Homotopy
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Definition
Given topological spaces [ilmath](X,\mathcal{ J })[/ilmath] and [ilmath](Y,\mathcal{ K })[/ilmath], and any set [ilmath]A\in\mathcal{P}(X)[/ilmath]^{[Note 1]} a homotopy (relative to [ilmath]A[/ilmath]) is any continuous function:
 [ilmath]H:X\times I\rightarrow Y[/ilmath] (where [ilmath]I:=[0,1]\subset\mathbb{R}[/ilmath]) such that:
 [ilmath]\forall s,t\in I\ \forall a\in A[H(a,s)=H(a,t)][/ilmath]^{[Note 2]}
If [ilmath]A=\emptyset[/ilmath]^{[Note 2]} then we say [ilmath]H[/ilmath] is a free homotopy (or just a homotopy).
If [ilmath]A\neq \emptyset[/ilmath] then we speak of a homotopy rel [ilmath]A[/ilmath] or homotopy relative to [ilmath]A[/ilmath].
Stages of a homotopy
For a homotopy, [ilmath]H:X\times I\rightarrow Y\ (\text{rel }A)[/ilmath], a stage of the homotopy [ilmath]H[/ilmath] is a map:
 [ilmath]h_t:X\rightarrow Y[/ilmath] for some [ilmath]t\in I[/ilmath] given by [ilmath]h_t:x\mapsto H(x,t)[/ilmath]
The family of maps, [ilmath]\{h_t:X\rightarrow Y\}_{t\in I} [/ilmath], are collectively called the stages of a homotopy
Homotopy of maps
Notes
 ↑ Recall [ilmath]\mathcal{P}(X)[/ilmath] denotes the power set of [ilmath]X[/ilmath]  the set containing all subsets of [ilmath]X[/ilmath]; [ilmath]A\subseteq X\iff A\in\mathcal{P}(X)[/ilmath].
 ↑ ^{2.0} ^{2.1} Note that if [ilmath]A=\emptyset[/ilmath] then [ilmath]\forall s,t\in I\ \forall a\in\emptyset[H(a,s)=H(a,t)][/ilmath] is trivially satisfied; it represents no condition. As there is no [ilmath]a\in\emptyset[/ilmath] we never require [ilmath]H(a,s)=H(a,t)[/ilmath].
References
OLD PAGE
Definition
Given two topological spaces, [ilmath](X,\mathcal{ J })[/ilmath] and [ilmath](Y,\mathcal{ K })[/ilmath] then a homotopy of maps (from [ilmath]X[/ilmath] to [ilmath]Y[/ilmath]) is a continuous function: [ilmath]F:X\times I\rightarrow Y[/ilmath] (where [ilmath]I[/ilmath] denotes the unit interval, [ilmath]I:=[0,1]\subset\mathbb{R}[/ilmath]). Note:
 The stages of the homotopy, [ilmath]F[/ilmath], are a family of functions, [ilmath]\{ f_t:X\rightarrow Y\ \vert\ t\in[0,1]\} [/ilmath] such that [ilmath]f_t:x\rightarrow F(x,t)[/ilmath]. The stages of a homotopy are continuous.
 [ilmath]f_0[/ilmath] and [ilmath]f_1[/ilmath] are examples of stages, and are often called the initial stage of the homotopy and final stage of the homotopy respectively.
Two (continuous) functions, [ilmath]g,h:X\rightarrow Y[/ilmath] are said to be homotopic if there exists a homotopy such that [ilmath]f_0=g[/ilmath] and [ilmath]f_1=h[/ilmath]
 Claim: homotopy of maps is an equivalence relation^{[Note 1]}
Notes
 ↑ Do not shorten this to "homotopy equivalence" as homotopy equivalence of spaces is something very different
References
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