We investigate the minimal surface problem in the three dimensional Heisenberg group, H , equipped with its standard Carnot-Carathéodory metric. Using a particular surface measure, we characterize minimal surfaces in terms of a sub-elliptic partial differential equation and prove an existence result for the Plateau problem in this setting. Further, we provide a link between our minimal surfaces...

We give a loop group formulation for the problem of isometric immersions with flat normal bundle of a simply connected pseudo-Riemannian manifold M c,r, of dimension m, constant sectional curvature c 6= 0, and signature r, into the pseudo-Euclidean space R s , of signature s ≥ r. In fact these immersions are obtained canonically from the loop group maps corresponding to isometric immersions of ...

Introduction Organization of the paper Sections A Basic Riemannian geometry B Examples of vanishing and estimating theorems C More Riemannian geometry D Proof of Theorem II E Bochner's theorem and estimating theorems in a more general setting F Proofs of the estimating theorems (sketch of) G From vanishing to estimating theorems : an account H Possible extensions of the methods I Final comments...

We prove the following three results in Hamiltonian dynamics. • The Weinstein conjecture holds true for every displaceable hypersurface of contact type. • Every magnetic flow on a closed Riemannian manifold has contractible closed orbits for a dense set of small energies. • Every closed Lagrangian submanifold whose fundamental group injects and which admits a Riemannian metric without closed ge...

Let (M, g) be a Riemannian surface (i.e. a Riemannian manifold of dimension 2), orientable or not. We assume that M is either compact or a compact perturbation of the euclidian space, so that the Sobolev embeddings are true. Consider ∆ = ∆ g the Laplace-Beltrami operator. In this paper we are interested in constructing WKB approximations for the non linear cubic Schrödinger equation i∂ t u(t, x...

Denote by γ the Gauss measure on R and by L the Ornstein– Uhlenbeck operator. In this paper we introduce a Hardy space h(γ) of Goldberg type and show that for each u in R\{0} and r > 0 the operator (rI+L) is unbounded from h(γ) to L(γ). This result is in sharp contrast both with the fact that (rI+L) is bounded from H(γ) to L(γ), where H(γ) denotes the Hardy type space introduced in [MM], and wi...

The Riemannian metric on the manifold of positive definite matrices is defined by a kernel function φ in the form K D(H,K) = ∑ i,j φ(λi, λj) −1TrPiHPjK when ∑ i λiPi is the spectral decomposition of the foot point D and the Hermitian matrices H,K are tangent vectors. For such kernel metrics the tangent space has an orthogonal decomposition. The pull-back of a kernel metric under a mapping D 7→ ...

We show how to construct an N = 1 superconformal vertex algebra (SCVA) from any Riemannian manifold. When the Riemannian manifold has special holonomy groups, we discuss the extended supersymmetry. When the manifold is complex or Kähler, we also generalize the construction to obtain N = 2 SCVA’s. We study the BRST cohomology groups of the topological vertex algebras obtained by the A twist and ...

In this paper first it is proved that if ξ is a nontrivial closed conformal vector field on an n-dimensional compact Riemannian manifold (M, g) with constant scalar curvature S satisfying S ≤ λ1(n − 1), λ1 being first nonzero eigenvalue of the Laplacian operator ∆ on M and Ricci curvature in direction of a certain vector field is non-negative, then M is isometric to the n-sphere S(c), where S =...