Consider the topology τ on the category of schemes where a covering is a finite family of proper morphisms which are jointly surjective. (Dear reader: does this topology have a name?) For the purpose of this post proper hypercoverings will be τ-hypercoverings as defined in the chapter on hypercoverings. Proper hypercoverings are discussed specifically in Brian Conrad’s write up. In this post I wanted to explain an example which I was recently discussing with Bhargav on email. I’d love to hear about other “explicit” examples that you know about; please leave a comment.

The example is an example of proper hypercovering for curves. Namely, consider a separable degree 2 map X —> Y of projective nonsingular curves over an algebraically closed field and let y be a ramification point. The simplicial scheme X_* with X_i = normalization of (i + 1)st fibre product of X over Y is NOT a proper hypercovering of Y. Namely, consider the fibre above y (recall that the base change of a proper hypercovering is a proper hypercovering). Then we see that X_0 has one point above y, X_1 has 2 points above y, and X_2 has 4 points above y. But if X_2 is supposed to surject onto the degree 2 part of cosk_1(X_1 => X_0) then the fibre of X_2 over y has to have at least 8 points!!!!

Namely cosk_1(S —> *) where S is a set and * is a singleton set is the simplicial set with S^3 in degree 2, S^6 in degree 3, etc because an n-simplex should exist for any collection of (n + 1 choose 2) 1-simplices since each of the 1-simplices bounds the unique 0-simplex on both sides, see for example Remark 0189. So I think that to construct the proper hypercovering we have to throw in some extra points in simplicial degree 2 which sort of glue the two components of X_1.

Now, as X_* does work over the complement of the ramification locus in Y, I think you can argue that it really does suffice to add finite sets of points to X_* (over ramification points) to get a proper hypercovering!

PS: Proper hypercoverings are interesting since they can be used to express the cohomology of a (singular) variety in terms of cohomologies of smooth varieties. But that’s for another post.

Theorem. Let f : X —> Y be a proper morphism of varieties and let y ∈ Y with f^{-1}(y) finite. Then there exists a neighborhood V of y in Y such that f^{-1}(V) —> V is finite.

If X is quasi-projective, then there is a simple proof: Choose an affine open U of X containing f^{-1}(y); this uses X quasi-projective. Using properness of f, find an affine open V ⊂ Y such that f^{-1}V ⊂ U. Then f^{-1}V = V x_Y U is affine as Y is separated. Hence f^{-1}V —> V is a proper morphism of affines varieties. Such a morphism is finite, see Lemma Tag 01WM for an elementary argument.

I do not know a truly simple proof for the general case. (Ravi explained a proof to me that avoids most cohomological machinery, but unfortunately I forgot what the exact method was; it may even be one of the arguments I list below.) Here are some different approaches.

(A) One can give a proof using cohomology and the theorem on formal functions, see Lemma Tag 020H.

Let ZMT be Grothendieck’s algebraic version of Zariski’s main theorem, see Theorem Tag 00Q9.

(B) One can prove the result using ZMT and etale localization. Namely, one proves that given any finite type morphism X —> Y with finite fibre over y, there is after etale localization on Y, a decomposition X = U ∐ W with U finite over Y and the fibre W_y empty (see Section Tag 04HF). In the proper case it follows that W is empty after shrinking Y. Finally, etale descent of the property “being finite” finishes the argument. This method proves a general version of the result, see Lemma Tag 02LS.

(C) A mixture of the above two arguments using ZMT and a characterization of affines:

1. Show that after replacing Y by a neighborhood of y we may assume that all fibers of f are finite. This requires showing that dimensions of fibres go up under specialization. You can prove this using generic flatness and the dimension formula (as in Eisenbud for example) or using ZMT.
2. Let X’ —> Y be the normalization of Y in the function field of X. Then X’ —> Y is finite and X’ and X are birational over Y. Finiteness of X’ over Y requires finiteness of integral closure of finite type domains over fields, which follows from Noether normalization + epsilon.
3. Let W ⊂ X x_Y X’ be the closure of the graph of the birational rational map from X to X’. Then W —> X is finite and birational and W —> X’ is proper with finite fibres and birational.
4. Using ZMT one shows that W —> X’ is an isomorphism. Namely, a corollary of ZMT is that separated quasi-finite birational morphisms towards normal varieties are open immersions.
5. Now we have X’ —> X —> Y with the first arrow finite birational and the composition finite too. After shrinking Y we may assume Y and X’ are affine. If X is affine, then we win as O(X) would be a subalgebra ofa finite O(Y)-algebra.
6. Show that X is affine because it is the target of a finite surjective morphism from an affine. Usually one proves this using cohomology. The Noetherian case is Lemma Tag 01YQ (this uses less of the cohomological machinery but still uses the devissage of coherent modules on Noetherian schemes). In fact, the target of a surjective integral morphism from an affine is affine, see Lemma Tag 05YU.

This post is another attempt to explain how incredibly useful the notion of a cocontinuous functor of sites really is. I already tried once here.

Let u : C —> D be a functor between sites. We say u is cocontinuous if for every object U of C and every covering {V_j —> u(U)} in D there exists a covering {U_i —> U} in C such that {u(U_i) —> U} refines {V_j —> u(U)}. This is the direct translation of SGA 4, II, Defintion 2.1 into the language of sites as used in the stacks project and in Artin’s notes on Grothendieck topologies. Note that we do not require that u transforms coverings into coverings, i.e., we do not assume u is continuous. Often the condition of cocontinuity is trivial to check.

Lemma Tag 00XO A cocontinuous functor defines a morphism of topoi g : Sh(C) —> Sh(D) such that g^{-1}G is the sheaf associated to U |—> G(u(U)).

The reader should contrast this with the “default” which is morphisms of topoi associated to continuous functors (where one has to check the exactness of the pull back functor explicitly in each case!). Let’s discuss some examples where the lemma applies.

The standard example is the functor Sch/X —> Sch/Y associated to a morphism of schemes X —> Y for any of the topologies Zariski, etale, smooth, syntomic, fppf. This defines functoriality for the big topoi. This also works to give functoriality for big topoi of algebraic spaces and algebraic stacks. In exactly the same way we get functoriality of the big crystalline topoi.

Another example is any functor u : C —> D between categories endowed with the chaotic topology, i.e., such that sheaves = presheaves. Then u is cocontinuous and we get a morphism of topoi Sh(C) —> Sh(D).

Finally, an important example is localization. Let C be a site and let K be a sheaf of sets. Let C/K be the category of pairs (U, s) where U is an object of C and s ∈ K(U). Endow C/K with the induced topology, i.e., such that {(U_i, s_i) —> (U, s)} is a covering in C/K if and only if {U_i —> U} is a covering in C. Then C/K —> C is cocontinuous (and continuous too) and we obtain a morphism of topoi Sh(C/K) —> Sh(C) whose pullback functor is restriction.

What I am absolutely not saying is that the lemma above is a “great” result. What I am saying is that, in algebraic geometry, the lemma is easy to use (no additional conditions to check) and situations where it applies come up frequently and naturally.

PS: Warning: In some references a cocontinuous functor is a functor between categories (not sites) is defined as a functor that commutes with colimits. This is a different notion. Too bad!