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  2. Base (topology) - Wikipedia

    en.wikipedia.org/wiki/Base_(topology)

    The Zariski topology on the spectrum of a ring has a base consisting of open sets that have specific useful properties. For the usual base for this topology, every finite intersection of basic open sets is a basic open set. The Zariski topology of is the topology that has the algebraic sets as closed sets

  3. Open and closed maps - Wikipedia

    en.wikipedia.org/wiki/Open_and_closed_maps

    In mathematics, more specifically in topology, an open map is a function between two topological spaces that maps open sets to open sets. [1] [2] [3] That is, a function : is open if for any open set in , the image is open in . Likewise, a closed map is a function that maps closed sets to closed sets.

  4. Open set - Wikipedia

    en.wikipedia.org/wiki/Open_set

    The union of any number of open sets, or infinitely many open sets, is open. [4] The intersection of a finite number of open sets is open. [4] A complement of an open set (relative to the space that the topology is defined on) is called a closed set. A set may be both open and closed (a clopen set).

  5. Topological space - Wikipedia

    en.wikipedia.org/wiki/Topological_space

    Any set can be given the cofinite topology in which the open sets are the empty set and the sets whose complement is finite. This is the smallest T 1 topology on any infinite set. [13] Any set can be given the cocountable topology, in which a set is defined as open if it is either empty or its complement is countable. When the set is ...

  6. General topology - Wikipedia

    en.wikipedia.org/wiki/General_topology

    Any set can be given the cofinite topology in which the open sets are the empty set and the sets whose complement is finite. This is the smallest T 1 topology on any infinite set. Any set can be given the cocountable topology, in which a set is defined as open if it is either empty or its complement is countable. When the set is uncountable ...

  7. Baire space (set theory) - Wikipedia

    en.wikipedia.org/wiki/Baire_space_(set_theory)

    The basic open sets of the product topology are cylinder sets.These can be characterized as: If any finite set of natural number coordinates I={i} is selected, and for each i a particular natural number value v i is selected, then the set of all infinite sequences of natural numbers that have value v i at position i is a basic open set.

  8. Neighbourhood (mathematics) - Wikipedia

    en.wikipedia.org/wiki/Neighbourhood_(mathematics)

    A set in the plane is a neighbourhood of a point if a small disc around is contained in . The small disc around is an open set .. In topology and related areas of mathematics, a neighbourhood (or neighborhood) is one of the basic concepts in a topological space.

  9. Regular open set - Wikipedia

    en.wikipedia.org/wiki/Regular_open_set

    The interior of a closed subset of is a regular open subset of and likewise, the closure of an open subset of is a regular closed subset of . [2] The intersection (but not necessarily the union) of two regular open sets is a regular open set. Similarly, the union (but not necessarily the intersection) of two regular closed sets is a regular ...