# Homotopy extension property

In mathematics, in the area of algebraic topology, the homotopy extension property indicates which homotopies defined on a subspace can be extended to a homotopy defined on a larger space.

## Definition

Let $X\,\!$ be a topological space, and let $A \subset X$. We say that the pair $(X,A)\,\!$ has the homotopy extension property if, given a homotopy $f_t\colon A \rightarrow Y$ and a map $F_0\colon X \rightarrow Y$ such that $F_0 |_A = f_0$, there exists an extension of $F_0$ to a homotopy $F_t\colon X \rightarrow Y$ such that $F_t|_A = f_t$. [1]

That is, the pair $(X,A)\,\!$ has the homotopy extension property if any map $G\colon ((X\times \{0\}) \cup (A\times I)) \rightarrow Y$ can be extended to a map $G'\colon X\times I \rightarrow Y$ (i.e. $G\,\!$ and $G'\,\!$ agree on their common domain).

If the pair has this property only for a certain codomain $Y\,\!$, we say that $(X,A)\,\!$ has the homotopy extension property with respect to $Y\,\!$.

## Visualisation

The homotopy extension property is depicted in the following diagram

If the above diagram (without the dashed map) commutes, which is equivalent to the conditions above, then there exists a map $\tilde{f}$ which makes the diagram commute. By currying, note that a map $\tilde{f} \colon X \to Y^I$ is the same as a map $\tilde{f} \colon X\times I \to Y$.

Also compare this to the visualization of the homotopy lifting property.

## Properties

• If $X\,\!$ is a cell complex and $A\,\!$ is a subcomplex of $X\,\!$, then the pair $(X,A)\,\!$ has the homotopy extension property.
• A pair $(X,A)\,\!$ has the homotopy extension property if and only if $(X\times \{0\} \cup A\times I)$ is a retract of $X\times I.$

## Other

If $\mathbf{\mathit{(X,A)}}$ has the homotopy extension property, then the simple inclusion map $i: A \to X$ is a cofibration.

In fact, if you consider any cofibration $i: Y \to Z$, then we have that $\mathbf{\mathit{Y}}$ is homeomorphic to its image under $\mathbf{\mathit{i}}$. This implies that any cofibration can be treated as an inclusion map, and therefore it can be treated as having the homotopy extension property.