is a prefilter;
- is a prefilter;
- ;
- meshes with
and moreover, if
is a prefilter then so is
If
and if
denotes the inclusion map then the trace of
is equal to
This observation allows the results in this subsection to be applied to investigating the trace on a set.
Bijections, injections, and surjections
All properties involving filters are preserved under bijections. This means that if
is a bijection, then
is a prefilter (resp. ultra, ultra prefilter, filter on
ultrafilter on
filter subbase, –system, ideal on
etc.) if and only if the same is true of
A map
is injective if and only if for all prefilters
is equivalent to
The image of an ultra family of sets under an injection is again ultra.
The map
is a
surjection
In mathematics, a surjective function (also known as surjection, or onto function) is a function that every element can be mapped from element so that . In other words, every element of the function's codomain is the image of one element of ...
if and only if whenever
is a prefilter on
then the same is true of
(this result does not require the ultrafilter lemma).
Subordination is preserved by images and preimages
The relation
is preserved under both images and preimages of families of sets.
This means that for families
Moreover, the following relations always hold for family of sets
:
where equality will hold if
is surjective.
Furthermore,
If
then
and
where equality will hold if
is injective.
Products of prefilters
Suppose
is a family of one or more non–empty sets, whose product will be denoted by
and for every index
let
denote the canonical projection.
Let
be non−empty families, also indexed by
such that
for each
The of the families
is defined identically to how the basic open subsets of the
product topology
In topology and related areas of mathematics, a product space is the Cartesian product of a family of topological spaces equipped with a natural topology called the product topology. This topology differs from another, perhaps more natural-seem ...
are defined (had all of these
been topologies). That is, both the notations
denote the family of all subsets
such that
for all but finitely many
and where
for any one of these finitely many exceptions (that is, for any
such that
necessarily
).
When every
is a filter subbase then the family
is a filter subbase for the filter on
generated by
If
is a filter subbase then the filter on
that it generates is called the .
If every
is a prefilter on
then
will be a prefilter on
and moreover, this prefilter is equal to the coarsest prefilter
such that
for every
However,
may fail to be a filter on
even if every
is a filter on
Set subtraction and some examples
Set subtracting away a subset of the kernel
If
is a prefilter on
then
is a prefilter, where this latter set is a filter if and only if
is a filter and
In particular, if
is a neighborhood basis at a point
in a topological space
having at least 2 points, then
is a prefilter on
This construction is used to define
in terms of prefilter convergence.
Using duality between ideals and dual ideals
There is a dual relation or which is defined to mean that every
some
Explicitly, this means that for every
, there is some
such that
This relation is dual to
in sense that
if and only if
The relation
is closely related to the downward closure of a family in a manner similar to how
is related to the upward closure family.
For an example that uses this duality, suppose
is a map and
Define
which contains the empty set if and only if
does. It is possible for
to be an ultrafilter and for
to be empty or not closed under finite intersections (see footnote for example).
[Suppose has more than one point, is a constant map, and then will consist of all non–empty subsets of ] Although
does not preserve properties of filters very well, if
is downward closed (resp. closed under finite unions, an ideal) then this will also be true for
Using the duality between ideals and dual ideals allows for a construction of the following filter.
Suppose
is a filter on
and let
be its dual in
If
then
's dual
will be a filter.
Other examples
Example: The set
of all dense open subsets of a topological space is a proper –system and a prefilter. If the space is a
Baire space
In mathematics, a topological space X is said to be a Baire space if countable unions of closed sets with empty interior also have empty interior.
According to the Baire category theorem, compact Hausdorff spaces and complete metric spaces are e ...
, then the set of all countable intersections of dense open subsets is a –system and a prefilter that is finer than
Example: The family
of all dense open sets of
having finite Lebesgue measure is a proper –system and a free prefilter. The prefilter
is properly contained in, and not equivalent to, the prefilter consisting of all dense open subsets of
Since
is a
Baire space
In mathematics, a topological space X is said to be a Baire space if countable unions of closed sets with empty interior also have empty interior.
