𝑝-adic Hodge Theory (2019)

Overview of talks

The talks at the conference consisted of several organized pairs of lectures, covering important recent developments, as well as individual talks on topics related to the themes of the workshop.

Inspired by the complex theory developed by Simpson and others giving a correspondence between complex local systems and Higgs fields on algebraic varieties over C, Faltings proposed in 2005 an analogue in the 𝑝-adic context using 𝑝-adic Hodge theory. Gerd Faltings (A 𝑝-adic Simpson Correspondence) and Ahmed Abbes (Local Structure of Almost-étale φ-modules) discussed developments in this area since then.

Kęstutis Česnavičius and Peter Scholze gave a two-talk series entitled “Cohomological Purity in Bad Characteristics and Some Results on the Cohomology of Finite Flat Group Schemes,” explaining their recent proof of cohomological purity for finite flat commutative group schemes. Remarkably, their proof uses Dieudonné theory over perfectoid rings to prove results over noetherian rings.

In a two-talk series titled “Coefficients in Integral 𝑝-adic Hodge Theory via Generalized Ainf-representations and q-connections,” Matthew Morrow and Takeshi Tsuji explained their recent work developing a theory of integral 𝑝-adic Hodge Theory, in the sense of earlier work of Bhatt, Morrow and Scholze, with coefficients.

Dating back to work of Coleman on 𝑝-adic integration, it has long been known that 𝑝-adic Hodge theory of the fundamental group can be used to study rational points on algebraic varieties. This has in recent years been studied by many people in a program initiated by Minhyong Kim. At the symposium, talks in this direction were given by Minhyong Kim (“Principal Bundles and Diophantine Geometry: Some Remarks on Effectivity”) and Amnon Besser (“Vologodsky Integration on Curves with Semi-Stable Reduction: Progress Report”).

A key feature of Hodge theory with coefficients and in families is the Riemann-Hilbert correspondence relating complex local systems to D-modules. At the workshop, Xinwen Zhu and Ruochuan Liu explained their recent work toward a 𝑝-adic version. The title of their lectures was “P-adic Riemann-Hilbert Functors and Applications.”

Faltings’ 𝑝-adic Simpson correspondence can be strengthened in the case of vector bundles corresponding to Higgs bundles with vanishing Higgs field. Christopher Deninger and Annette Werner have constructed a functorial parallel transport for vector bundles on 𝑝-adic varieties satisfying a condition on their reduction. They explained this work at the conference in two talks titled “Vector Bundles and P-adic Representations.”

The remaining talks were individual presentations on topics related to the workshop.

In his talk “Monodromy and Galois Actions on Deformation Rings,” Daniel Litt spoke about recent work related to results of Deligne about the fundamental question of which representations of the fundamental group of a variety over C come from geometry, in the sense that they appear as subquotients of the cohomology of a variety over the function field.

Related to this, in her talk “Arithmetic Subspaces of Moduli Spaces of Rank One Local Systems,” Hélène Esnault spoke about loci in the character variety for the fundamental group for which the associated local systems are motivic. This is related to a conjecture of Carlos Simpson.

Ofer Gabber discussed various duality results in 𝑝-adic Hodge theory, including Poincare duality for mod 𝑝 cohomology of smooth proper rigid analytic spaces. The title of his talk was “Almost Duality for Nearby Cycles of \(O^+/p\).”

Jacob Lurie discussed, in his talk “Full Level Structures on Elliptic Curves,” an integral version of a result of Scholze showing that the limit of the tower of moduli spaces for elliptic curves with level structure is perfectoid.

Another approach to 𝑝-adic cohomology of algebraic varieties is using the de Rham-Witt complex. Arthur Ogus discussed recent developments in this area in his talk “The Saturated de Rham-Witt Complex, after Bhatt, Lurie, Mathew.” There is some hope that this new de Rham-Witt complex is better behaved than the classical one in the case of singular varieties, and Ogus presented new results in this direction for toric varieties.

