Adjunction space

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In mathematics, an adjunction space is a common construction in topology where one topological space is attached or "glued" onto another. Specifically, let X and Y be a topological spaces with A a subspace of Y. Let f : A → X be continuous map (called the attaching map). One forms the adjunction space X ∪f Y by taking the disjoint union of X and Y and identifying x ∼ f(x) for all x in A. Schematically,

X\cup_f Y = (X\amalg Y) / \{f(A) \sim A\}

Sometimes, the adjunction is written as X+\!_f \,Y. Intuitively, we think of Y as being glued onto X via the map f.

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. 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.

[edit] Examples

  • 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.

[edit] 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 following commutative diagram:

Image:AdjunctionSpace-01.png

Here i is the inclusion map and φX, φY 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.

[edit] References

  • Stephen Willard, General Topology, (1970) Addison-Wesley Publishing Company, Reading Massachusetts. (Provides a very brief introduction.)
  • Adjunction space on PlanetMath
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