No Arabic abstract
We define the secondary invariants L^2- eta and -rho forms for families of generalized Dirac operators on normal coverings of fibre bundles. On the covering family we assume transversally smooth spectral projections, and Novikov--Shubin invariants bigger than 3(dim B+1) to treat the large time asymptotic for general operators. In the particular case of a bundle of spin manifolds, we study the L^2- rho class in relation to the space of positive scalar curvature vertical metrics.
We give explicit examples of degree 3 cohomology classes not Poincare dual to submanifolds, and discuss the realisability of homology classes by submanifolds with Spin-C normal bundles.
In this paper, we prove a singular version of the Donaldson-Uhlenbeck-Yau theorem over normal projective varieties and normal complex subvarieties of compact Kahler manifolds that are smooth outside a codimension three analytic subset. As a consequence, we deduce the polystability of (dual) tensor products of stable reflexive sheaves, and we give a new proof of the Bogomolov-Gieseker inequality over such spaces, along with a precise characterization of the case of equality. In addition, we improve several previously known algebro-geometric results on normalized tautological classes. We also study the limiting behavior of semistable bundles over a degenerating family of normal projective varieties. In the case of a family of stable bundles, we explain how the singular Hermitian-Yang-Mills connections obtained here fit into the degeneration picture. These can also be characterized from the algebro-geometric perspective. As an application, we apply the results to the degeneration of stable bundles through the deformation to projective cones, and we explain how our results are related to the Mehta-Ramanathan restriction theorem.
We present a classification theorem for closed smooth spin 2-connected 7-manifolds M. This builds on the almost-smooth classification from the first authors thesis. The main additional ingredient is an extension of the Eells-Kuiper invariant for any closed spin 7-manifold, regardless of whether the spin characteristic class p_M in the fourth integral cohomology of M is torsion. In addition we determine the inertia group of 2-connected M - equivalently the number of oriented smooth structures on the underlying topological manifold - in terms of p_M and the torsion linking form.
We study simple branched coverings of degree d of the 2- and 3- dimensional sphere branched over oriented links. We demonstrate how to use braid charts to develop embeddings of these into $S^k times D^2$ for $k=2,3 when $d=2,3$. This is an initial part of our study and represents the manuscript submitted to the RIMS workshop at Intelligence of Low Dimensional Topology.
For a virtual knot $K$ and an integer $rgeq 0$, the $r$-covering $K^{(r)}$ is defined by using the indices of chords on a Gauss diagram of $K$. In this paper, we prove that for any finite set of virtual knots $J_0,J_2,J_3,dots,J_m$, there is a virtual knot $K$ such that $K^{(r)}=J_r$ $(r=0mbox{ and }2leq rleq m)$, $K^{(1)}=K$, and otherwise $K^{(r)}=J_0$.