I apologize in advance : what follows is a bit of a mess. Also, I think it might be a big tautology, but i don't see it yet. My question is about the rapport of Cech cohomology and cohomology of a (poset-)category with values in a functor. I recall some definitions. In what follows $k$ is a (possibly commutative) unital ring, $X$ is a topological space and $\mathcal{F}$ is a (pre-)sheaf of $k$-modules on $X$.
- Cohomology of a small category with values in a functor. Let $\mathcal{C}$ be a small category. The functor category $$k\mathcal{C}\mathrm{Mod}:=\mathrm{Fun}\big(\mathcal{C},k\mathrm{Mod}\big)$$ (with functors as objects, and natural transformations as morphisms) is an abelian category with enough injectives and projectives. Therefore functors $F,G:\mathcal{C}\to k\mathrm{Mod}$ admit projective and injective resolutions, one may define Ext groups $$\mathrm{Ext}^*_{k\mathcal{C}\mathrm{Mod}}(F,G)$$ by appliying either $\mathrm{Hom}_{k\mathcal{C}\mathrm{Mod}}(F,\:\:)$ to an injective resolution $0\to G\to I_*$ of $G$ and taking homology, or by appliying $\mathrm{Hom}_{k\mathcal{C}\mathrm{Mod}}(\:\:,G)$ to a projective resolution $P_*\to F\to 0$ of $F$ and, again, taking homology. The cohomology of $\mathcal{C}$ with values in $F$ is defined as $$\mathbf{H}^*(\mathcal{C},F):=\mathrm{Ext}^*(\underline{k},F)$$ where $\underline{k}$ is the constant functor equal to $k$ : it sends all objects of $\mathcal{C}$ to $k$, and all morphisms to the identity $\mathrm{id}:k\to k$.
- Cohomology group of a sheaf relative to a cover. (I'm not certain that's the correct terminology.) Let $\mathcal{U}$ be an open cover of $X$. Usually when defining Cech cohomology this means "a set $I$ and a map $U:I\to\lbrace$ all open subsets of $X\:\rbrace=\tau_X$ such that $\bigcup_{i\in I}U_i=X$", but I shall mean "a self-indexed open cover", i.e. a subset $\mathcal{U}\subset\tau_X$ with $\bigcup\mathcal{U}=X$. There are cohomology groups $$H^*(\mathcal{U},\mathcal{F})$$ associated to the cover and the sheaf : these are the homology groups of the Cech complex $$\prod_{u_0\in\mathcal{U}}\mathcal{F}(u_0)\longrightarrow\prod_{(u_0,u_1)\in\mathcal{U}^2}\mathcal{F}(u_0\cap u_1)\longrightarrow\prod_{(u_0,u_1,u_2)\in\mathcal{U}^3}\mathcal{F}(u_0\cap u_1\cap u_2)\longrightarrow\cdots$$
- Any poset $(P,\prec)$ may be considered a small category whith objects the points of $P$ and morphism sets defined as follows : for any objects $x,y\in P$ $$\mathrm{mor}(x,y)=\begin{cases}\lbrace * \rbrace & \text{if }x\preceq y\\ \emptyset & \text{if }x\npreceq y\end{cases}$$ An open cover (of the self-indexed kind) is a poset with the order relation $\prec$ defined as inclusion $\subset$.
- The sheaf $\mathcal{F}$ induces a functor $$\mathcal{F}\big|_{\mathcal{U}}:\mathcal{U}^{\mathrm{op}}\to k\mathrm{Mod}$$ and we can define its cohomology groups $$\mathbf{H}^*\left(\mathcal{U}^{\mathrm{op}},\mathcal{F}\big|_{\mathcal{U}}\right)$$
The obvious question now is :
Question 1 : Are the cohomology groups $$H^*(\mathcal{U},\mathcal{F})\quad\text{and}\quad \mathbf{H}^*\left(\mathcal{U}^{\mathrm{op}},\mathcal{F}\big|_{\mathcal{U}}\right)$$ isomorphic?
If that is so, then presumably there is also a functor from the poset category $Cov(X)$ of (self-indexed) open covers of $X$ to that of graded-commutative algebras that takes an open cover $\mathcal{U}$ to $\mathbf{H}^*\left(\mathcal{U}^{\mathrm{op}},\mathcal{F}\big|_{\mathcal{U}}\right)$.
As in the case Cech cohomology, one would first have to define such a functor on the category $\widetilde{Cov}(X)$ of open covers where "morphisms = choice functions" (if a cover $\mathcal{V}$ is finer than a cover $\mathcal{V}$, there are choice functions from the finer cover to the coarser cover that take an open set from the fine cover and hand back an open set in the coarse cover that containing it).
Question 2 : Is the Cech cohomology isomorphic to the cohomology of the poset category of open cover with values in the functor $\mathcal{U}\mapsto \mathbf{H}(\mathcal{U}^{\mathrm{op}},\mathcal{F}\big|_{\mathcal{U}})$, i.e. is $$\check{H}^*(X,\mathcal{F})\simeq \mathbf{H}^*\Big(Cov(X),\mathbf{H}\big(\:?^{\mathrm{op}}\:,\mathcal{F}\big|_{\mathcal{\:?\:}}\big)\Big)$$
In some notes by Brian Conrad, it is shown that the augmented chain complex associated to the simplicial set $WI$ is acyclic, i.e. that it is a free resolution of $k$.
$\Big[I$ is an arbitrary (non-empty) set, $WI$ is the simplicial set with $(WI)_0=I$, $(WI)_1=I\times I$, $(WI)_2=I\times I\times I$, $\dots$, with face maps $d_i:(WI)_n\to(WI)_{n-1}$ that cancel the $i$-th coordinate, degeneracies $s_i:(WI)_n\to(WI)_{n+1}$ that duplicate the $i$-th coordinate.$\Big]$
Question 3 : is this free resolution (when $I=\mathcal{U}$ is an open cover) somehow linked to a projective resolution of the constant functor $\underline{k}:\mathcal{U}^{\mathrm{op}}\to k\mathrm{Mod}$? If so, how?
Edit : to make things clearer, cohomology of a category with values in a functor is bold : $$\mathbf{H}^*(\mathrm{category},\mathrm{functor}).$$
yes
? Where can I read about the third question? $\endgroup$