PlanetPhysics/Cubically Thin Homotopy 2

Cubically thin homotopy

Let $u,u'$ be squares in $X$ with common vertices.

1. A {\it cubically thin homotopy} $U:u\equiv _{T}^{\square }u'$

between $u$ and $u'$ is a cube $U\in R_{3}^{\square }(X)$ such that

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• $U$ is a homotopy between $u$ and $u',$

  i.e. Failed to parse (unknown function "\enskip"): {\displaystyle \partial^{-}_1 (U)=u,\enskip \partial^{+}_1 (U)=u',}

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• $U$ is rel. vertices of $I^{2},$

  i.e. Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "http://localhost:6011/en.wikiversity.org/v1/":): {\displaystyle \partial^{-}_2\partial^{-}_2 (U),\enskip\partial^{-}_2 \partial^{+}_2 (U),\enskip \partial^{+}_2\partial^{-}_2 (U),\enskip\partial^{+}_2 \partial^{+}_2 (U)} are constant,

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• the faces $\partial _{i}^{\alpha }(U)$ are thin for $\alpha =\pm 1,\ i=1,2$.

1. The square $u$ is {\it cubically} $T$-{\it equivalent} to

$u',$ denoted $u\equiv _{T}^{\square }u'$ if there is a cubically thin homotopy between $u$ and $u'.$

This definition enables one to construct ${\boldsymbol {\rho }}_{2}^{\square }(X)$ , by defining a relation of cubically thin homotopy on the set $R_{2}^{\square }(X)$ of squares.