According to the Baire category theorem, compact Hausdorff spaces and complete metric spaces are e ...
, every countable intersection of sets in
is dense in
(and also
comeagre and non–meager) so the set of all countable intersections of elements of
is a prefilter and –system; it is also finer than, and not equivalent to,
Filters and nets
This section will describe the relationships between prefilters and nets in great detail because of how important these details are applying
filters to topology − particularly in switching from utilizing nets to utilizing filters and vice verse − and because it to make it easier to understand later why subnets (with their most commonly used definitions) are not generally equivalent with "sub–prefilters".
Nets to prefilters
A
net is canonically associated with its prefilter of tails
If
is a map and
is a net in
then
Prefilters to nets
A is a pair
consisting of a non–empty set
and an element
For any family
let
Define a canonical
preorder
In mathematics, especially in order theory, a preorder or quasiorder is a binary relation that is reflexive and transitive. Preorders are more general than equivalence relations and (non-strict) partial orders, both of which are special cas ...
on pointed sets by declaring
If
even if
so this preorder is not
antisymmetric and given any family of sets
is
partially ordered
In mathematics, especially order theory, a partially ordered set (also poset) formalizes and generalizes the intuitive concept of an ordering, sequencing, or arrangement of the elements of a set. A poset consists of a set together with a binary r ...
if and only if
consists entirely of singleton sets.
If
is a
maximal element
In mathematics, especially in order theory, a maximal element of a subset ''S'' of some preordered set is an element of ''S'' that is not smaller than any other element in ''S''. A minimal element of a subset ''S'' of some preordered set is defin ...
of
; moreover, all maximal elements are of this form.
If
is a
greatest element
In mathematics, especially in order theory, the greatest element of a subset S of a partially ordered set (poset) is an element of S that is greater than every other element of S. The term least element is defined dually, that is, it is an elem ...
if and only if
in which case
is the set of all greatest elements. However, a greatest element
is a maximal element if and only if
so there is at most one element that is both maximal and greatest.
There is a canonical map
defined by
If
then the tail of the assignment
starting at
is
Although
is not, in general, a partially ordered set, it is a
directed set
In mathematics, a directed set (or a directed preorder or a filtered set) is a nonempty set A together with a reflexive and transitive binary relation \,\leq\, (that is, a preorder), with the additional property that every pair of elements has ...
if (and only if)
is a prefilter.
So the most immediate choice for the definition of "the net in
induced by a prefilter
" is the assignment
from
into
:\;&& (\operatorname(\mathcal), \leq) &&\,\to \;& X \\
&& (B, b) &&\,\mapsto\;& b \\
\end
that is,
If
is a prefilter on
is a net in
and the prefilter associated with
is
; that is:
[The set equality holds more generally: if the family of sets then the family of tails of the map (defined by ) is equal to ]
This would not necessarily be true had
been defined on a proper subset of
For example, suppose
has at least two distinct elements,
is the indiscrete filter, and
is arbitrary. Had
instead been defined on the singleton set
where the restriction of
to
will temporarily be denote by
then the prefilter of tails associated with
would be the principal prefilter
rather than the original filter
;
this means that the equality
is , so unlike
the prefilter
can be recovered from
Worse still, while
is the unique filter on
the prefilter
instead generates a filter (that is, an ultrafilter) on
However, if
is a net in
then it is in general true that
is equal to
because, for example, the domain of
may be of a completely different cardinality than that of
(since unlike the domain of
the domain of an arbitrary net in
could have cardinality).
Ultranets and ultra prefilters
A net
is called an or in
if for every subset
is
eventually in
or it is eventually in
;
this happens if and only if
is an ultra prefilter.
A prefilter
is an ultra prefilter if and only if
is an ultranet in
Partially ordered net
The domain of the canonical net
is in general not partially ordered. However, in 1955 Bruns and Schmidt discovered
[Bruns G., Schmidt J.,Zur Aquivalenz von Moore-Smith-Folgen und Filtern, Math. Nachr. 13 (1955), 169-186.] a construction that allows for the canonical net to have a domain that is both partially ordered and directed; this was independently rediscovered by
Albert Wilansky Albert "Tommy" Wilansky (13 September 1921, St Johns, Newfoundland – 3 July 2017, Bethlehem, Pennsylvania) was a Canadian-American mathematician, known for introducing Smith numbers.