Vadim Vologodsky discussed a new approach to defining crystalline cohomology of differential graded algebras over positive characteristic fields, using topological cyclic homology. This work is related to work of Bhatt, Morrow and Scholze and some of the central themes of the first meeting in the symposium series.

Finally, Jared Weinstein discussed a conjecture of Kottwitz making precise the Local Langlands Correspondence for representations of reductive 𝑝-adic groups. He explained this recent work in his talk “The Cohomology of Local Shtuka Spaces.”

New Developments and Future Directions

The symposium was very active, with an informal and interactive atmosphere. Many of the presenters discussed both finished work as well as work in progress, and there were a lot of smaller conversations about important technical points, improvements to arguments, extensions of results, etc. — the kinds of conversations that often lead to substantial advances and require a strong group of experts in the area. For example, one series of questions during a talk led to substantial improvements in the arguments for one of the main results presented; another speaker credited the mere potential presence of an audience member as having led to the discovery of an error in a proof while the speaker was preparing the talk, eventually leading to a better proof. While several important finished results were presented at the symposium, the small size of the meeting and the informal atmosphere fostered numerous discussions about work in progress and future directions.

*Participants may explore the hotel property and its surrounding areas as well as engage in informal discussion with other participants.

Ahmed Abbes
Centre national de la recherche scientifique & Institut des Hautes Études Scientifiques

Local Structure of Almost-étale \(\varphi\)-modules

In the appendix of his 2002 Astérisque article, Faltings sketched a proof of a relative version of his main comparison theorem in 𝑝-adic Hodge theory. Abbes will report on a key ingredient of this proof describing the local structure of certain almost-étale \(\varphi\)-modules, which is interesting in itself. This is joint work with Michel Gros.

Amnon Besser
Ben Gurion University

Vologodsky Integration on Curves with Semi-Stable Reduction: Progress Report

Vologodsky extended Coleman’s integration theory to give canonical paths on the unipotent de Rham fundamental groupoid of any smooth variety over a 𝑝-adic field. Besser expects this to have a strong connection with the unipotent Albanese map for such varieties and, therefore, for the study of rational points using non-abelian techniques at primes of bad reduction. Besser will provide some evidence for that. A major problem with Vologodsky integration is that it is less explicit than Coleman integration. Besser will report on progress, due in part to work with several researchers, on the relation between Vologodsky integration and Coleman integration.

Kęstutis Česnavičius
Centre national de la recherche scientifique, Université Paris-Sud

Cohomological Purity in Bad Characteristics

An fppf cohomological purity conjecture predicts that for a regular (or, more generally, complete intersection) Noetherian local ring (R, m) and a commutative finite flat R-group scheme G, one should have \(H^i_m(R, G) = 0\) for \(i < dim(R)\). Česnavičius will discuss several cases of this conjecture. The talk is based on joint work with Peter Scholze.

Christopher Deninger
Universität Münster
-and-
Annette Werner
Goethe-Universität Frankfurt

Vector Bundles and 𝑝-adic Representations I and II

Deninger and Werner introduce the category of vector bundles with numerically flat reduction on a 𝑝-adic variety and construct a functorial étale parallel transport for such bundles. This strengthens Faltings’ higher dimensional 𝑝-adic Simpson correspondence in the case of vanishing Higgs fields. The first talk by Annette Werner is expository and also explains some stronger results and examples in the case of curves, while the second, by Christopher Deninger, outlines the proof.

Hélène Esnault
Freie Universität Berlin

Arithmetic Subspaces of Moduli Spaces of Rank One Local Systems

Esnault shows that closed subsets of the character variety of a complex variety with negatively weighted homology, which are 𝑝-adically integral and Galois invariant, are motivic. This is joint work with Moritz Kerz.

