Cofiniteness

In mathematics, a cofinite subset of a set "X" is a subset "A" whose complement in "X" is a finite set. 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.
Boolean algebras.
The set of all subsets of "X" that are either finite or cofinite forms a Boolean algebra, i.e., it is closed under the operations of union, intersection, and complementation. This Boolean algebra is the finite–cofinite algebra on "X". A Boolean algebra "A" has a unique non-principal ultrafilter (i.e. a maximal filter not generated by a single element of the algebra) if and only if there is 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 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
This topology occurs naturally in the context of the Zariski topology. Since polynomials in one variable over a field "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; it is not true, for example, for "XY" = 0 in the plane.
Double-pointed cofinite topology.
The double-pointed cofinite topology is the cofinite topology with every point doubled; that is, it is the topological product 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. 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"{"x", "x"+1} if "x" is an even number, and "G""x" = "O"{"x"-1, "x"} if "x" is odd. Then the basis sets of "X" are generated by finite intersections, that is, for finite "A", the open sets of the topology are
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, since each "U"A contains all but finitely many points.
Other examples.
Product topology.
The product topology on a product of topological spaces formula_3
has basis formula_4 where formula_5 is open, and cofinitely many formula_6.
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 formula_7 are sequences formula_8 where cofinitely many formula_9.
The analog (without requiring that cofinitely many are zero) is the direct product.