For any prime p, one has the Frobenius homomorphism Fp defined on rings of characteristic p.


Is there any kind of object, say U, with a "universal Frobenius map" F such that for any prime p and any ring R of characteristic p we can view the Frobenius Fp over R as "the" base change of F from U to R?


I have the following picture in mind: In some sense it should be possible to view the category of Z-algebras as a sheaf of categories over Spec Z such that the fibre over Spec Fp is just the category of F\_p-algebras. A natural transformation f of the identity functor on the category of Z-algebras should restrict to a natural transformation fp of the identity functor on the category of Fp-algebras. In this naive picture one cannot expect the existence of an f such that fp is the Frobenius on Fp-algebras for all primes p. But is there way to make this picture work?


Another possible way to answer my question could be the following: Is there a classifying topos of, say, algebras with a Frobenius action? By this I mean the following: Is there a topos E with a fixed ring object R and an algebra A over it and an R-linear endomorphism f of A such that for any other topos E' with similar data R', A' there is a unique morphism of topoi E' -> E that pulls back R, A to R', A' and such that f is pulled back to the Frobenius fp of A' in case R' is of prime characteristic.


(Feel free to modify my two pictures to make them work.)



Does a universal Frobenius map exist?
I don't really get the categorical picture of what you are asking, but it feels something very similar to the relation between finite fields of characteristic $p$ and the ring (of characteristic 0) of $p$-typical Witt vectors.


You might want to have a look at Borger and Wieland work on [pleythistic algebras](http://wwwmaths.anu.edu.au/~borger/papers/03/paper03.html). The "lifting of all Frobeniuses at the same time" gives you an structure of $\Lambda$-ring, so the paper [The basic geometry of Witt vectors](http://wwwmaths.anu.edu.au/~borger/papers/05/paper05.html) by Borger might also be useful.


