## Periodic Reporting for period 4 - K3CRYSTAL (Moduli of Crystals and K3 Surfaces)

Reporting period: 2021-04-01 to 2021-09-30

PROJECT DESCRIPTION

Algebraic geometry deals with algebraic varieties, that is, systems of polynomial equations and their geometric interpretation. Its ultimate goal is the classification of all algebraic varieties. For a detailed understanding, one has to construct their moduli spaces, and eventually study them over the integers, that is, in the arithmetic situation. So far, the best results are available for curves and Abelian varieties.

To go beyond the aforementioned classes, I want to study arithmetic moduli spaces of the only other classes that are currently within reach, namely, K3 surfaces, Enriques surfaces, and Hyperkähler varieties. I expect this study to lead to finer invariants, to new stratifications of moduli spaces, and to open new research areas in arithmetic algebraic geometry.

Next, I propose a systematic study of supersingular varieties, which are the most mysterious class of varieties in positive characteristic. Again, a good theory is available only for Abelian varieties, but recently, I established a general framework via deformations controlled by formal group laws. I expect to extend this also to constructions in complex geometry, such as twistor space, which would link so far completely unrelated fields of research.

I want to accompany these projects by developing a general theory of period maps and period domains for F-crystals, with an emphasis on the supersingular ones to start with. This will be the framework for Torelli theorems that translate the geometry and moduli of K3 surfaces, Enriques surfaces, and Hyperkähler varieties into explicit linear algebra problems, thereby establishing new tools in algebraic geometry.

Algebraic geometry deals with algebraic varieties, that is, systems of polynomial equations and their geometric interpretation. Its ultimate goal is the classification of all algebraic varieties. For a detailed understanding, one has to construct their moduli spaces, and eventually study them over the integers, that is, in the arithmetic situation. So far, the best results are available for curves and Abelian varieties.

To go beyond the aforementioned classes, I want to study arithmetic moduli spaces of the only other classes that are currently within reach, namely, K3 surfaces, Enriques surfaces, and Hyperkähler varieties. I expect this study to lead to finer invariants, to new stratifications of moduli spaces, and to open new research areas in arithmetic algebraic geometry.

Next, I propose a systematic study of supersingular varieties, which are the most mysterious class of varieties in positive characteristic. Again, a good theory is available only for Abelian varieties, but recently, I established a general framework via deformations controlled by formal group laws. I expect to extend this also to constructions in complex geometry, such as twistor space, which would link so far completely unrelated fields of research.

I want to accompany these projects by developing a general theory of period maps and period domains for F-crystals, with an emphasis on the supersingular ones to start with. This will be the framework for Torelli theorems that translate the geometry and moduli of K3 surfaces, Enriques surfaces, and Hyperkähler varieties into explicit linear algebra problems, thereby establishing new tools in algebraic geometry.

Before one can study Torelli theorems, one often first needs theorems on "good reduction", which are well-known for Abelian varieties. For K3 surfaces, a complete and thorough treatment of such Néron-Ogg-Shafarevich theorems has now been achieved in (12), (17), and (18). Such results are important in themselves, but also crucial for understanding the images of period maps. A Torelli theorem for ordinary Enriques surfaces was established in (6), the supersingular case is dealt with (4) and (5).

When studying supersingular and uniruled varieties, one has to understand existence and deformations of rational curves on these varieties. In (14), we studied the deformation theory of rational curves and established rigidity criteria, which is an important step towards understanding supersingular and uniruled varieties. Moreover, in (3), (7) and (8), we studied deformations, degenerations, and regenerations of arbitrary curves on K3 surfaces, which uses rational curves, and which has led to a proof of the longstanding conjecture that every K3 surface in characteristic zero contains infinitely many rational curves.

To understand period maps near the boundary, one should study maximally degenerating varieties. In (16), we studied this problem for Abelian varieties from the point of view of filtered (phi,N)-modules. This led to disproving a conjecture of Raskind. In (10), Picard numbers of Abelian varieties are studied in positive characteristic. The study of crystals for K3 surfaces relies upon a better understanding of the deRham-Witt complex and using this language, foundational results were established in (11) and (15).

Finite group schemes, torsors under them, and quotients by them are an important tool in characteristic p geometry. Here, we studied quotient singularities and torsors over them in (1) and (2), which opens new directions.

EVENTS RELATED TO THE PROJECT

In April 2018, we organised a conference "Crystals and Geometry in Characteristic p" with 50 participants, among them 12 speakers from all over the world.

In July 2019, we organised a workshop on "Logarithmic Algebraic Geometry" with 11 participants and 9 mostly speakers from Germany and Italy.

