Yesterday, I gave a talk on the “Seminaire de Theorie Ergodique” at the Universite de Paris 13 (Villetaneuse) about a short note entitled “An example of a Teichmuller disk in genus 4 with degenerate Kontsevich-Zorich spectrum” that G. Forni and me just posted on the arXiv.

The *Teichmuller geodesic flow* acts (as any respectable geodesic flow) on the unit cotangent bundle of the Teichmuller space by parallel transport of points and vectors along geodesics of the Teichmuller metric (here one should be careful because the Teichmuller metric is *not* a Riemannian metric but only a Finsler metric).

Using the results of Alhfors and Bers in Riemann surface theory, it is possible to identify the unit cotangent bundle of the Teichmuller space with the space of holomorphic quadratic differentials on a Riemann surface of genus with area 1 modulo the group of orientation-preserving diffeomorphisms of which are isotopic to the identity. Note that acts naturally on by linear transformations on the pairs of Abelian differentials. The interesting consequence of this point of view is that it turns out that the *Teichmuller geodesic flow* on is simply the action of the *diagonal subgroup* on .

Now let me make a few general remarks about the fine structure of :

- is stratified into analytic spaces obtained by fixing the multiplicities of the zeroes of the quadratic differentials (here in view of the Riemann-Hurwitz theorem);
- can be endowed with a natural notion of ‘Lebesgue measure’: for each stratum there is a -invariant probability measure in the same class of the Lebesgue measure on the local charts given by the period map;

Once we know that has a good structure, we can start doing some Ergodic Theory. We consider the Teichmuller flow on a stratum . Although Veech showed that the strata are not always connected (see the works of Kontsevich, Zorich and Lanneau for the complete classification of the connected components), Masur and Veech (independently) managed to show that is *ergodic* on each connected component of and, more recently, Avila, Gouezel and Yoccoz proved that is *exponentially mixing*.

Next, we ask about the Lyapounov spectrum (i.e., the collection of Lyapounov exponents) of . In order to adress properly this question, Kontsevich and Zorich introduced the so-called *Kontsevich-Zorich cocycle* given by the *quotient* of the *trivial* cocycle by the mapping class group. It is know that is symplectic so that its Lyapounov spectrum with respect to any ergodic measure is *symmetric*:

.

Moreover, one can show that the Lyapounov exponents of are determined by the first (non-negative) Lyapounov exponents of .

After several numerical experiments, Kontsevich and Zorich conjectured that the Lyapounov exponents of with respect to the canonical ‘Lebesgue’ measure are all non-zero (i.e., is *non-uniformly hyperbolic*) and *simple* (i.e., multiplicity 1). Nowadays, we know that this conjecture is true due to the results of G. Forni (who proved the non-uniform hyperbolicity of ) and A. Avila, M. Viana (who showed that the simplicity of the Lyapounov spectrum). In particular, the *Kontsevich-Zorich spectrum* (i.e., the Lyapounov spectrum of ) of a *generic* point is well-understood.

A natural question (posed by Veech) related to this result concerns the Kontsevich-Zorich spectrum of *non-generic* points: how bad (or ‘degenerate’) can it be? This question was firstly answered by G. Forni who showed the existence of a *Veech surface* of genus 3 so that any -invariant measure supported on the -orbit of this surface has (moreover, it *seems* that there are no such examples in genus 2).

At this point, we are ready to state the following result:

**Theorem**(G. Forni, —)**.** Any -invariant measure supported on the -orbit of the genus 4 Riemann surface associated to the algebraic equation

has ‘degenerate’ Kontsevich-Zorich spectrum: .

Roughly speaking, the basic idea here is: this Riemann surface is ‘sufficiently symmetric’ (i.e., it has a ‘good’ automorphism group); on the other hand, by Forni’s method, the presence of symmetries to show some cancellations of the Lyapounov exponents (and the more symmetries you have, the more cancellations you get); finally, in the case of this surface, it turns out that the cyclic group of automorphisms generated by the symmetry (where is a 6-th root of unity) suffices to completely annilihate the Lyapounov spectrum (except for the ‘trivial’ exponent ).

**Remark 1. **It turns out that Forni’s method automatically implies that the cocycle *along* the -orbit of this surface is *isometric* (and moreover the cocycle is trivial in the sense that it is conjugated to constant). This is an interesting phenomenon if you compare with the ‘*chaotic*‘ behavior exhibited by *generic* points.

**Remark 2. **It is not hard to see (via simple arithmetic arguments) that *the method of our paper* does not produce any new examples of the type

where , and (the first two conditions are imposed to guarantee good symmetries while the third condition is necessary to get a connected Riemann surface). On the other hand, M. Möller told us that there are no further such examples among Veech surfaces (in any genus).

Of course, this theorem is just the tip of the iceberg: for instance, it would be interesting to know whether one can find examples of surfaces with a prescribed number of non-zero Lyapounov exponents (that is, given any , there is a surface with ?). It is worth to observe that Forni’s method has a weak point: while we can detect surfaces with *all* (but one) Lyapounov exponents equal to zero, we can’t prove that a prescribed part of the spectrum vanishes for a given surface and the basic reason is the lack of an explicit formula for the sum of the first exponents although Forni has an explicit formula for the sum of *all* exponents. Currently, I’m working with my post-doc advisor J.-C. Yoccoz in order to come around this problem by the usage of other methods, but this is still a work in progress… So, I think here is a good point to end this post!

I hope to see you soon in the proof of Asaoka’s theorem! Bye!

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