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In mathematics, the disjoint union (or discriminated union) of the sets A and B is the set formed from the elements of A and B labelled (indexed) with the name of the set from which they come. So, an element belonging to both A and B appears twice in the disjoint union, with two different labels.
The disjoint union space X, together with the canonical injections, can be characterized by the following universal property: If Y is a topological space, and f i : X i → Y is a continuous map for each i ∈ I, then there exists precisely one continuous map f : X → Y such that the following set of diagrams commute:
The continuous maps h : X ∪ f Y → Z are in 1-1 correspondence with the pairs of continuous maps h X : X → Z and h Y : Y → Z that satisfy h X (f(a))=h Y (a) for all a in A.. In the case where A is a closed subspace of Y one can show that the map X → X ∪ f Y is a closed embedding and (Y − A) → X ∪ f Y is an open embedding.
Let be the least uncountable ordinal.In an analog of Baire space derived from the -fold cartesian product of with itself, any closed set is the disjoint union of an -perfect set and a set of cardinality, where -closedness of a set is defined via a topological game in which members of are played.
Geometric join of two line segments.The original spaces are shown in green and blue. The join is a three-dimensional solid, a disphenoid, in gray.. In topology, a field of mathematics, the join of two topological spaces and , often denoted by or , is a topological space formed by taking the disjoint union of the two spaces, and attaching line segments joining every point in to every point in .
A wedge sum of two circles. In topology, the wedge sum is a "one-point union" of a family of topological spaces.Specifically, if X and Y are pointed spaces (i.e. topological spaces with distinguished basepoints and ) the wedge sum of X and Y is the quotient space of the disjoint union of X and Y by the identification : = /,
The pushout of f and g is the disjoint union of X and Y, where elements sharing a common preimage (in Z) are identified, together with the morphisms i 1, i 2 from X and Y, i.e. = / where ~ is the finest equivalence relation (cf. also this) such that f(z) ~ g(z) for all z in Z.
In general, an n-dimensional CW complex is constructed by taking the disjoint union of a k-dimensional CW complex (for some <) with one or more copies of the n-dimensional ball. For each copy, there is a map that "glues" its boundary (the ( n − 1 ) {\displaystyle (n-1)} -dimensional sphere ) to elements of the k {\displaystyle k} -dimensional ...