In January 2020, we organised a one day conference "Moduli Day" with 15 participants and 4 speakers from France, Germany, and Switzerland

In April 2020, we had planned a workshop on "Periods, motives, and differential equations". Due to the Corona pandemic, it had to be postponed. New date: April 2022.

PROJECT RELATED PREPRINTS AND PUBLICATIONS

(1) Christian Liedtke, Yuya Matsumoto, Gebhard Martin, Torsors over the rational double points in characteristic p, preprint,76 pages.

(2) Christian Liedtke, Yuya Matsumoto, Gebhard Martin, Linearly reductive quotient singularities, preprint, 53 pages.

(3) Xi Chen, Frank Gounelas, Curves of maximal moduli on K3 surfaces, preprint, 22 pages.

(4) Kai Behrens, On the number of Enriques quotients for supersingular K3 surfaces, preprint, 27 pages.

(5) Kai Behrens, A moduli space for supersingular Enriques surfaces, preprint, 27 pages.

(6) Roberto Laface, Sofia Tirabassi, Ordinary Enriques surfaces in positive characteristic, preprint, 12 pages.

(7) Xi Chen, Frank Gounelas, Christian Liedtke, Curves on K3 surfaces, preprint (submitted), 38 pages.

(8) Xi Chen, Frank Gounelas, Christian Liedtke, Rational curves on lattice-polarised K3 surfaces, preprint (submitted), 37 pages.

(9) Olof Bergvall, Frank Gounelas, Cohomology of moduli spaces of Del Pezzo surfaces, preprint, 26 pages.

(10) Roberto Laface, On the Picard numbers of Abelian varieties in positive characteristic, preprint (submitted), 17 pages.

(11) Oliver Gregory, Andreas Langer, Higher displays arising from filtered de Rham-Witt complexes, preprint (submitted), 13 pages.

(12) Bruno Chiarellotto, Christopher Lazda, Christian Liedtke, Good Reduction of K3 Surfaces in Equicharacteristic p, to appear in Ann. Sc. Norm. Super. Pisa Cl. Sci.

(13) Frank Gounelas, John Ottem, Remarks on the positivity of the cotangent bundle of a K3 surface, Épijournal Géom. Algébrique 4 (2020), Art. 8, 16 pp.

(14) Kazuhiro Ito, Tetsushi Ito, Kazuhiro Ito, Deformations of Rational Curves in Positive Characteristic, J. Reine Angew. Math. 769 (2020), 55-86.

(15) Oliver Gregory, Andreas Langer, Overconvergent de Rham-Witt cohomology for semistable varieties, Münster J. Math. 13 (2020), no. 2, 541–571.

(16) Oliver Gregory, Christian Liedtke, p-adic Tate conjectures and abeloid varieties, Doc. Math. 24 (2019), 1879-1934.

(17) Bruno Chiarellotto, Christopher Lazda, Christian Liedtke, A Néron-Ogg-Shafarevich criterion for K3 surfaces, Proc. Lond. Math. Soc. 119 (2019), 469-514.

(18) Christian Liedtke, Yuya Matsumoto, Good Reduction of K3 Surfaces, Compos. Math. 154 (2018), 1–35.

When studying supersingular and uniruled varieties, one has to understand existence and deformations of rational curves on these varieties. In (14), we studied the deformation theory of rational curves and established rigidity criteria, which is an important step towards understanding supersingular and uniruled varieties. Moreover, in (3), (7) and (8), we studied deformations, degenerations, and regenerations of arbitrary curves on K3 surfaces, which uses rational curves, and which has led to a proof of the longstanding conjecture that every K3 surface in characteristic zero contains infinitely many rational curves.

To understand period maps near the boundary, one should study maximally degenerating varieties. In (16), we studied this problem for Abelian varieties from the point of view of filtered (phi,N)-modules. This led to disproving a conjecture of Raskind. In (10), Picard numbers of Abelian varieties are studied in positive characteristic. The study of crystals for K3 surfaces relies upon a better understanding of the deRham-Witt complex and using this language, foundational results were established in (11) and (15).

Finite group schemes, torsors under them, and quotients by them are an important tool in characteristic p geometry. Here, we studied quotient singularities and torsors over them in (1) and (2), which opens new directions.

EVENTS RELATED TO THE PROJECT

In April 2018, we organised a conference "Crystals and Geometry in Characteristic p" with 50 participants, among them 12 speakers from all over the world.

In July 2019, we organised a workshop on "Logarithmic Algebraic Geometry" with 11 participants and 9 mostly speakers from Germany and Italy.

In January 2020, we organised a one day conference "Moduli Day" with 15 participants and 4 speakers from France, Germany, and Switzerland

In April 2020, we had planned a workshop on "Periods, motives, and differential equations". Due to the Corona pandemic, it had to be postponed. New date: April 2022.

