Equivariant basic cohomology under deformations

By methods of Ghys and Haefliger–Salem it is possible to deform a Riemannian foliation on a simply connected compact manifold, or more generally a Killing foliation, into a closed foliation, i.e., a foliation whose leaves are all closed. Certain transverse geometric and topological properties are preserved under these deformations, as previously shown by the authors. For instance the Euler characteristic of basic cohomology is invariant, whereas its Betti numbers are not. In this article we show that the equivariant basic cohomology ring structure is invariant. This leads to a sufficient algebraic condition, namely equivariant formality, for the Betti numbers to be preserved as well. In particular, this is true for the deformation of the Reeb orbit foliation of a K-contact manifold. Another consequence is that there is a universal bound on the sum of basic Betti numbers of any equivariantly formal, positively curved Killing foliation of a given codimension. We also show that a Killing foliation with negative transverse Ricci curvature is closed. If the transverse sectional curvature is negative we show, furthermore, that its fundamental group has exponential growth. Finally, we obtain a transverse generalization of Synge’s theorem to Killing foliations.