Les strates ne possèdent pas de variétés complètes

We compute many new classes of effective divisors in M¯g,n coming from the strata of Abelian differentials. Our method utilizes maps between moduli spaces and the degeneration of Abelian differentials.

We study the transcendence of periods of abelian differentials, both at the arithmetic and functional level, from the point of view of the natural bi-algebraic structure on strata of abelian differentials. We characterize geometrically the arithmetic points, study their distribution, and prove that in many cases the bi-algebraic curves are the linear ones.

This paper focuses on the interplay between the intersection theoryand the Teichmüller dynamics on the moduli space of curves. Asapplications, we study the cycle class of strata of the Hodge bundle,present an algebraic method to calculate the class of the divisorparameterizing abelian differentials with a nonsimple zero, andverify a number of extremal effective divisors on the moduli space ofpointed curves in low genus.

An abelian differential on a surface defines a flat metric and a vector field on the complement of a finite set of points. The vertical flow that can be defined on the surface has two kinds of invariant closed sets (i.e. invariant components) — periodic components and minimal components. We give upper bounds on the number of minimal components, on the number of periodic components and on the total number of invariant components in every stratum of abelian differentials. We also show that these bounds are tight in every stratum.

On the space of ergodic measures for the horocycle flow on strata of Abelian differentials

We show that an algebraic subvariety of the moduli space of genus g Riemann surfaces is coarsely dense with respect to the Teichmüller metric (or Thurston metric) if and only if it has full dimension. We apply this to determine which strata of abelian differentials have coarsely dense projection to moduli space. Furthermore, we prove a result on coarse density of projections of GL 2 (ℝ)-orbit closures in the space of abelian differentials.

A cyclic cover of the complex projective line branched at four appropriate points has a natural structure of a square-tiled surface. We describe the combinatorics of such a square-tiled surface, the geometry of the corresponding Teichmüller curve, and compute the Lyapunov exponents of the determinant bundle over the Teichmüller curve with respect to the geodesic flow. This paper includes a new example (announced by G. Forni and C. Matheus in [17] of a Teichmüller curve of a square-tiled cyclic cover in a stratum of Abelian differentials in genus four with a maximally degenerate Kontsevich--Zorich spectrum (the only known example in genus three found previously by Forni also corresponds to a square-tiled cyclic cover [15]. We present several new examples of Teichmüller curves in strata of holomorphic and meromorphic quadratic differentials with a maximally degenerate Kontsevich--Zorich spectrum. Presumably, these examples cover all possible Teichmüller curves with maximally degenerate spectra. We prove that this is indeed the case within the class of square-tiled cyclic covers.

We show that for many strata of Abelian differentials in low genus the sum of Lyapunov exponents for the Teichmuller geodesic flow is the same for all Teichmuller curves in that stratum, hence equal to the sum of Lyapunov exponents for the whole stratum. This behavior is due to the disjointness property of Teichmuller curves with various geometrically defined divisors on moduli spaces of curves. 14H10; 37D40, 14H51

We show that the Masur–Veech volumes and area Siegel–Veech constants can be obtained using intersection theory on strata of Abelian differentials with prescribed orders of zeros. As applications, we evaluate their large genus limits and compute the saddle connection Siegel–Veech constants for all strata. We also show that the same results hold for the spin and hyperelliptic components of the strata.

We introduce a twisted cohomology cocycle over the Teichmüller flow and prove a “spectral gap” for its Lyapunov spectrum with respect to the Masur–Veech measures. We then derive Hölder estimates on spectral measures and bounds on the speed of weak mixing for almost all translation flows in every stratum of Abelian differentials on Riemann surfaces, as well as bounds on the deviation of ergodic averages for product translation flows on the product of a translation surface with a circle.

On the effective cone of [formula omitted

Birational Geometry and Logarithmic Forms

Introduction This paper surveys some aspects of the theory of abelian varieties relevant to the Pila–Zannier proof of the Manin–Mumford conjecture and to the Andre– Oort conjecture. An abelian variety is a complete algebraic variety with a group law. The geometry of abelian varieties is tightly constrained and well-behaved, and they are important tools in algebraic geometry. Abelian varieties defined over number fields pose interesting arithmetic problems, for example concerning their rational points and associated Galois representations. The paper is in three parts: (1) an introduction to abelian varieties; (2) an outline of moduli spaces of principally polarised abelian varieties, which are the fundamental examples of Shimura varieties; (3) a detailed proof of the Ax–Lindemann–Weierstrass theorem for abelian varieties, following amethod using o-minimal geometry due to Pila, Ullmo and Yafaev. The first part assumes only an elementary knowledge of algebraic varieties and complex analytic geometry. The second part makes heavier use of algebraic geometry, but still at the level of varieties, and a little algebraic number theory. Like the first part, the algebraic geometry in the third part is elementary; the third part also assumes familiarity with the concept of semialgebraic sets, and uses cell decomposition for semialgebraic sets and the Pila–Wilkie theorem as black boxes. The second and third parts are independent of each other, so the reader interested primarily in the Ax–Lindemann–Weierstrass theorem may skip the second part (sections 4 to 6). In the first part of the paper (sections 2 and 3), we introduce abelian varieties over fields of characteristic zero, and especially over the complex numbers. The theory of abelian varieties over fields of positive characteristic introduces additional complications which we will not discuss. Our choice of topics is driven by Pila and Zannier's proof of the Manin–Mumford conjecture using o-minimal geometry. We will not discuss the Manin–Mumford conjecture or its proofs directly in this paper; aspects of the proof, and its generalisation to Shimura varieties, are discussed in other papers in this volume.

It is shown that any compact semistable quotient (in the sense of Heinzner and Snow) of a normal algebraic variety by a complex reductive Lie group $G$ is a good quotient. This reduces the investigation and classification of such complex-analytic quotients to the corresponding questions in the algebraic category. As a consequence of our main result, we show that every compact space in Nemirovski's class $\mathscr{Q}_G$ has a realisation as a good quotient, and that every complete algebraic variety in $\mathscr{Q}_G$ is unirational with finitely generated Cox ring and at worst rational singularities. In particular, every compact space in class $\mathscr{Q}_T$, where $T$ is an algebraic torus, is a toric variety.

We prove that any finite set of n-dimensional isolated algebraic singularities can be afforded on a simply connected projective variety.