H topology


In algebraic geometry, the h topology is a Grothendieck topology introduced by Vladimir Voevodsky to study the homology of schemes. It combines several good properties possessed by its related "sub"topologies, such as the qfh and cdh topologies. It has subsequently been used by Beilinson to study p-adic Hodge theory, in Bhatt and Scholze's work on projectivity of the affine Grassmannian, Huber and Jörder's study of differential forms, etc.

Definition

Voevodsky defined the h topology to be the topology associated to finite families of morphisms of finite type such that is a universal topological epimorphism. Voevodsky worked with this topology exclusively on categories of schemes of finite type over a Noetherian base scheme S.
Bhatt-Scholze define the h topology on the category of schemes of finite presentation over a qcqs base scheme to be generated by -covers of finite presentation. They show that the h topology is generated by:
  1. fppf-coverings, and
  2. families of the form where
  3. # is a proper morphism of finite presentation,
  4. # is a closed immersion of finite presentation, and
  5. # is an isomorphism over.
Note that is allowed in an abstract blowup, in which case Z is a nilimmersion of finite presentation.

Examples

The h-topology is not subcanonical, so representable presheaves are almost never h-sheaves. However, the h-sheafification of representable sheaves are interesting and useful objects; while presheaves of relative cycles are not representable, their associated h-sheaves are representable in the sense that there exists a disjoint union of quasi-projective schemes whose h-sheafifications agree with these h-sheaves of relative cycles.
Any h-sheaf in positive characteristic satisfies where we interpret as the colimit over the Frobenii. In fact, the h-sheafification of the structure sheaf is given by. So the structure sheaf "is an h-sheaf on the category of perfect schemes". In characteristic zero similar results hold with perfection replaced by semi-normalisation.
Huber-Jörder study the h-sheafification of the presheaf of Kähler differentials on categories of schemes of finite type over a characteristic zero base field. They show that if X is smooth, then, and for various nice non-smooth X, the sheaf recovers objects such as reflexive differentials and torsion-free differentials. Since the Frobenius is an h-covering, in positive characteristic we get for, but analogous results are true if we replace the h-topology with the cdh-topology.
By the Nullstellensatz, a morphism of finite presentation towards the spectrum of a field admits a section up to finite extension. That is, there exists a finite field extension and a factorisation. Consequently, for any presheaf and field we have where, resp., denotes the h-sheafification, resp. etale sheafification.

Properties

As mentioned above, in positive characteristic, any h-sheaf satisfies. In characteristic zero, we have where is the semi-normalisation.
Since the h-topology is finer than the Zariski topology, every scheme admits an h-covering by affine schemes.
Using abstract blowups and Noetherian induction, if is a field admitting resolution of singularities then any scheme of finite type over admits an h-covering by smooth -schemes. More generally, in any situation where de Jong's theorem on alterations is valid we can find h-coverings by regular schemes.
Since finite morphisms are h-coverings, algebraic correspondences are finite sums of morphisms.

''cdh'' topology

The cdh topology on the category of schemes of finite presentation over a qcqs base scheme is generated by:
  1. Nisnevich coverings, and
  2. families of the form where
  3. # is a proper morphism of finite presentation,
  4. # is a closed immersion of finite presentation, and
  5. # is an isomorphism over.
It is the universal topology with a "good" theory of compact supports.
The cd stands for completely decomposed. As mentioned in the examples section, over a field admitting resolution of singularities, any variety admits a cdh-covering by smooth varieties. This topology is heavily used in the study of Voevodsky motives with integral coefficients.
Since the Frobenius is not a cdh-covering, the cdh-topology is also a useful replacement for the h-topology in the study of differentials in positive characteristic.
Rather confusingly, there are completely decomposed h-coverings, which are not cdh-coverings, for example the completely decomposed family of flat morphisms.

Relation to v-topology and arc-topology

The v-topology is equivalent to the h-topology on the category of schemes of finite type over a Noetherian base scheme S. Indeed, a morphism in is universally subtrusive if and only if it is universally submersive. In other words,
More generally, on the category of all qcqs schemes, neither of the v- nor the h- topologies are finer than the other: and. There are v-covers which are not h-covers and h-covers which are not v-covers.
However, we could define an h-analogue of the fpqc topology by saying that an hqc-covering is a family such that for each affine open there exists a finite set K, a map and affine opens such that is universally submersive. Then every v-covering is an hqc-covering.
Indeed, any subtrusive morphism is submersive.
By a theorem of Rydh, for a map of qcqs schemes with Noetherian, is a v-cover if and only if it is an arc-cover. That is, in the Noetherian setting everything said above for the v-topology is valid for the arc-topology.