Differential Geometry

Equivalences between conformal foliations on Euclidean 3 -space, Hermitian structures on Euclidean 4 -space, shear-free ray congruences on Minkowski 4 -space, and holomorphic foliations on complex 4 -space are explained geometrically and twistorially; these are used to show that 1) any real-analytic complex-valued harmonic morphism without critical points defined on an open subset of Minkowski space is conformally equivalent to the direction vector field of a shear-free ray congruence, 2) the boundary values at infinity of a complex-valued harmonic morphism on hyperbolic 4 -space define a real-analytic conformal foliation by curves of an open subset of Euclidean 3 -space and all such foliations arise this way. This gives an explicit method of finding such foliations; some examples are given.

Uhlenbeck introduced an invariant, the (minimal) uniton number, of harmonic 2-spheres in a Lie group G and proved that when G=SU(n) the uniton number cannot exceed n-1. In this paper, using new methods inspired by Morse Theory, we explain this result and extend it to an arbitrary compact group G. The same methods also yield Weierstrass formulae for these harmonic maps and simple proofs of most of the known classification theorems for harmonic 2-spheres in symmetric spaces.

In this paper we continue our study on the moduli spaces of flat G-bundles, for any semi-simple Lie group G, over a Riemann surface by using heat kernel and Reidemeister torsion. Formulas for intersection numbers on the moduli spaces over a Riemann surface with several boundary components, over non-orientable Riemann surfaces are obtained. Some general vanishing theorems about characteristic numbers of the moduli spaces are proved. We also extend our method to study Higgs moduli spaces, to introduce invariants for knots and 3-manifolds.

We consider a closed odd-dimensional oriented manifold M together with an acyclic flat hermitean vector bundle $\cF$. We form the trivial fibre bundle with fibre M over the manifold of all Riemannian metrics on M . It has a natural flat connection and a vertical Riemannian metric. The higher analytic torsion form of Bismut/Lott associated to the situation is invariant with respect to the connected component of the identity of the diffeomorphism group of M . Using that the space of Riemannian metrics is contractible we define continuous cohomology classes of the diffeomorphism group and its Lie algebra. For the circle we compute this classes in degree 2 and show that the group cohomology class is non-trivial, while the Lie algebra cohomology class vanishes.

Using supervector fields and graded forms along a morphism, we study the geometry of ordinary differential superequations, extend the formalism of higher order Lagrangian mechanics to the graded context and prove a generalization of Noether's theorem.

For a continuous curve of families of Dirac type operators we define a higher spectral flow as a K -group element. We show that this higher spectral flow can be computed analytically by $\heta$-forms, and is related to the family index in the same way as the spectral flow is related to the index. We introduce a notion of Toeplitz family and relate its index to the higher spectral flow. Applications to family indices for manifolds with boundary are also given.

The Lagrangian formalism on a arbitrary non-fibrating manifold is considered. The kinematical description of this generic situation is based on the concept of (higher-order) Grassmann manifolds which is the factorization of the regular velocity manifold to the action of the differential group. Here we introduce in this context the basic concepts of the Lagrangian formalism as Lagrange, Euler-Lagrange and Helmholtz-Sonin forms. These objects come in pairs, namely we have homogeneous objects (defined on the regular velocity manifold) and non-homogeneous objects (defined on the Grassmann manifold). We will establish the connection between the homogeneous objects and their non-homogeneous counterparts. As a result we will conclude that the generic expressions for a variationally trivial Lagrangian and for a locally variational differential equation remain the same as in the fibrating case.

We give elementary constructions for Satake-Furstenberg, Martin and Karpelevich boundaries of symmetric spaces. We also consruct some "new" boundaries

We introduce Morse-type inequalities for a holomorphic circle action on a holomorphic vector bundle over a compact Kaehler manifold. Our inequalities produce bounds on the multiplicities of weights occurring in the twisted Dolbeault cohomology in terms of the data of the fixed points and of the symplectic reduction. This result generalizes both Wu-Zhang extension of Witten's holomorphic Morse inequalities and Tian-Zhang Morse-type inequalities for symplectic reduction. As an application we get a new proof of the Tian-Zhang relative index theorem for symplectic quotients.

The holomorphic torsion of a compact locally symmetric manifold is expressed as a special value of a zeta function built out of geometric data (closed geodesics) of the manifold.