Integral formulae on foliated symmetric spaces

In this article, we generalize known integral formulae (due to Brito–Langevin–Rosenberg, Ranjan and the second author) for foliations of codimension 1 or unit vector fields and obtain an infinite series of such formulae involving invariants of the Weingarten operator of a unit vector field, of the Jacobi operator in its direction, and their products. We write several such formulae explicitly, on locally symmetric spaces as well as on arbitrary Riemannian manifolds where they involve also covariant derivatives of the Jacobi operator. We work also with foliations of codimension 1 (or vector fields) which admit “good” (in a sense) singularities.     

Extrinsic Geometric Flows on Codimension-One Foliations

We study the geometry of a codimension-one foliation with a time-dependent Riemannian metric. The work begins with formulae for deformations of geometric quantities as the Riemannian metric varies along the leaves of a foliation. Then the Extrinsic Geometric Flow depending on the second fundamental form of the foliation is introduced. Under suitable assumptions, this evolution yields the second-order parabolic PDEs, for which the existence/uniqueness and in some cases convergence of a solution are shown. Applications to the problem of prescribing the mean curvature function of a codimension-one foliation, and examples with harmonic and umbilical foliations (e.g., foliated surfaces) and with twisted product metrics are given.     

Integral formulae for a Riemannian manifold with two orthogonal distributions

We obtain a series of new integral formulae for a distribution of arbitrary codimension (and its orthogonal complement) given on a closed Riemannian manifold, which start from the formula by Walczak (1990) and generalize ones for foliations by several authors. For foliations on space forms our formulae reduce to the classical type formulae by Brito-Langevin-Rosenberg (1981) and Brito-Naveira (2000). The integral formulae involve the conullity tensor of a distribution, and certain components of the curvature tensor. The formulae also deal with a set of arbitrary functions depending on the scalar invariants of the co-nullity tensor. For a special choice of the functions our formulae involve the Newton transformations of the co-nullity tensor.     

Hyperfoliations on compact 3-manifolds with restrictions on the external curvature of leaves

C∞-foliations of codimension 1 on compact Riemannian 3-manifolds are studied. New classes of foliations, namely hyperbolic, elliptic, and parabolic foliations, are considered. Examples of such foliations are presented. In particular, aC∞-metric of nonnegative sectional curvature onS ³ such that the Reeb foliation is parabolic with respect to this metric is constructed. Analytic 3-manifolds with sectional curvature of constant sign admitting parabolic foliations are classified. Translated fromMatematicheskie Zametki, Vol. 63, No. 5, pp. 651–659, May, 1998. The author wishes to express his thanks to Professor A. A. Borisenko for his supervision, and to Yu. A. Nikolaevskii for useful advice in the process of preparing the present paper.     

Courbure totale des feuilletages des surfaces

The sum of the total curvatures of two orientable orthogonal foliations on the unit sphereS ²⊂R ³ is at least 4Π. The total curvature of a foliation with saddle singularities on a closed hyperbolic surfaceM is at least (12 Log 2–6 Log 3) ... |χ(M)|.      

The Noether–Fano inequalities for codimension one singular holomorphic foliations

The idea of the proof of the classical Noether–Fano inequalities can be adapted to the domain of codimension one singular holomorphic foliations of the projective space. We obtained criteria for proving that the degree of a foliation on the plane is minimal in the birational class of the foliation and for the non-existence of birational symmetries of generic foliations (except automorphisms). Moreover, we give several examples of birational symmetries of special foliations illustrating our results.      

The singularities of the wave trace of the basic Laplacian of a Riemannian foliation

We apply techniques of microlocal analysis to the study of the transverse geometry of Riemannian foliations in order to analyze spectral invariants of the basic Laplacian acting on functions on a Riemannian foliation with a bundle-like metric. In particular, we consider the trace of the basic wave operator when the mean curvature form is basic. We extend the concept of basic functions to distributions and demonstrate the existence of the basic wave kernel. The singularities of the trace of this basic wave kernel occur at the lengths of certain geodesic arcs which are orthogonal to the closures of the leaves of the foliation. In cases when the foliation has regular closure, a complete representation of the trace of the basic wave kernel can be computed for t≠0. Otherwise, a partial trace formula over a certain set of lengths of well-behaved geodesic arcs is obtained.     

Total Extrinsic Curvature of Certain Distributions on Closed Spaces of Constant Curvature

An integral formula for symmetric functions of curvature ofdistributions on closed constant nonnegative sectional curvature spacesis proved. The distributions under consideration are orthogonal to atotally geodesic foliation and the main theorem extends a previousresult concerning the total curvature of codimension-one foliations.     

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.     

