Join of topological spaces

Definition

Given two topological spaces $X$ and $Y$, the join of $X$ and $Y$, denoted $X*Y$, is defined as follows: it is the quotient of the space $X\times Y\times I$ under the identifications:

$(x,y_1,0)\sim (x,y_2,0)\mathrm{\forall }x\in X,y_1,y_2\in Y$

and

$(x_1,y,1)\sim (x_2,y,1)\mathrm{\forall }{x}_1,x_2\in X,y\in Y$

Pictorially, we can think of this as the space of all line segments joining points in $X$ and $Y$, with two line segments meeting only at common endpoints.

Particular cases

Cone space

Further information: Cone space

The cone space of a topological space $X$ can be viewed as the join of $X$ with a one-point space.

Suspension

Further information: suspension

The suspension of a topological space $X$ can be viewed as the join of $X$ with a two-point space.

Simplex

The $n$-simplex can be viewed, at least topologically, as the join of $n$ one-point spaces.

Operation properties

There is a canonical isomorphism between $X*Y$ and $Y*X$, sending $(x,y,t)$ to (y,x,1-t)</math>.

There is a canonical isomorphism between $(X*Y)*Z$ and $X*(Y*Z)$.