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A Weil cohomology theory is a functor that assigns to a smooth projective variety $X$ of dimension $d$ over a field $k$ a graded ring of cohomology groups with values in a field $K$ of characteristic $0$, so that

  1. $H^{i}(X)$ is finite-dimensional for all $i$.
  2. $H^{i}(X)=0$ if $i<0$ or $i>2d$.
  3. $H^{2d}(X)\cong K$.
  4. There exists a perfect pairing $H^{i}(X)\times H^{2d-i}(X)\longrightarrow H^{2d}(X)=K$.
  5. The map $H^{*}(X)\otimes H^{*}(Y)\longrightarrow H^{*}(X\otimes Y)$ is an isomorphism.
  6. Existence of the cycle map $\gamma_{r}:Z^{r}(X)\longrightarrow H^{2r}(X)$ where $Z^{r}(X)$ is the group of algebraic cycles of codimension $r$.
  7. If $W\subseteq X$ is a smooth hyperplane section of the ambient projective space then the maps $H^{i}(X)\longrightarrow H^{i}(W)$ is an injection for $i\leq d-1$ and an isomorphism for $i\leq d-2$.
  8. If $W\subseteq X$ is a smooth hyperplane section of the ambient projective space then the map $L:H^{d-i}(X)\longrightarrow H^{d+i}(X)$ defined by $x\longrightarrow\gamma_{X}(W)\cdot x$ is an isomorphism for $i\in\left\{1,\dotsc,n\right\}$.

I have been pondering for quite some time now the question whether or not it is possible to generalize this to a setting where $X$ is defined not over $k$ but over any ring $R$.

Furthermore I am asking myself what would happen if the groups $H^{i}(X)$ were not defined over some field $K$ but over an arbitrary ring $S$, as long as one replaces "finite-dimensional" by "finitely generated over $S$" in condition 1, or even if one allows the $H^{i}(X)$ to take values in an arbitrary Abelian category as long as the Grothendieck group $K^{0}$ of that category is isomorphic to $\mathbb{Z}$ (so that the notion of a "trace" makes sense).

Would the usual theorems such as Lefschetz's fixed point theorem, the trace formula etc., still hold?

I do believe that if one requires the modules in condition 1. to be finitely generated and projective the usual theorems would still hold, but this is no more than a gut feeling.

I also believe that if one only lets go of the "characteristic $0$" condition for $K$ and allows $K$ to be an arbitrary field, then the main theorems would still hold — but the resulting $L$ functions would not be "useful" anymore in the sense that one could not possibly conceive them as generating functions counting the sets of points that $X$ has at the geometric points.

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    $\begingroup$ To some extent, this already exists: if $\ell$ is a prime invertible on a base scheme $S$, then $R^if_* \mathbf Q_\ell$ (pushforward of (pro-)étale sheaves) is a global version of cohomology for any $S$-scheme $f \colon X \to S$. It's not quite a $\mathbf Q_\ell$-vector space, but a constructible sheaf of $\mathbf Q_\ell$-vector spaces. There are probably categorical reasons why that's the best thing you can expect, for instance if you want it to be functorial not just in $X$ but also in $S$. $\endgroup$ Commented Nov 24, 2022 at 22:52
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    $\begingroup$ The type of problem I would expect to run into is that $H^{2d}(X)$ does not want to be $K$, but really wants to be a rank 1 sheaf in some site, i.e. there is some global structure happening on $S = \operatorname{Spec} R$ that the category of "motives over $S$" already knows about. This can probably be made precise with modern tools such as Voevodsky motives (about which I know little, but which definitely exist over a large class of base schemes $S$), even in easy examples where $X$ is the projectivisation of a vector bundle on $S$. $\endgroup$ Commented Nov 24, 2022 at 23:03
  • $\begingroup$ As R. points out you have a cohomology sheaf $R^if_*\mathbb Q_\ell$ on the base $S$. You can take its cohomology or global sections and perhaps obtain the precise results you need in the cases of interest. The general results -dualities and Lefschetz theory- are contained in Kiehl-Weissauer's book on l-adic sheaves -cf p109 and p208 respectively. Regarding realizations from complexes of motivic sheaves you should see arxiv.org/pdf/1305.5361.pdf section 7. I think Bhatt-Scholze did the prismatic case. And Ayoub the de Rham (see his foliated papers for advanced material) and rigid cases. $\endgroup$
    – plm
    Commented Jul 1, 2023 at 15:21
  • $\begingroup$ For Chern classes of line bundles you can see Kiehl-Weissauer, p123 onwards. For families of cycle classes you should see Fulton's Intersection theory, the chapter starting p175, but for particular results you might be interested in i guess you would have to look in various places -without a single neat reference. The authors above, Ayoub in particular, have published alot on motives over a base, and you would surely find there what you need, if only as conjectures -but much is already proved. $\endgroup$
    – plm
    Commented Jul 1, 2023 at 15:21

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