Link to this article: http://journals.cambridge.org/abstract_S0143385712001101How to cite this article: MÁRIO BESSA and JORGE ROCHA (2013). Contributions to the geometric and ergodic theory of conservative ows.Abstract. We prove the following dichotomy for vector fields in a C 1 -residual subset of volume-preserving flows: for Lebesgue-almost every point, either all of its Lyapunov exponents are equal to zero or its orbit has a dominated splitting. Moreover, we prove that a volume-preserving and C 1 -stably ergodic flow can be C 1 -approximated by another volume-preserving flow which is non-uniformly hyperbolic.
Introduction and statement of the resultsLet M be a d-dimensional (with d ≥ 3), compact, connected and boundaryless Riemannian manifold endowed with a volume form ω, and let µ denote the Lebesgue measure associated to it. We denote by X 1 µ (M) the space of C 1 vector fields X over M which are divergence-free, that is, the associated flow X t of each X preserves the measure µ. We consider X 1 µ (M) endowed with the usual Whitney C 1 -topology. Given a flow X t , one usually deduces properties of it by studying its linear approximation. One way to do that is by considering the Lyapunov exponents, which, in broad terms, detect whether there is any exponential behavior of the linear tangent map along orbits. Given X ∈ X 1 µ (M), the existence of Lyapunov exponents for almost every point is guaranteed by Oseledets' theorem [31]. Positive (or negative) exponents ensure, on average, an exponential rate of divergence (or convergence) of two neighboring trajectories, whereas zero exponents tell us there is a lack of any kind of average exponential behavior. A flow is said to be non-uniformly hyperbolic if its Lyapunov exponents are all different from zero. In [25], Hu et al gave examples of non-uniformly hyperbolic flows in any compact manifold. Non-zero exponents, together with some smoothness assumptions on the flow, allow us to obtain invariant manifolds that are dynamically defined (see [32]). Since this stable/unstable manifold theory forms the basis