Simion Filip

Papers

• Gaps in the support of canonical currents on projective K3 surfaces

  
  with V. Tosatti
  [ arXiv ] Abstract ±
  We construct examples of canonical closed positive currents on projective K3 surfaces that are not fully supported on the complex points. The currents are the unique positive representatives in their cohomology classes and have vanishing self-intersection. The only previously known such examples were due to McMullen on non-projective K3 surfaces and were constructed using positive entropy automorphisms with a Siegel disk. Our construction is based on a Zassenhaus-type estimate for commutators of automorphisms.

• Translation Surfaces: Dynamics and Hodge Theory

  
  [ pdf ] Abstract ±
  A translation surface is a multifaceted object that can be studied with the tools of dynamics, analysis, or algebraic geometry. Moduli spaces of translation surfaces exhibit equally rich features. This survey provides an introduction to the subject and describes some developments that make use of Hodge theory to establish algebraization and finiteness statements in moduli spaces of translation surfaces.

• Uniformization of some weight 3 variations of Hodge structure, Anosov representations, and Lyapunov exponents

  
  [ arXiv ] Abstract ±
  We develop a class of uniformizations for certain weight 3 variations of Hodge structure (VHS). The analytic properties of the VHS are used to establish a conjecture of Eskin, Kontsevich, Möller, and Zorich on Lyapunov exponents. Additionally, we prove that the monodromy representations are log-Anosov, a dynamical property that has a number of global consequences for the VHS. We establish a strong Torelli theorem for the VHS and describe appropriate domains of discontinuity.
  Additionally, we classify the hypergeometric differential equations that satisfy our assumptions. We obtain several multi-parameter families of equations, which include the mirror quintic as well as the six other thin cases of Doran-Morgan and Brav-Thomas.

• A cyclotomic family of thin hypergeometric monodromy groups in \(\operatorname{Sp}_4(\mathbb{R})\)

  
  with C. Fougeron
  [ arXiv ] Abstract ±
  We exhibit an infinite family of discrete subgroups of \(\operatorname{Sp}_4(\mathbb{R})\) which have a number of remarkable properties. Our results are established by showing that each group plays ping-pong on an appropriate set of cones. The groups arise as the monodromy of hypergeometric differential equations with parameters \(\left(\tfrac{N-3}{2N},\tfrac{N-1}{2N},\tfrac{N+1}{2N},\tfrac{N+3}{2N}\right)\) at infinity and maximal unipotent monodromy at zero, for any integer \(N\geq 4\). Additionally, we relate the cones used for ping-pong in \(\mathbb{R}^4\) with crooked surfaces, which we then use to exhibit domains of discontinuity for the monodromy groups in the Lagrangian Grassmannian.

• Canonical currents and heights for K3 surfaces

  
  with V. Tosatti
  accepted, Cambridge Journal of Mathematics
  [ arXiv ] Abstract ±
  We construct canonical positive currents and heights on the boundary of the ample cone of a K3 surface. These are equivariant for the automorphism group and fit together into a continuous family, defined over an enlarged boundary of the ample cone. Along the way, we construct preferred representatives for certain height functions and currents on elliptically fibered surfaces.

• Asymptotic shifting numbers in triangulated categories

  
  with Y.-W. Fan
  [ arXiv ] Abstract ±
  We introduce invariants, called shifting numbers, that measure the asymptotic amount by which an autoequivalence of a triangulated category translates inside the category. The invariants are analogous to Poincare translation numbers that are widely used in dynamical systems. We additionally establish that in some examples the shifting numbers provide a quasimorphism on the group of autoequivalences. Additionally, we relate our shifting numbers to the entropy function introduced by Dimitrov, Haiden, Katzarkov, and Kontsevich.

• On pseudo-Anosov autoequivalences

  
  with Y.-W. Fan, F. Haiden, L. Katzarkov, Y. Liu
  Advances in Mathematics (2021), vol. 384
  [ arXiv | journal ] Abstract ±
  Motivated by results of Thurston, we prove that any autoequivalence on a triangulated category induces a canonical filtration by triangulated subcategories, provided the existence of Bridgeland stability conditions. We then propose a new definition of pseudo-Anosov autoequivalences, and prove that our definition is more general than the one previously proposed by Dimitrov, Haiden, Katzarkov, and Kontsevich. We construct new examples of pseudo-Anosov autoequivalences on the derived categories of quintic Calabi-Yau threefolds and quiver Calabi-Yau categories. Finally, we prove that these new examples of pseudo-Anosov autoequivalences on quiver 3-Calabi-Yau categories act hyperbolically on the space of Bridgeland stability conditions.

