# Homotopy class

## Definition

The relation of paths being end-point-preserving homotopic is an Equivalence relation[1]

That is [ilmath]\alpha\simeq\beta\text{ rel}\{0,1\} [/ilmath] where [ilmath]\alpha[/ilmath] and [ilmath]\beta[/ilmath] are paths from [ilmath]a[/ilmath] to [ilmath]b[/ilmath] (which are not necessarily distinct as it may be a loop) is an equivalence relation, which is to say:

• Reflexive: [ilmath]\alpha\simeq\alpha\text{ rel}\{0,1\} [/ilmath]
• Symmetric: [ilmath]\alpha\simeq\beta\text{ rel}\{0,1\}\implies \beta\simeq\alpha\text{ rel}\{0,1\} [/ilmath]
• Transitive: [ilmath]\alpha\simeq\beta\text{ rel}\{0,1\}\wedge\beta\simeq\gamma\text{ rel}\{0,1\}\implies \alpha\simeq\gamma\text{ rel}\{0,1\} [/ilmath]

The equivalence class of [ilmath]\alpha[/ilmath] is denoted (as is usual) by [ilmath][\alpha][/ilmath]

## Important properties

[ilmath]\alpha,\ \beta[/ilmath] and [ilmath]\gamma[/ilmath] denote paths

• For a continuous map [ilmath]p:[0,1]\rightarrow[0,1][/ilmath] with [ilmath]p(0)=0[/ilmath] and [ilmath]p(1)=1[/ilmath] we have:
[ilmath][\alpha\circ p]=[\alpha][/ilmath] - that is any reparametrisation of [ilmath]\alpha[/ilmath] is homotopic to [ilmath]\alpha[/ilmath]
• $[\alpha_1]=[\alpha_2]\wedge[\beta_1]=[\beta_2]\implies[\alpha_1\beta_1]=[\alpha_2\beta_2]$
• This allows us to define multiplication
• $(\alpha\beta)\gamma\simeq\alpha(\beta\gamma)\text{ rel}\{0,1\}$ or $([\alpha][\beta])[\gamma]=[\alpha]([\beta][\gamma])$
• This allows us to define associativity
• Where [ilmath]a[/ilmath] is the constant loop at [ilmath]a[/ilmath] (ie [ilmath]a(t)=a\ \forall t\in[0,1][/ilmath]) we have
$a\alpha\simeq\alpha\simeq\alpha b\text{ rel}\{0,1\}$ or $[a][\alpha]=[\alpha]=[\alpha][b]$
• if [ilmath]\alpha^{-1} [/ilmath] is the reverse path of [ilmath]\alpha[/ilmath], literally [ilmath]\alpha^{-1}(t)=\alpha(1-t)[/ilmath] then
• [ilmath][\alpha_0]=[\alpha_1]\implies[\alpha_0^{-1}]=[\alpha_1^{-1}][/ilmath]
• we can now define the inverse, [ilmath][\alpha^{-1}]=[\alpha]^{-1}[/ilmath]
• [ilmath][\alpha][\alpha]^{-1}=[a][/ilmath]

TODO: Proofs for all of these p117