Progress in the theory of singular Riemannian foliations

A singular foliation is called a singular Riemannian foliation (SRF) if every geodesic that is perpendicular to one leaf is perpendicular to every leaf it meets. A typical example is the partition of a complete Riemannian manifold into orbits of an isometric action.In this survey, we provide an introduction to the theory of SRFs, leading from the foundations to recent developments. Sketches of proofs are included and useful techniques are emphasized. We study the local structure of SRFs in general and under curvature conditions in particular. We also review the solution of the Palais–Terng problem on integrability of the horizontal distribution. Important special classes of SRFs, like polar and variationally complete foliations and their relations, are treated. A characterization of SRFs whose leaf space is an orbifold is given. Moreover, desingularizations of SRFs are studied and applications, e.g., to Molino?s conjecture, are presented.     

Spectral geometry for Riemannian foliations

Let F be a Riemannian foliation on a Riemannian manifold (M, g), with bundle-like metric g. Aside from the Laplacian △_g associated to the metric g, there is another differential operator, the Jacobi operator J▽, which is a second order elliptic operator acting on sections of the normal bundle. Its spectrum is discrete as a consequence of the compactness of M. Hence one has two spectra, spec (M, g) = spectrum of △_g (acting on functions), and spec (F, J▽) = spectrum of J▽. We discuss the following problem: Which geometric properties of a Riemannian foliation F on a Riemannian manifold (M, g) are determined by the two types of spectral invariants?     

Totally geodesic foliations close to Riemannian foliations

The relation of the curvature and topology of totally geodesic foliations close to Riemannian ones is studied. The main result complements Ferus's famous theorem on totally geodesic foliations.Translated from Ukrainskii Geometricheskii Sbornik, No. 35, pp. 114–118, 1992.     

Transversal harmonic transformations for Riemannian foliations

The main purpose of the present paper is to study geometric properties of transversal (infinitesimal) harmonic transformations for Riemannian foliations. For the point foliation these notions are discussed in [14]. Especially we treat transversal infinitesimal harmonic transformations from the standpoint of λ-automorphisms. Our results extend those obtained in [6, 7, 15] for the case of harmonic foliations. Mathematics Subject Classifications (2000): Primary 53C20, Secondary 57R30.     

Minimal singular Riemannian foliations

We prove that a singular foliation on a compact manifold admitting an adapted Riemannian metric for which all leaves are minimal must be regular. To cite this article: V. Miquel, R.A. Wolak, C. R. Acad. Sci. Paris, Ser. I 342 (2006).     

Weitzenböck formulas for Riemannian foliations

In this paper we study the interplay between adiabatic limits of a Riemannian foliation and the classical Weitzenböck formula. For the leafwise part, our study leads to a vanishing result for the first order term of differential spectral sequence associated with the foliation. For the transversal part we obtain a Weitzenböck type formula which is an extension of the previous formula for basic forms due to Ph. Tondeur, M. Min-Oo, and E. Ruh, and is also more general than a Weitzenböck formula for transverse fiber bundle due to Y. Kordyukov.     

Riemannian foliations of bounded geometry

Continuing the study of bounded geometry for Riemannian foliations, begun by Sanguiao, we introduce a chart‐free definition of this concept. Our main theorem states that it is equivalent to a condition involving certain normal foliation charts. For this type of charts, it is also shown that the derivatives of the changes of coordinates are uniformly bounded, and there are nice partitions of unity. Applications to a trace formula for foliated flows will be given in a forthcoming paper.     

Minimizability of developable Riemannian foliations

Let (M,F){(M,\mathcal{F})} be a closed manifold with a Riemannian foliation. We show that the secondary characteristic classes of the Molino’s commuting sheaf of (M,F){(M,\mathcal{F})} vanish if (M,F){(M,\mathcal{F})} is developable and π ₁ M is of polynomial growth. By theorems of álvarez López in (álvarez López, Ann. Global Anal. Geom., 10:179–194, 1992) and (álvarez López, Ann. Pol. Math., 64:253–265, 1996), our result implies that (M,F){(M,\mathcal{F})} is minimizable under the same conditions. As a corollary, we show that (M,F){(M,\mathcal{F})} is minimizable if F{\mathcal{F}} is of codimension 2 and π ₁ M is of polynomial growth.     

Basic forms for transversely integrable singular Riemannian foliations

Basic forms for a transversely integrable singular Riemannian foliation with compact leaves are in one-to-one correspondence with ``Weyl"-invariant differential forms on a generalized section of the foliation.

     

Riemannian foliations admitting transversal conformal fields

Let be a closed, connected Riemannian manifold with a foliation of codimension q and a bundle-like metric g _M . We study the relationship between several infinitesimal automorphisms. Moreover under the some curvature condition, if M admits a transversal conformal field, then is transversally isometric to the action of a finite subgroup of O(q) acting on the q-sphere of constant curvature.      

Geometric resolution of singular Riemannian foliations

We prove that an isometric action of a Lie group on a Riemannian manifold admits a resolution preserving the transverse geometry if and only if the action is infinitesimally polar. We provide applications concerning topological simplicity of several classes of isometric actions, including polar and variationally complete ones. All results are proven in the more general case of singular Riemannian foliations.     

Spectral sequences and taut Riemannian foliations

Using a relation between the terms of the spectral sequence of a Riemannian foliation and its adiabatic limit, we obtain Bochner type techniques for this special setting and, as a consequence, in the special case of a Riemannian flow we obtain vanishing conditions for the top dimensional group of the basic cohomology \(H_{b}^{q}(\mathcal{F})\)-which is related to the property of being geodesible. We also extend a Weitzenböck type formula for the leafwise Laplacian and, for the particular class of compact foliations, we obtain a generalization of a result due to Ph. Tondeur, M. Min-Oo, and E. Ruh concerning the vanishing of the basic cohomology under the assumption that certain curvature operators are positive definite. In the final part we present an example.     

Index formulas for geometric Dirac operators in Riemannian foliations

With regards to certain Riemannian foliations we consider Kasparov pairings of leafwise and transverse Dirac operators. Relative to a pairing with a transversal class we commence by establishing an index formula for foliations with leaves of nonpositive sectional curvature. The underlying ideas are then developed in a more general setting leading to pairings of images under the Baum-Connes map in geometricK-theory with transversal classes. Several ideas implicit in the work of Connes and Hilsum-Skandalis are formulated in the context of Riemannian foliations. From these we establish the notion of a dual pairing inK-homology and a theorem of the Grothendieck-Riemann-Roch type.R. G. D. was supported by The National Science Foundation under Grant No. DMS-9304283.J. F. G. and F. W. K. were supported in part by The National Science Foundation under Grant No. DMS-9208182.F. W. K. was also supported in part by an Arnold O. Beckman Research Award from the Research Board of the University of Illinois.     

Vanishing theorem for transverse Dirac operators on Riemannian foliations

We obtain a vanishing theorem for the half-kernel of a transverse Spin ^c Dirac operator on a compact manifold endowed with a transversely almost complex Riemannian foliation twisted by a sufficiently large power of a line bundle, whose curvature vanishes along the leaves and is transversely non-degenerate at any point of the ambient manifold.      

Finslerian foliations of compact manifolds are Riemannian

We prove that a Finslerian foliation of a compact manifold is Riemannian.