Tight embeddings of simply connected 4-manifolds.
We consider a unit speed timelike curve in Minkowski 4-space and denote the Frenet frame of by . We say that is a generalized helix if one of the unit vector fields of the Frenet frame has constant scalar product with a fixed direction of . In this work we study those helices where the function is constant and we give different characterizations of such curves.
We characterize the Christoffel pairs of timelike isothermic surfaces in the four-dimensional split-quaternions. When restricting the receiving space to the three-dimensional imaginary split-quaternions, we establish an equivalent condition for a timelike surface in ℝ³₂ to be real or complex isothermic in terms of the existence of integrating factors.
Dans cet article, nous présentons une famille à un paramètre de 5-tissus plans exceptionnels. Dans leur présentation la plus naturelle, les tissus de cette famille sont formés d’un système harmonique de quatre pinceaux de droites parallèles et du feuilletages d’intégrale première, la fonction , où est une fonction elliptique de Jacobi.
It is known that, for a regular riemannian foliation on a compact manifold, the properties of its basic cohomology (non-vanishing of the top-dimensional group and Poincaré duality) and the tautness of the foliation are closely related. If we consider singular riemannian foliations, there is little or no relation between these properties. We present an example of a singular isometric flow for which the top-dimensional basic cohomology group is non-trivial, but the basic cohomology does not satisfy...
In this paper we study the topological and metric rigidity of hypersurfaces in , the -dimensional hyperbolic space of sectional curvature . We find conditions to ensure a complete connected oriented hypersurface in to be diffeomorphic to a Euclidean sphere. We also give sufficient conditions for a complete connected oriented closed hypersurface with constant norm of the second fundamental form to be totally umbilic.
In this paper, we consider the problem of the existence of conformal metrics with prescribed scalar curvature on the standard sphere S n, n ≥ 3. We give new existence and multiplicity results based on a new Euler-Hopf formula type. Our argument also has the advantage of extending well known results due to Y. Li [16].