Gerd Faltings
Max Planck Institute for Mathematics

A 𝑝-adic Simpson Correspondence

Faltings explains his old paper in the title. The main method is a local calculation in 𝑝-adic Hodge theory, which implies the result for small representations, but there are also some interesting complications for big representations.

Ofer Gabber
(CNRS & IHES)

Almost duality for nearby cycles of \(O^+/p\)

We discuss several points in the approaches of Faltings and Scholze to p-adic Hodge theory and Grothendieck duality in this context. Let \(K\) be an algebraically closed complete rank 1 valued field with valuation ring \(O_K\) of mixed characteristic \((0, p)\), \(X\) a proper smooth connected rigid analytic space over \(K\) of dimension \(d\) with normal formal model \(\mathcal{X}\) over Spf \(O_K\). To show Poincaré duality for \(H^∗(X_{et},\mathbb{Z}/p)\) one considers the “nearby cycle” complex \(R\psi_∗(O^+/p)\) on \(\mathcal{X}/p\) ; it has bounded almost coherent cohomology and one observes that \(\mathcal{H}^dR\psi_∗(O^+/p)(d)\) has a canonical almost map to the dualizing sheaf; we show that this induces an almost autoduality by means of local uniformization by quotients of nice formal models by finite groups. We also discuss how to get a duality theorem for proper morphisms of rigid analytic spaces and complexes of \(\mathbb{Z}/p^n\) sheaves with locally bounded constructible cohomology.

Minhyong Kim
Oxford University

Principal Bundles and Diophantine Geometry: Some Remarks on Effectivity

Much of the interest surrounding the non-abelian method of Chabauty comes from the hope of using it to compute completely the set of rational points on a curve of higher genus. Kim will make some observations on this.

Daniel Litt
Institute for Advanced Study

Monodromy and Galois Actions on Deformation Rings

Let K be a number field (resp. function field of a curve over a fixed finite field k) of degree (resp. gonality) g. The torsion conjecture predicts that there is a constant N=N(g,d) such that if A is a (traceless) d-dimensional Abelian variety over K, then \(\# |A(K)|_{\mathrm{tors}} < N\). The Frey-Mazur conjecture predicts that there is a constant N′=N′(g,d) such that if A₁, A₂ are (traceless) d-dimensional Abelian varieties over K, then A₁ is isogenous to A₂ if and only if A₁[lʳ] is isomorphic to A₂[lʳ], for lʳ > N′ and prime to the characteristic of K. Litt discusses new results toward the l-primary versions of these conjectures in the function field setting.

Jacob Lurie
Harvard University

Full Level Structures on Elliptic Curves

Let p be a prime number, and let M denote the moduli stack of elliptic curves equipped with an infinite level structure at the prime p. Scholze has shown that the generic fiber of M is a perfectoid space. In this talk, Lurie will discuss an integral refinement of this result, as well as some ‘approximate’ results that hold at finite level.

Matthew Morrow
Centre national de la recherche scientifique & Institut de Mathématiques de Jussieu-Paris Rive Gauche
-and-
Takeshi Tsuji
University of Tokyo

Coefficients in Integral 𝑝-adic Hodge Theory via Generalized A_inf-representations and q-connections

In these talks, Morrow and Tsuji will present an overview of their work on coefficients in integral 𝑝-adic Hodge theory. To any smooth formal scheme over the ring of integers of the completed algebraic closure of a 𝑝-adic field, they associate a category of ‘relative Breuil-Kisin-Fargues’ modules; these are defined as certain locally free modules over the A_inf-period sheaf on the pro-étale site of the generic fiber of X. Any relative BKF module gives rise to a vector bundle with connection, an F-crystal on the special fiber, and a crystalline lisse Z_p-sheaf on the generic fiber; their cohomologies are then intertwined by a relative form of the A_inf-cohomology from Bhatt-M.-Scholze. These relative BKF modules are closely related to other notions: they include Faltings’ relative Fontaine-Laffaille modules; on any small opening they are given by certain small generalized representations as in Faltings’ and Gros-Abbes-T.’s work on the 𝑝-adic Simpsons correspondence, and given a choice of coordinates, they correspond to modules with a q-connection, in the sense of q-de Rham cohomology, as a result of which they also admit an interpretation in terms of Bhatt-Scholze’s theory of prismatic cohomology.