• Tropical Dynamics of area-preserving maps

  
  Journal of Modern Dynamics 14 (2019), pp. 179–226
  [ arXiv | journal ] Abstract ±
  We consider a class of area-preserving, piecewise affine maps on the 2-sphere. These maps encode degenerating families of K3 surface automorphisms and are profitably studied using techniques from tropical and Berkovich geometries.

• Kummer rigidity for K3 surface automorphisms via Ricci-flat metrics

  
  with V. Tosatti
  American Journal of Mathematics (2021) vol. 143, no. 5, pp. 1431–1462
  [ arXiv | journal ] Abstract ±
  We give an alternative proof of a result of Cantat and Dupont, showing that any automorphism of a K3 surface with measure of maximal entropy in the Lebesgue class must be a Kummer example. Our method exploits the existence of Ricci-flat metrics on K3s and also covers the non-projective case.

• Smooth and Rough Positive Currents

  with V. Tosatti
  Ann. Inst. Fourier, Grenoble (2018), vol. 68, no. 7, pp. 2981-2999
  [ arXiv | journal ] Abstract ±
  We study the different notions of semipositivity for (1,1) cohomology classes on K3 surfaces. We first show that every big and nef class (and every nef and rational class) is semiample, and in particular it contains a smooth semipositive representative. By contrast, a result of Cantat and Dupont implies that there exist irrational nef classes with no closed positive current representative which is smooth outside a proper analytic subset. We use this to answer negatively two questions of the second-named author.

• The algebraic hull of the Kontsevich-Zorich cocycle

  with A. Eskin and A. Wright
  Ann. of Math. (2) 188 (2018), no. 1, 281–313
  [ arXiv | journal ] Abstract ±
  We compute the algebraic hull of the Kontsevich-Zorich cocycle over any \(GL^+_2(R)\) invariant subvariety of the Hodge bundle, and derive from this finiteness results on such subvarieties.

• Counting special Lagrangian fibrations in twistor families of K3 surfaces

  
  with an appendix by N. Bergeron and C. Matheus
  Ann. Sci. Éc. Norm. Supér. (4), 53(3):713–750, 2020
  [ arXiv | journal ] Abstract ±
  The number of closed billiard trajectories in a rational-angled polygon grows quadratically in the length. This paper gives an analogue on K3 surfaces, by considering special Lagrangian tori. The analogue of the angle of a billiard trajectory is a point on a twistor sphere, and the number of directions admitting a special Lagrangian torus fibration with volume bounded by V grows like \(V^{20}\).

• Families of K3 surfaces and Lyapunov exponents

  
  Israel J. Math. 226 (2018), no. 1, 29–69
  [ arXiv | journal ] Abstract ±
  Consider a family of K3 surfaces over a hyperbolic curve (i.e. Riemann surface). Their second cohomology groups form a local system, and we show that its top Lyapunov exponent is a rational number. One proof uses the Kuga-Satake construction, which reduces the question to Hodge structures of weight 1. A second proof uses integration by parts. The case of maximal Lyapunov exponent corresponds to modular families, given by the Kummer construction on a product of isogenous elliptic curves.

• Quaternionic covers and monodromy of the Kontsevich-Zorich cocycle in orthogonal groups

  with G. Forni and C. Matheus
  J. Eur. Math. Soc. (JEMS) 20 (2018), no. 1, 165–198
  [ arXiv | journal ] Abstract ±
  We give an example of a Teichmüller curve which contains, in a factor of its monodromy, a group which was not observed before. Namely, it has Zariski closure equal to the group \(\operatorname{SO}^*(6) \) in its standard representation; up to finite index, this is the same as \(\operatorname{SU}(3,1)\) in its second exterior power representation. The example is constructed using origamis (i.e. square-tiled surfaces). It can be generalized to give monodromy inside the group \( \operatorname{SO}^*(2n)\) for all n, but in the general case the monodromy might split further inside the group. Also, we take the opportunity to compute the multiplicities of representations in the (0,1) part of the cohomology of regular origamis, answering a question of Matheus-Yoccoz-Zmiaikou.

