No Arabic abstract
An affine surface is said to be an affine Zoll surface if all affine geodesics close smoothly. It is said to be an affine almost Zoll surface if thru any point, every affine geodesic but one closes smoothly (the exceptional geodesic is said to be alienated as it does not return). We exhibit an affine structure on the cylinder which is almost Zoll. This structure is geodesically complete, affine Killing complete, and affine symmetric.
If $mathcal{M}=(M, abla)$ is an affine surface, let $mathcal{Q}(mathcal{M}):=ker(mathcal{H}+frac1{m-1}rho_s)$ be the space of solutions to the quasi-Einstein equation for the crucial eigenvalue. Let $tilde{mathcal{M}}=(M,tilde abla)$ be another affine structure on $M$ which is strongly projectively flat. We show that $mathcal{Q}(mathcal{M})=mathcal{Q}(tilde{mathcal{M}})$ if and only if $ abla=tilde abla$ and that $mathcal{Q}(mathcal{M})$ is linearly equivalent to $mathcal{Q}(tilde{mathcal{M}})$ if and only if $mathcal{M}$ is linearly equivalent to $tilde{mathcal{M}}$. We use these observations to classify the flat Type~$mathcal{A}$ connections up to linear equivalence, to classify the Type~$mathcal{A}$ connections where the Ricci tensor has rank 1 up to linear equivalence, and to study the moduli spaces of Type~$mathcal{A}$ connections where the Ricci tensor is non-degenerate up to affine equivalence.
We prove an almost splitting theorem for the warped product space with warped function $f(r)=coshleft(rsqrt{frac{lambda}{n-2}}right)$.
We show that any closed spin manifold not diffeomorphic to the two-sphere admits a sequence of volume-one-Riemannian metrics for which the smallest non-zero Dirac eigenvalue tends to zero. As an application, we compare the Dirac spectrum with the conformal volume.
We investigate the geometry of almost Robinson manifolds, Lorentzian analogues of Hermitian manifolds, defined by Nurowski and Trautman as Lorentzian manifolds of even dimension equipped with a totally null complex distribution of maximal rank. Associated to such a structure, there is a congruence of null curves, which, in dimension four, is geodesic and non-shearing if and only if the complex distribution is involutive. Under suitable conditions, the distribution gives rise to an almost Cauchy--Riemann structure on the leaf space of the congruence. We give a comprehensive classification of such manifolds on the basis of their intrinsic torsion. This includes an investigation of the relation between an almost Robinson structure and the geometric properties of the leaf space of its congruence. We also obtain conformally invariant properties of such a structure, and we finally study an analogue of so-called generalised optical geometries as introduced by Robinson and Trautman.
We find a new class of invariant metrics existing on the tangent bundle of any given almost-Hermitian manifold. We focus here on the case of Riemannian surfaces, which yield new examples of Kahlerian Ricci-flat manifolds in four real dimensions.