Displacements

Given a functor $p:E\to B$ and an object $e\in E$ , we define a \emph{displacement} of $e$ along a morphism $\epsilon :p(e)\to b$, as a map $e\to {\mathrm{\nabla }}_{\epsilon }(e)$ satisfying a universal property analogue to that of a \emph{cocartesian lifting} (pushforward) \emph{`a la} B'enabou-Grothendieck-Street. There are many difficulties in geometry that come from the fact that forgetful functors such as $p: Var($\mathbf{C}$) \rightarrow Top $ don’t have displacements of objects along arbitrary maps. And this can be already seen abstractly, since the existence of a left adjoint to $p$, can be reduced to the existence of all displacements of the initial object. However some \emph{schematization functors} exist as approximations. In a broader context, if $B$ is a model category and $p$ is a right adjoint, then the right-induced model category on $E$ exists if and only if all displacements along any trivial cofibration $\epsilon$, are weak $p$-equivalences. In these notes we provide some categorical lemmas that will be necessary for future applications. The idea is to have a \emph{homotopy descent process} for \emph{elementary displacements} when $p$ has a \emph{presentation} as a $2$-pullback of a family $\{p_i:E_i\to B{\}}_{i\in J}$. When suitably applied it should lead to techniques similar to Mumford’s GIT through homotopy theory (simplicial presheaves).