Biography
Wilansky was educated as an undergraduate at Dalhous ...
in 1970.
It begins with the construction of a
strict partial order (meaning a transitive and
irreflexive relation)
on a subset of
that is similar to the
lexicographical order
In mathematics, the lexicographic or lexicographical order (also known as lexical order, or dictionary order) is a generalization of the alphabetical order of the dictionaries to sequences of ordered symbols or, more generally, of elements of ...
on
of the strict partial orders
For any
in
declare that
if and only if
or equivalently, if and only if
The
non−strict partial order associated with
denoted by
is defined by declaring that
Unwinding these definitions gives the following characterization:
which shows that
is just the
lexicographical order
In mathematics, the lexicographic or lexicographical order (also known as lexical order, or dictionary order) is a generalization of the alphabetical order of the dictionaries to sequences of ordered symbols or, more generally, of elements of ...
on
induced by
where
is partially ordered by equality
[Explicitly, the partial order on induced by equality refers to the diagonal which is a ]homogeneous relation
In mathematics, a homogeneous relation (also called endorelation) over a set ''X'' is a binary relation over ''X'' and itself, i.e. it is a subset of the Cartesian product . This is commonly phrased as "a relation on ''X''" or "a (binary) relation ...
on that makes into a partially ordered set
In mathematics, especially order theory, a partially ordered set (also poset) formalizes and generalizes the intuitive concept of an ordering, sequencing, or arrangement of the elements of a set. A poset consists of a set together with a binary ...
. If this partial order is denoted by the more familiar symbol (that is, define ) then for any which shows that (and thus also ) is nothing more than a new symbol for equality on that is, The notation is used because it avoids the unnecessary introduction of a new symbol for the diagonal.
Both
are
serial and neither possesses a
greatest element
In mathematics, especially in order theory, the greatest element of a subset S of a partially ordered set (poset) is an element of S that is greater than every other element of S. The term least element is defined dually, that is, it is an elem ...
or a
maximal element
In mathematics, especially in order theory, a maximal element of a subset ''S'' of some preordered set is an element of ''S'' that is not smaller than any other element in ''S''. A minimal element of a subset ''S'' of some preordered set is defin ...
; this remains true if they are each restricted to the subset of
defined by
where it will henceforth be assumed that they are.
Denote the assignment
from this subset by:
If
then just as with
before, the tail of the
starting at
is equal to
If
is a prefilter on
then
is a net in
whose domain
is a partially ordered set and moreover,
Because the tails of
are identical (since both are equal to the prefilter
), there is typically nothing lost by assuming that the domain of the net associated with a prefilter is both directed partially ordered. If the set
is replaced with the positive rational numbers then the strict partial order
will also be a
dense order In mathematics, a partial order or total order < on a is said to be dense if, for all .
Subordinate filters and subnets
The notion of "
is subordinate to
" (written
) is for filters and prefilters what "
is a
subsequence
In mathematics, a subsequence of a given sequence is a sequence that can be derived from the given sequence by deleting some or no elements without changing the order of the remaining elements. For example, the sequence \langle A,B,D \rangle is a ...
of
" is for sequences.
For example, if
denotes the set of tails of
and if
denotes the set of tails of the subsequence
(where
) then
(that is,
) is true but
is in general false.
Non–equivalence of subnets and subordinate filters
A subset
of a
preorder
In mathematics, especially in order theory, a preorder or quasiorder is a binary relation that is reflexive and transitive. Preorders are more general than equivalence relations and (non-strict) partial orders, both of which are special cas ...
ed space
is or in
if for every
there exists some
If
contains a tail of
then
is said to be or ; explicitly, this means that there exists some
(that is,
). An eventual set is necessarily not empty. A subset is eventual if and only if its complement is not frequent (which is termed ).
