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In mathematics, a cofinite subset of a set X is a subset A whose
complement A complement is something that completes something else. Complement may refer specifically to: The arts * Complement (music), an interval that, when added to another, spans an octave ** Aggregate complementation, the separation of pitch-class ...
in X is a
finite set In mathematics, particularly set theory, a finite set is a set that has a finite number of elements. Informally, a finite set is a set which one could in principle count and finish counting. For example, :\ is a finite set with five elements. ...
. In other words, A contains all but finitely many elements of X. If the complement is not finite, but it is countable, then one says the set is cocountable. These arise naturally when generalizing structures on finite sets to infinite sets, particularly on infinite products, as in the product topology or
direct sum The direct sum is an operation between structures in abstract algebra, a branch of mathematics. It is defined differently, but analogously, for different kinds of structures. To see how the direct sum is used in abstract algebra, consider a more ...
. This use of the prefix "" to describe a property possessed by a set's mplement is consistent with its use in other terms such as " meagre set".


Boolean algebras

The set of all subsets of X that are either finite or cofinite forms a
Boolean algebra In mathematics and mathematical logic, Boolean algebra is a branch of algebra. It differs from elementary algebra in two ways. First, the values of the variables are the truth values ''true'' and ''false'', usually denoted 1 and 0, whereas i ...
, which means that it is closed under the operations of
union Union commonly refers to: * Trade union, an organization of workers * Union (set theory), in mathematics, a fundamental operation on sets Union may also refer to: Arts and entertainment Music * Union (band), an American rock group ** ''U ...
, intersection, and complementation. This Boolean algebra is the on X. A Boolean algebra A has a unique non-principal
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 on ...
(that is, a maximal filter not generated by a single element of the algebra) if and only if there exists an infinite set X such that A is isomorphic to the finite–cofinite algebra on X. In this case, the non-principal ultrafilter is the set of all cofinite sets.


Cofinite topology

The cofinite topology (sometimes called the finite complement topology) is a topology that can be defined on every set X. It has precisely the
empty set In mathematics, the empty set is the unique set having no elements; its size or cardinality (count of elements in a set) is zero. Some axiomatic set theories ensure that the empty set exists by including an axiom of empty set, while in other ...
and all cofinite subsets of X as open sets. As a consequence, in the cofinite topology, the only closed subsets are finite sets, or the whole of X. Symbolically, one writes the topology as \mathcal = \. This topology occurs naturally in the context of the Zariski topology. Since polynomials in one variable over a
field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a grass ...
K are zero on finite sets, or the whole of K, the Zariski topology on K (considered as ''affine line'') is the cofinite topology. The same is true for any '' irreducible''
algebraic curve In mathematics, an affine algebraic plane curve is the zero set of a polynomial in two variables. A projective algebraic plane curve is the zero set in a projective plane of a homogeneous polynomial in three variables. An affine algebraic plane ...
; it is not true, for example, for XY = 0 in the plane.


Properties

* Subspaces: Every subspace topology of the cofinite topology is also a cofinite topology. * Compactness: Since every
open set In mathematics, open sets are a generalization of open intervals in the real line. In a metric space (a set along with a distance defined between any two points), open sets are the sets that, with every point , contain all points that are ...
contains all but finitely many points of X, the space X is
compact Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to: * Interstate compact * Blood compact, an ancient ritual of the Philippines * Compact government, a type of colonial rule utilized in Britis ...
and sequentially compact. * Separation: The cofinite topology is the
coarsest topology In topology and related areas of mathematics, the set of all possible topologies on a given set forms a partially ordered set. This order relation can be used for comparison of the topologies. Definition A topology on a set may be defined as th ...
satisfying the T1 axiom; that is, it is the smallest topology for which every singleton set is closed. In fact, an arbitrary topology on X satisfies the T1 axiom if and only if it contains the cofinite topology. If X is finite then the cofinite topology is simply the
discrete topology In topology, a discrete space is a particularly simple example of a topological space or similar structure, one in which the points form a , meaning they are '' isolated'' from each other in a certain sense. The discrete topology is the finest to ...
. If X is not finite then this topology is not Hausdorff (T2), regular or normal because no two nonempty open sets are disjoint (that is, it is
hyperconnected In the mathematical field of topology, a hyperconnected space or irreducible space is a topological space ''X'' that cannot be written as the union of two proper closed sets (whether disjoint or non-disjoint). The name ''irreducible space'' is pre ...
).


Double-pointed cofinite topology

The double-pointed cofinite topology is the cofinite topology with every point doubled; that is, it is the
topological product 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-seemi ...
of the cofinite topology with the indiscrete topology on a two-element set. It is not T0 or T1, since the points of the doublet are
topologically indistinguishable In topology, two points of a topological space ''X'' are topologically indistinguishable if they have exactly the same neighborhoods. That is, if ''x'' and ''y'' are points in ''X'', and ''Nx'' is the set of all neighborhoods that contain ''x'', ...
. It is, however, R0 since the topologically distinguishable points are separable. An example of a countable double-pointed cofinite topology is the set of even and odd integers, with a topology that groups them together. Let X be the set of integers, and let O_A be a subset of the integers whose complement is the set A. Define a subbase of open sets G_x for any integer x to be G_x = O_ if x is an even number, and G_x = O_ if x is odd. Then the
basis Basis may refer to: Finance and accounting *Adjusted basis, the net cost of an asset after adjusting for various tax-related items *Basis point, 0.01%, often used in the context of interest rates *Basis trading, a trading strategy consisting of ...
sets of X are generated by finite intersections, that is, for finite A, the open sets of the topology are U_A := \bigcap_ G_x The resulting space is not T0 (and hence not T1), because the points x and x + 1 (for x even) are topologically indistinguishable. The space is, however, a
compact space In mathematics, specifically general topology, compactness is a property that seeks to generalize the notion of a closed and bounded subset of Euclidean space by making precise the idea of a space having no "punctures" or "missing endpoints", ...
, since each U_A contains all but finitely many points.


Other examples


Product topology

The product topology on a product of topological spaces \prod X_i has
basis Basis may refer to: Finance and accounting *Adjusted basis, the net cost of an asset after adjusting for various tax-related items *Basis point, 0.01%, often used in the context of interest rates *Basis trading, a trading strategy consisting of ...
\prod U_i where U_i \subseteq X_i is open, and cofinitely many U_i = X_i. The analog (without requiring that cofinitely many are the whole space) is the box topology.


Direct sum

The elements of the direct sum of modules \bigoplus M_i are sequences \alpha_i \in M_i where cofinitely many \alpha_i = 0. The analog (without requiring that cofinitely many are zero) is the
direct product In mathematics, one can often define a direct product of objects already known, giving a new one. This generalizes the Cartesian product of the underlying sets, together with a suitably defined structure on the product set. More abstractly, one t ...
.


See also

* * *


References

* {{Citation, last1=Steen, first1=Lynn Arthur, author1-link=Lynn Arthur Steen, last2=Seebach, first2=J. Arthur Jr., author2-link=J. Arthur Seebach, Jr., title=
Counterexamples in Topology ''Counterexamples in Topology'' (1970, 2nd ed. 1978) is a book on mathematics by topologists Lynn Steen and J. Arthur Seebach, Jr. In the process of working on problems like the metrization problem, topologists (including Steen and Seebach) h ...
, orig-year=1978, publisher= Springer-Verlag, location=Berlin, New York, edition= Dover reprint of 1978, isbn=978-0-486-68735-3, mr=507446, year=1995 ''(See example 18)'' Basic concepts in infinite set theory General topology