Adjunction space 📖 Wikipedia

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 ${\displaystyle X}$ and ${\displaystyle Y}$ be topological spaces, and let ${\displaystyle A}$ be a subspace of ${\displaystyle Y}$. Let ${\displaystyle f:A\rightarrow X}$ be a continuous map (called the attaching map). One forms the adjunction space ${\displaystyle X\cup _{f}Y}$ (sometimes also written as ${\displaystyle X+_{f}Y}$) by taking the disjoint union of ${\displaystyle X}$ and ${\displaystyle Y}$ and identifying ${\displaystyle a}$ with ${\displaystyle f(a)}$ for all ${\displaystyle a}$ in ${\displaystyle A}$. Formally,

${\displaystyle X\cup _{f}Y=(X\sqcup Y)/\sim }$

where the equivalence relation ${\displaystyle \sim }$ is generated by ${\displaystyle a\sim f(a)}$ for all ${\displaystyle a}$ in ${\displaystyle A}$, and the quotient is given the quotient topology. As a set, ${\displaystyle X\cup _{f}Y}$ consists of the disjoint union of ${\displaystyle X}$ and (${\displaystyle Y-A}$). The topology, however, is specified by the quotient construction.

Intuitively, one may think of ${\displaystyle Y}$ as being glued onto ${\displaystyle X}$ via the map ${\displaystyle f}$.

• A common example of an adjunction space is given when Y is a closed n-ball (or cell) and A is the boundary of the ball, the (n−1)-sphere. Inductively attaching cells along their spherical boundaries to this space results in an example of a CW complex.
• Adjunction spaces are also used to define connected sums of manifolds. Here, one first removes open balls from X and Y before attaching the boundaries of the removed balls along an attaching map.
• If A is a space with one point then the adjunction is the wedge sum of X and Y.
• If X is a space with one point then the adjunction is the quotient Y/A.

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.

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.

• Quotient space
• Mapping cylinder

• Stephen Willard, General Topology, (1970) Addison-Wesley Publishing Company, Reading Massachusetts. (Provides a very brief introduction.)
• "Adjunction space". PlanetMath.
• Ronald Brown, "Topology and Groupoids" pdf available , (2006) available from amazon sites. Discusses the homotopy type of adjunction spaces, and uses adjunction spaces as an introduction to (finite) cell complexes.
• J.H.C. Whitehead "Note on a theorem due to Borsuk" Bull AMS 54 (1948), 1125-1132 is the earliest outside reference I know of using the term "adjuction space".