Adjunction space

In mathematics, an adjunction space (or attaching space) is a common construction in topology where one topological space is attached or "glued" onto another. Specifically, let "X" and "Y" be topological spaces, and let "A" be a subspace of "Y". Let "f" : "A" → "X" be a continuous map (called the attaching map). One forms the adjunction space "X" ∪"f" "Y" (sometimes also written as "X" +"f" "Y") by taking the disjoint union of "X" and "Y" and identifying "a" with "f"("a") for all "a" in "A". Formally,
where the equivalence relation ~ is generated by "a" ~ "f"("a") for all "a" in "A", and the quotient is given the quotient topology. As a set, "X" ∪"f" "Y" consists of the disjoint union of "X" and ("Y" − "A"). The topology, however, is specified by the quotient construction. 
Intuitively, one may think of "Y" as being glued onto "X" via the map "f".
Properties.
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.
Categorical description.
The attaching construction is an example of a pushout in the category of topological spaces. That is to say, the adjunction space is universal with respect to the following commutative diagram:
Here "i" is the inclusion map and "ϕ""X", "ϕ""Y" are the maps obtained by composing the quotient map with the canonical injections into the disjoint union of "X" and "Y". One can form a more general pushout by replacing "i" with an arbitrary continuous map "g"—the construction is similar. Conversely, if "f" is also an inclusion the attaching construction is to simply glue "X" and "Y" together along their common subspace.