Commuting holonomies and rigidity of holomorphic foliations

In this article we study deformations of a holomorphic foliationwith a generic non-rational first integral in the complex plane.We consider two vanishing cycles in a regular fiber of the firstintegral with a non-zero self intersection and with vanishingpaths that intersect each other only at their start points.It is proved that if the deformed holonomies of such vanishingcycles commute then the deformed foliation has also a firstintegral. Our result generalizes a similar result of Ilyashenkoon the rigidity of holomorphic foliations with a persistentcenter singularity. The main tools of the proof are Picard–Lefschetztheory and the theory of iterated integrals for such deformations.     

Bakry–Emery Curvature Operator and Ricci Flow

In this paper, we introduce some techniques of Bakry–Emery curvature operator to Ricci flow and prove the evolution equation for the Bakry–Emery scalar curvature. As its application, we can easily derive the Perelman’s entropy functional monotonicity formula. We also discuss some gradient estimates of Ricci curvature and L ²– estimates of scalar curvature.Project partially supported by Yumiao Fund of Putian University.     

Horizontal Connection and Horizontal Mean Curvature in Carnot Groups

In this paper we give a geometric interpretation of the notion of the horizontal mean curvature which is introduced by Danielli Garofalo-Nhieu and Pauls who recently introduced sub- Riemannian minimal surfaces in Carnot groups. This will be done by introducing a natural nonholonomic connection which is the restriction （projection） of the natural Riemannian connection on the horizontal bundle. For this nonholonomic connection and （intrinsic） regular hypersurfaces we introduce the notions of the horizontal second fundamental form and the horizontal shape operator. It turns out that the horizontal mean curvature is the trace of the horizontal shape operator.     

Holonomy transformations for singular foliations

In order to understand the linearization problem around a leaf of a singular foliation, we extend the familiar holonomy map from the case of regular foliations to the case of singular foliations. To this aim we introduce the notion of holonomy transformation. Unlike the regular case, holonomy transformations cannot be attached to classes of paths in the foliation, but rather to elements of the holonomy groupoid of the singular foliation.     

Existence and Uniqueness of Constant Mean Curvature Foliation of Asymptotically Hyperbolic 3-Manifolds

We prove existence and uniqueness of foliations by stable spheres with constant mean curvature for 3-manifolds which are asymptotic to anti-de Sitter–Schwarzschild metrics with positive mass. These metrics arise naturally as spacelike timeslices for solutions of the Einstein equation with a negative cosmological constant.     

Total curvature and volume of foliations on the sphere S 2

In this paper we study a curvature integral associated with a pair of orthogonal foliations on the Riemann sphere S ² and we compute the minimal value of the volume of meromorphic foliations.     

Theorem on the density of separatrix connections for polynomial foliations in ℂP 2

In this paper, we prove that in the space of polynomial foliations of a fixed degree of the complex two-dimensional space, foliations with separatrix connection, i.e., foliations in which any two distinct points have a common separatrix, are dense. The main tool of the proof is the analysis of the monodromy group of the foliation in a neighborhood of the infinitely distant point of the ambient projective space. __________ Translated from Fundamentalnaya i Prikladnaya Matematika, Vol. 12, No. 4, pp. 53–64, 2006.     

Fibers of the Baum-Bott map for foliations of degree two on ?2

The Baum-Bott map associates to a foliation the Baum-Bott indexes of their singularities. In this paper we study the fibers of the Baum-Bott map in the space of foliations of degree two on the projective plane ℙ². In the main result we prove that its generic fiber contains exactly 240 orbits of the natural action of Aut(ℙ²) onthespace of foliations.     

Complete foliations of space forms by hypersurfaces

We study foliations of space forms by complete hypersurfaces, under some mild conditions on its higher order mean curvatures. In particular, in Euclidean space we obtain a Bernstein-type theorem for graphs whose mean and scalar curvature do not change sign but may otherwise be nonconstant. We also establish the nonexistence of foliations of the standard sphere whose leaves are complete and have constant scalar curvature, thus extending a theorem of Barbosa, Kenmotsu and Oshikiri. For the more general case of r-minimal foliations of the Euclidean space, possibly with a singular set, we are able to invoke a theorem of Ferus to give conditions under which the non- singular leaves are foliated by hyperplanes.     

On the degree of polar transformations. An approach through logarithmic foliations

We investigate the degree of the polar transformations associated to a certain class of multi-valued homogeneous functions. In particular we prove that the degree of the preimage of generic linear spaces by a polar transformation associated to a homogeneous polynomial F is determined by the zero locus of F. For zero dimensional-dimensional linear spaces this was conjectured by Dolgachev and proved by Dimca–Papadima using topological arguments. Our methods are algebro-geometric and rely on the study of the Gauss map of naturally associated logarithmic foliations.