A map
between two preordered sets is if whenever
Subnets in the sense of Willard and
subnets in the sense of Kelley are the most commonly used definitions of "
subnet
A subnetwork or subnet is a logical subdivision of an IP network. Updated by RFC 6918. The practice of dividing a network into two or more networks is called subnetting.
Computers that belong to the same subnet are addressed with an identical ...
."
The first definition of a subnet was introduced by
John L. Kelley in 1955.
Stephen Willard
Stephen Willard (born 27 August 1958 in Swindon) is a former professional English darts player. Who played in Professional Darts Corporation events.
He won a PDC Tour Card in 2015, which was the year he also won the Saints Open defeating G ...
introduced his own variant of Kelley's definition of subnet in 1970.
AA–subnets were introduced independently by Smiley (1957), Aarnes and Andenaes (1972), and Murdeshwar (1983); AA–subnets were studied in great detail by Aarnes and Andenaes but they are not often used.
Kelley did not require the map
to be order preserving while the definition of an AA–subnet does away entirely with any map between the two nets' domains and instead focuses entirely on
− the nets' common codomain.
Every Willard–subnet is a Kelley–subnet and both are AA–subnets.
In particular, if
is a Willard–subnet or a Kelley–subnet of
then
- Example: Let and let be a constant sequence, say Let and so that is a net on Then is an AA-subnet of because But is not a Willard-subnet of because there does not exist any map whose image is a cofinal subset of Nor is a Kelley-subnet of because if is any map then is a cofinal subset of but is not eventually in
AA–subnets have a defining characterization that immediately shows that they are fully interchangeable with sub(ordinate)filters.
Explicitly, what is meant is that the following statement is true for AA–subnets:
If
are prefilters then
is an AA–subnet of
If "AA–subnet" is replaced by "Willard–subnet" or "Kelley–subnet" then the above statement becomes . In particular, the problem is that the following statement is in general false:
statement: If
are prefilters such that
is a Kelley–subnet of
Since every Willard–subnet is a Kelley–subnet, this statement remains false if the word "Kelley–subnet" is replaced with "Willard–subnet".
- : For all let Let which is a proper –system, and let where both families are prefilters on the
natural number
In mathematics, the natural numbers are those numbers used for counting (as in "there are ''six'' coins on the table") and ordering (as in "this is the ''third'' largest city in the country").
Numbers used for counting are called '' cardinal ...
s
Because is to as a subsequence is to a sequence.
So ideally, should be a subnet of
Let be the domain of so contains a cofinal subset that is order isomorphic to and consequently contains neither a maximal nor greatest element.
Let is both a maximal and greatest element of
The directed set also contains a subset that is order isomorphic to (because it contains which contains such a subset) but no such subset can be cofinal in because of the maximal element
Consequently, any order–preserving map must be eventually constant (with value ) where is then a greatest element of the range
Because of this, there can be no order preserving map that satisfies the conditions required for to be a Willard–subnet of (because the range of such a map cannot be cofinal in ).
Suppose for the sake of contradiction that there exists a map such that is eventually in for all
Because there exist such that
For every because is eventually in it is necessary that
In particular, if then which by definition is equivalent to which is false.
Consequently, is not a Kelley–subnet of
If "subnet" is defined to mean Willard–subnet or Kelley–subnet then nets and filters are not completely interchangeable because there exists a filter–sub(ordinate)filter relationships that cannot be expressed in terms of a net–subnet relationship between the two induced nets. In particular, the problem is that Kelley–subnets and Willard–subnets are fully interchangeable with subordinate filters. If the notion of "subnet" is not used or if "subnet" is defined to mean AA–subnet, then this ceases to be a problem and so it becomes correct to say that nets and filters are interchangeable. Despite the fact that AA–subnets do not have the problem that Willard and Kelley subnets have, they are not widely used or known about.
See also
*
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Notes
Proofs
Citations
References
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* (Provides an introductory review of filters in topology and in metric spaces.)
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{{Set theory
General topology
Order theory
Set theory