PROJECT RELATED PREPRINTS AND PUBLICATIONS

(1) Christian Liedtke, Yuya Matsumoto, Gebhard Martin, Torsors over the rational double points in characteristic p, preprint,76 pages.

(2) Christian Liedtke, Yuya Matsumoto, Gebhard Martin, Linearly reductive quotient singularities, preprint, 53 pages.

(3) Xi Chen, Frank Gounelas, Curves of maximal moduli on K3 surfaces, preprint, 22 pages.

(4) Kai Behrens, On the number of Enriques quotients for supersingular K3 surfaces, preprint, 27 pages.

(5) Kai Behrens, A moduli space for supersingular Enriques surfaces, preprint, 27 pages.

(6) Roberto Laface, Sofia Tirabassi, Ordinary Enriques surfaces in positive characteristic, preprint, 12 pages.

(7) Xi Chen, Frank Gounelas, Christian Liedtke, Curves on K3 surfaces, preprint (submitted), 38 pages.

(8) Xi Chen, Frank Gounelas, Christian Liedtke, Rational curves on lattice-polarised K3 surfaces, preprint (submitted), 37 pages.

(9) Olof Bergvall, Frank Gounelas, Cohomology of moduli spaces of Del Pezzo surfaces, preprint, 26 pages.

(10) Roberto Laface, On the Picard numbers of Abelian varieties in positive characteristic, preprint (submitted), 17 pages.

(11) Oliver Gregory, Andreas Langer, Higher displays arising from filtered de Rham-Witt complexes, preprint (submitted), 13 pages.

(12) Bruno Chiarellotto, Christopher Lazda, Christian Liedtke, Good Reduction of K3 Surfaces in Equicharacteristic p, to appear in Ann. Sc. Norm. Super. Pisa Cl. Sci.

(13) Frank Gounelas, John Ottem, Remarks on the positivity of the cotangent bundle of a K3 surface, Épijournal Géom. Algébrique 4 (2020), Art. 8, 16 pp.

(14) Kazuhiro Ito, Tetsushi Ito, Kazuhiro Ito, Deformations of Rational Curves in Positive Characteristic, J. Reine Angew. Math. 769 (2020), 55-86.

(15) Oliver Gregory, Andreas Langer, Overconvergent de Rham-Witt cohomology for semistable varieties, Münster J. Math. 13 (2020), no. 2, 541–571.

(16) Oliver Gregory, Christian Liedtke, p-adic Tate conjectures and abeloid varieties, Doc. Math. 24 (2019), 1879-1934.

(17) Bruno Chiarellotto, Christopher Lazda, Christian Liedtke, A Néron-Ogg-Shafarevich criterion for K3 surfaces, Proc. Lond. Math. Soc. 119 (2019), 469-514.

(18) Christian Liedtke, Yuya Matsumoto, Good Reduction of K3 Surfaces, Compos. Math. 154 (2018), 1–35.

CRITERIA FOR GOOD REDUCTION AND TOWARDS TORELLI THEOREMS

In (12), (17), and (18), a Néron-Ogg-Shafarevich criterion for K3 surfaces was found: important in itself, but also for Torelli theorems. In (16), we explored period maps under maximal degeneration and disproved a conjecture of Raskind. In (5) and (6), Torelli theorems for Enriques surfaces in characteristic p>0 were established.

EXISTENCE AND DEFORMATIONS OF CURVES

An approach to unirationality via deformations of rational curves was established in (14). In (3), (7) and (8), we explored a new deformation and a new degeneration technique for curves on K3 surfaces. As an application, which established a long-standing conjecture concerning curves on K3 surfaces. In (10), the situation of Picard ranks for Abelian varieties in characteristic p was clarified.

TORSORS AND FINITE GROUP SCHEMES

To understand geometry in characteristic p, (1) and (2) address group scheme quotients, torsors over singularities, and techniques to classify them.

In (12), (17), and (18), a Néron-Ogg-Shafarevich criterion for K3 surfaces was found: important in itself, but also for Torelli theorems. In (16), we explored period maps under maximal degeneration and disproved a conjecture of Raskind. In (5) and (6), Torelli theorems for Enriques surfaces in characteristic p>0 were established.

EXISTENCE AND DEFORMATIONS OF CURVES

An approach to unirationality via deformations of rational curves was established in (14). In (3), (7) and (8), we explored a new deformation and a new degeneration technique for curves on K3 surfaces. As an application, which established a long-standing conjecture concerning curves on K3 surfaces. In (10), the situation of Picard ranks for Abelian varieties in characteristic p was clarified.

TORSORS AND FINITE GROUP SCHEMES

To understand geometry in characteristic p, (1) and (2) address group scheme quotients, torsors over singularities, and techniques to classify them.