• Zero Lyapunov exponents and monodromy of the Kontsevich-Zorich cocycle

  
  Duke Math. J. 166 (2017), no. 4, 657–706
  [ arXiv | journal ] Abstract ±
  We describe the situations in which the Kontsevich-Zorich cocycle has zero Lyapunov exponents. Confirming a conjecture of Forni, Matheus, and Zorich, this only occurs when the cocycle satisfies additional geometric constraints. We also describe the real Lie groups which can appear in the monodromy of the Kontsevich-Zorich cocycle. The number of zero exponents is then as small as possible, given its monodromy.

• Splitting mixed Hodge structures over affine invariant manifolds

  
  Ann. of Math. (2) 183 (2016), no. 2, 681–713
  [ arXiv | journal | Blog post ] Abstract ±
  We prove that affine invariant manifolds in strata of flat surfaces are algebraic varieties. The result is deduced from a generalization of a theorem of Möller. Namely, we prove that the image of a certain twisted Abel-Jacobi map lands in the torsion of a factor of the Jacobians. This statement can be viewed as a splitting of certain mixed Hodge structures.

• Semisimplicity and rigidity of the Kontsevich-Zorich Cocycle

  
  Invent. Math. 205 (2016), no. 3, 617–670
  [ arXiv | journal | Blog post ] Abstract ±
  We prove that invariant subbundles of the Kontsevich-Zorich cocycle respect the Hodge structure. In particular, we establish a version of Deligne semisimplicity in this context. This implies that invariant subbundles must vary polynomially on affine manifolds. All results apply to tensor powers of the cocycle and this implies that the measurable and real-analytic algebraic hulls coincide. We also prove that affine manifolds parametrize Jacobians with non-trivial endomorphisms. Typically a factor has real multiplication. The tools involve curvature properties of the Hodge bundles and estimates from random walks. In the appendix, we explain how methods from ergodic theory imply some of the global consequences of Schmid's work on variations of Hodge structures. We also derive the Kontsevich-Forni formula using differential geometry.

• On Hölder-continuity of Oseledets subspaces

  with V. Araujo and A. Bufetov
  J. Lond. Math. Soc. (2) 93 (2016), no. 1, 194–218
  [ arXiv | journal ] Abstract ±
  For Hölder cocycles over a Lipschitz base transformation, possibly non-invertible, we show that the subbundles given by the Oseledets Theorem are Hoelder-continuous on compact sets of measure arbitrarily close to 1. The results extend to vector bundle automorphisms, as well as to the Kontsevich-Zorich cocycle over the Teichmüller flow on the moduli space of abelian differentials. Following a recent result of Chaika-Eskin, our results also extend to any given Teichmüller disk.

Teaching, Notes, Seminars, etc.

BiSTRO seminar

Teaching

Expository

• Translation Surfaces: Dynamics and Hodge Theory

  
  [ pdf ] Abstract ±
  A translation surface is a multifaceted object that can be studied with the tools of dynamics, analysis, or algebraic geometry. Moduli spaces of translation surfaces exhibit equally rich features. This survey provides an introduction to the subject and describes some developments that make use of Hodge theory to establish algebraization and finiteness statements in moduli spaces of translation surfaces.

• An introduction to hypergeometric equations, via D-modules

  
  Abstract ±
  In progress, available upon request.

• Geometry and dynamics on Riemann and K3 surfaces

  
  Eur. Math. Soc. Mag. 119 (2021), 17–22
  [ journal ] Abstract ±
  Surfaces are some of the simplest yet geometrically rich manifolds. Geometric structures on surfaces illuminate their topology and are useful for studying dynamical systems on surfaces. We illustrate below how some of these concepts blend together, and relate them to algebraic geometry.

• An introduction to K3 surfaces and their dynamics

  
  [ pdf ] Abstract ±
  These notes provide an introduction to the geometry of K3 surfaces and the dynamics of their automorphisms. The notes are based on lectures delivered in Grenoble in July 2018, and in Beijing in July 2019.

• Lectures on the Oseledets Multiplicative Ergodic Theorem

  
  Ergodic Theory and Dynamical Systems (2019) vol. 39(5), 1153-1189
  [ pdf | arxiv | journal version ] Abstract ±
  The Oseledets Multiplicative Ergodic theorem is a basic result with numerous applications throughout dynamical systems. These notes provide an introduction to this theorem, as well as subsequent generalizations. They are based on lectures at summer schools in Brazil and France.

• The Lie algebra of the fundamental group of a surface as a symplectic module

  
  [ arXiv ] Abstract ±
  This note provides a formula for the character of the Lie algebra of the fundamental group of a surface, viewed as a module over the symplectic group.

Student Seminars (old)