Arthur Ogus
University of California, Berkeley

The Saturated de Rham-Witt Complex, after Bhatt, Lurie, Mathew

Let X/k be a smooth scheme over a perfect field of characteristic p. The de Rham-Witt complex, constructed by Illusie (with roots in work by Bloch, Lubkin and Deligne) is a canonical sheaf of differential graded algebras on the Zariski site, which computes the crystalline cohomology of X/W and which reveals a great deal of information about the action of the Frobenius endomorphism of X. Bhatt, Lurie and Mathew have recently given a simple, new construction of this complex which, unlike the original, seems to give reasonable answers for (at least some) singular schemes. Ogus will explain the main points of the new construction and how it works for schemes with toric singularities.

Peter Scholze
Mathematisches Institut der Universität Bonn

Some Results on the Cohomology of Finite Flat Group Schemes

Scholze proves some results on the cohomology of finite flat group schemes, such as fpqc descent, formal gluing and some purity results. This gives new proofs of Grothendieck’s purity conjectures in etale cohomology and for the Brauer group, and some new results (e.g., that, on complete intersections, torsion line bundles extend over codimension 3, and Brauer classes extend over codimension 4, as conjectured by Gabber). The proofs proceed by reduction to perfectoid rings. This is joint with Kęstutis Česnavičius.

Vadim Vologodsky
Higher School of Economics

On the Periodic Topological Cyclic Homology of (DG) Algebras in Characteristic p

Vologodsky proves that the periodic topological cyclic homology of a smooth proper DG algebra over F_p is isomorphic to the periodic (algebraic) cyclic homology of a lifting of the algebra over Z_p. This is joint work with Alexander Petrov.

Jared Weinstein
Boston University

The Cohomology of Local Shtuka Spaces

The local Langlands correspondence predicts that representations of a reductive group G over a 𝑝-adic field are related to Galois representations into the Langlands dual of G. A suitably generalized conjecture of Kottwitz asserts that this relationship appears in a precise way in the cohomology of Scholze’s local shtuka spaces. We don’t know how Galois acts on this cohomology yet, but we can verify much of the rest of the conjecture, using a Lefschetz-Verdier fixed point formula. This is joint work with Tasho Kaletha.

Xinwen Zhu
California Institute of Technology

𝑝-adic Riemann-Hilbert Functors and Applications

Zhu will review his recent works in the construction of 𝑝-adic Riemann-Hilbert functors for 𝑝-adic local systems on rigid analytic varieties. He will also discuss a few applications of the general theory. The talk is based on joint work with R. Liu, H. Diao, K.W. Lan and R. Liu.

Ahmed Abbes
Centre national de la recherche scientifique & Institut des Hautes Études Scientifiques
Local Structure of Almost-étale \(\varphi\)-modules (PDF)

Daniel Litt
Institute for Advanced Study
Monodromy and Galois Actions on Deformation Rings (PDF)

Notes taken by Kestutis Cesnavicus:
Cesnavicius: Cohomological purity in bad characteristic (PDF)
Deninger: Vector bundles and 𝑝-adic parallel transport II (PDF)
Litt: Monodromy and Galois action on _pi_1^_et (PDF)
Liu: 𝑝-adic Riemann-Hilbert correspondence and applications (PDF)
Ogus: The saturated de Rham-Witt complex after Bhatt, Lurie, Mathew (PDF)
Scholze: Some results on the cohomology of finite flat group schemes (PDF)
Vologodsky: On periodic topological cyclic homology of (dg) algebras in characteristic p (PDF)
Weinstein: Cohomology of moduli spaces of local shtukas (PDF)