Disjoint union (topology)

Definition

Let {X**i : i ∈ I} be a family of topological spaces indexed by I. Let :X = \coprod_i X_i be the disjoint union of the underlying sets. For each i in I, let :\varphi_i : X_i \to X, be the canonical injection (defined by \varphi_i(x)=(x,i)). The disjoint union topology on X is defined as the finest topology on X for which all the canonical injections \varphi_i are continuous (i.e.: it is the final topology on X induced by the canonical injections).

Explicitly, the disjoint union topology can be described as follows. A subset U of X is open in X if and only if its preimage \varphi_i^{-1}(U) is open in X**i for each i ∈ I. Yet another formulation is that a subset V of X is open relative to X iff its intersection with Xi is open relative to Xi for each i.

Properties

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 fi : Xi → 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:

This shows that the disjoint union is the coproduct in the category of topological spaces. It follows from the above universal property that a map f : X → Y is continuous iff fi = f o φi is continuous for all i in I.

In addition to being continuous, the canonical injections φi : X**i → X are open and closed maps. It follows that the injections are topological embeddings so that each X**i may be canonically thought of as a subspace of X.

Examples

If each X**i is homeomorphic to a fixed space A, then the disjoint union X is homeomorphic to the product space A × I where I has the discrete topology.

Preservation of topological properties

• Every disjoint union of discrete spaces is discrete
• Separation
  • Every disjoint union of T0 spaces is T0
  • Every disjoint union of T1 spaces is T1
  • Every disjoint union of Hausdorff spaces is Hausdorff
• Connectedness
  • The disjoint union of two or more nonempty topological spaces is disconnected

References

References

1. "disjoint union topological space in nLab".