Local and nice structures of the groupoid of an equivalence relation
We introduce basic characteristic classes and numbers as new invariants for Riemannian foliations. If the ambient Riemannian manifold is complete, simply connected (or more generally if the foliation is a transversely orientable Killing foliation) and if the space of leaf closures is compact, then the basic characteristic numbers are determined by the infinitesimal dynamical behavior of the foliation at the union of its closed leaves. In fact, they can be computed with an Atiyah-Bott-Berline-Vergne-type...
This note is based on a theorem of Sacksteder which generalizes a classical result of Denjoy. Using this theorem and results on the existence of invariant measures, new results are obtained concerning minimal sets for groups of diffeomorphisms of the circle and for foliations of codimension one.
We compute the global multiplicity of a 1-dimensional foliation along an integral curve in projective spaces. We give a bound in the way of Poincaré problem for a complete intersection curves. In the projective plane, this bound give us a bound of the degree of non irreducible integral curves in function of the degree of the foliation.
A classification of natural liftings of foliations to the tangent bundle is given.
Damien Gaboriau a montré récemment que les nombres de Betti des feuilletages mesurés à feuilles contractiles sont des invariants de la relation d’équivalence associée. Sorin Popa a utilisé ce résultat joint à des propriétés de rigidité des facteurs de type II pour en déduire l’existence de facteurs de type II dont le groupe fondamental est trivial.