We prove that a Ricci curvature based method of triangulation of compact Riemannian manifolds, due to Grove and Petersen, extends to the context of weighted Riemannian manifolds and more general metric measure spaces. In both cases the role of the lower bound on Ricci curvature is replaced by the curvature-dimension condition CD(K,N). We show also that for weighted Riemannian manifolds the tria...

We classify all closed non-orientable P-irreducible manifolds having complexity up to 6 and we describe some having complexity 7. We show in particular that there is no such manifold with complexity less than 6, and that those having complexity 6 are precisely the 4 flat non-orientable ones. The manifolds having complexity 7 we describe are Seifert manifolds of type H × S and manifolds with non...

We give manifolds whose Riemann curvature operators commute, i.e. which satisfy R(x1, x2)R(x3, x4) = R(x3, x4)R(x1, x2) for all tangent vectors xi in both the Riemannian and the higher signature settings. These manifolds have global geometric phenomena which are quite different for higher signature manifolds than they are for Riemannian manifolds. Our focus is on global properties; questions of...

The discovery of the manifolds has long been a hot topic in computer vision. In many practical problems, highdimensional data poses a great obstacle to the researchers. But these data points are often sampled from several low-dimensional sub-manifolds. Therefore, charting the sub-manifolds in one coordinate system will help visualize them simultaneously. However, algorithms developed so far all...

We define and study branched shadows of 4-manifolds as a combination of branched spines of 3-manifolds and of Turaev’s shadows. We use these objects to combinatorially represent 4-manifolds equipped with Spinc-structures and homotopy classes of almost complex structures. We then use branched shadows to study complex 4-manifolds and prove that each almost complex structure on a 4-dimensional han...

Almost hypercomplex manifolds with Hermitian and Norden metrics and more specially the corresponding quaternionic Kähler manifolds are considered. Some necessary and sufficient conditions the investigated manifolds be isotropic hyper-Kählerian and flat are found. It is proved that the quaternionic Kähler manifolds with the considered metric structure are Einstein for dimension at least 8. The c...

We prove that when Hodge theory survives on non-compact symplectic manifolds, a compact symplectic Lie group action having fixed points is necessarily Hamiltonian, provided the associated almost complex structure preserves the space of harmonic one-forms. For example, this is the case for complete Kähler manifolds for which the symplectic form has an appropriate decay at infinity. This extends ...

The problem of geodesic connectedness in semi-Riemannian manifolds (i.e. the question whether each two points of the manifold can be joined by a geodesic) has been widely studied from very different viewpoints. Our purpose is to review these semi-Riemannian techniques, and possible extensions. In the Riemannian case, it is natural to state this problem on (incomplete) manifolds with (possibly n...

We prove that the rational blowdown, a surgery on smooth 4-manifolds introduced by Fintushel and Stern, can be performed in the symplectic category. As a consequence, interesting families of smooth 4-manifolds, including the exotic K3 surfaces of Gompf and Mrowka, admit symplectic structures. A basic problem in symplectic topology is to understand what smooth manifolds admit a symplectic struct...

We show that two-bridge knots and alternating fibered admit no purely cosmetic surgeries, i.e., pair of distinct Dehn surgeries on such a knot produce 3-manifolds are homeomorphic as oriented manifolds. Our argument, based recent result by Hanselman, uses several invariants or 3-manifolds; for knots, we study the signature some finite type invariants, 3-manifolds, deploy $SL(2,\mathbb{C})$ Cass...