# nLab straightening functor

This is a subentry of (infinity,1)-Grothendieck construction.

In model category theory? , in particular in the presentation of the (∞,0)-Grothendieck construction, for a simplicial set $S$, there is a pair of adjoint functors

$\left({\mathrm{St}}_{\varphi }⊣{\mathrm{Un}}_{\varphi }\right):\mathrm{sSet}/S\stackrel{\mathrm{St}}{\to }\left[{C}^{\mathrm{op}},\mathrm{sSet}\right]$(St_\phi\dashv Un_\phi) : sSet/S \stackrel{St}{\to} [C^{op}, sSet]

which (under an assumption on the parameter $\varphi$) can be shown to be a Quillen equivalence between the overcategory of simplicial sets equipped with the model structure for right fibrations (also called contravariant model structure in HTT) and the category of simplicial presheaves equipped with global projective model structure.

There is also a Quillen equivalence

$\left({\mathrm{St}}_{\varphi }⊣{\mathrm{Un}}_{\varphi }\right):{\mathrm{sSet}}^{+}/S\stackrel{\mathrm{St}}{\to }\left[{C}^{\mathrm{op}},{\mathrm{sSet}}^{+}\right]$(St_\phi\dashv Un_\phi) : sSet^+/S \stackrel{St}{\to} [C^{op}, sSet^+]

between the model structure for Cartesian fibrations and the global projective model structure on functors with values in the model structure on marked simplicial sets.

Here (in both cases) ${\mathrm{St}}_{\varphi }$ is called straightening functor and ${\mathrm{Un}}_{\varphi }$ is called unstraightening functor. These names have been chosen due to the fact that objects in the left hand category are defined be existential assertions and choices where on the right side these properties become coherence laws being part of the structure.

Revised on June 28, 2012 19:07:24 by Stephan Alexander Spahn (79.227.142.182)