Coherent topology

Let X be a topological space and let C = {C_α : α ∈ A} be a family of subsets of X with subspace topology. (Typically C will be a cover of X.) Then X is said to be coherent with C (or determined by C)[2] if the topology of X is recovered as the one coming from the final topology coinduced by the inclusion maps

${\displaystyle i_{\alpha }:C_{\alpha }\to X\qquad \alpha \in A.}$

By definition, this is the finest topology on (the underlying set of) X for which the inclusion maps are continuous. If C is a cover of X, then X is coherent with C if either of the following two equivalent conditions holds:

• A subset U is open in X if and only if U ∩ C_α is open in C_α for each α ∈ A.
• A subset U is closed in X if and only if U ∩ C_α is closed in C_α for each α ∈ A.

The above is not true if C does not cover X.

Given a topological space X and any family of subspaces C there is unique topology on (the underlying set of) X that is coherent with C. This topology will, in general, be finer than the given topology on X.

• A topological space X is coherent with every open cover of X.
• A topological space X is coherent with every locally finite closed cover of X.
• A discrete space is coherent with every family of subspaces (including the empty family).
• A topological space X is coherent with a partition of X if and only X is homeomorphic to the disjoint union of the elements of the partition.
• Finitely generated spaces are those determined by the family of all finite subspaces.
• Compactly generated spaces are those determined by the family of all compact subspaces.
• A CW complex X is coherent with its family of n-skeletons X_n.

Let ${\displaystyle \{X_{\alpha },\alpha \in A\}}$ be a family of (not necessarily disjoint) topological spaces such that the induced topologies agree on each intersection X_α ∩ X_β. Assume further that X_α ∩ X_β is closed in X_α for each α,β. Then the topological union X is the set-theoretic union

${\displaystyle X^{set}=\bigcup _{\alpha \in A}X_{\alpha }}$

endowed with the final topology coinduced by the inclusion maps ${\displaystyle i_{\alpha }:X_{\alpha }\to X^{set}}$. The inclusion maps will then be topological embeddings and X will be coherent with the subspaces {X_α}.

Conversely, if X is coherent with a family of subspaces {C_α} that cover X, then X is homeomorphic to the topological union of the family {C_α}.

One can form the topological union of an arbitrary family of topological spaces as above, but if the topologies do not agree on the intersections then the inclusions will not necessarily be embeddings.

One can also describe the topological union by means of the disjoint union. Specifically, if X is a topological union of the family {X_α}, then X is homeomorphic to the quotient of the disjoint union of the family {X_α} by the equivalence relation

${\displaystyle (x,\alpha )\sim (y,\beta )\Leftrightarrow x=y}$

for all α, β in A. That is,

${\displaystyle X\cong \coprod _{\alpha \in A}X_{\alpha }/\sim .}$

If the spaces {X_α} are all disjoint then the topological union is just the disjoint union.

Assume now that the set A is directed, in a way compatible with inclusion: ${\displaystyle \alpha \leq \beta }$ whenever ${\displaystyle X_{\alpha }\subset X_{\beta }}$. Then there is a unique map from ${\displaystyle \varinjlim X_{\alpha }}$ to X, which is in fact a homeomorphism. Here ${\displaystyle \varinjlim X_{\alpha }}$ is the direct (inductive) limit (colimit) of {X_α} in the category Top.

Let X be coherent with a family of subspaces {C_α}. A map f : X → Y is continuous if and only if the restrictions

${\displaystyle f|_{C_{\alpha }}:C_{\alpha }\to Y\,}$

are continuous for each α ∈ A. This universal property characterizes coherent topologies in the sense that a space X is coherent with C if and only if this property holds for all spaces Y and all functions f : X → Y.

Let X be determined by a cover C = {C_α}. Then

• If C is a refinement of a cover D, then X is determined by D.
• If D is a refinement of C and each C_α is determined by the family of all D_β contained in C_α then X is determined by D.

Let X be determined by {C_α} and let Y be an open or closed subspace of X. Then Y is determined by {Y ∩ C_α}.

Let X be determined by {C_α} and let f : X → Y be a quotient map. Then Y is determined by {f(C_α)}.

Let f : X → Y be a surjective map and suppose Y is determined by {D_α : α ∈ A}. For each α ∈ A let

${\displaystyle f_{\alpha }:f^{-1}(D_{\alpha })\to D_{\alpha }\,}$

be the restriction of f to f⁻¹(D_α). Then

• If f is continuous and each f_α is a quotient map, then f is a quotient map.
• f is a closed map (resp. open map) if and only if each f_α is closed (resp. open).

• Tanaka, Yoshio (2004). "Quotient Spaces and Decompositions". In K.P. Hart; J. Nagata; J.E. Vaughan (eds.). Encyclopedia of General Topology. Amsterdam: Elsevier Science. pp. 43–46. ISBN 0-444-50355-2.
• Willard, Stephen (1970). General Topology. Reading, Massachusetts: Addison-Wesley. ISBN 0-486-43479-6 (Dover edition).