Izu Vaisman

Generalized Hopf manifolds

Let (M,J,g) be a Hermitian manifold with complex structure J, metric g, and Kähler form Ω. Then g is locally conformal Kähler iff dΩ=ω ∧ Ω for some closed and non-exact 1-form ω. Moreover, if ω is a parallel form, M is called a generalized Hopf manifold. The main results of this paper are: (a) the description of the geometric structure of the compact locally conformal Kähler-flat manifolds; (b) the description of the geometric structure of the compact generalized Hopf manifolds on which a certain canonically defined foliation is regular; (c) a description of the harmonic forms and Betti numbers of a general compact generalized Hopf manifold; (d) a method for studying analytic vector fields on generalized Hopf manifolds; (e) conditions for submanifolds of generalized Hopf manifolds to belong to the same class.

LOCALLY CONFORMAL SYMPLECTIC MANIFOLDS

A locally conformal symplectic (l.c.s.) manifold is a pair (M2n,fl) where M2n(n > i) is a connected differentiable manifold, and a nondegenerate 2-form on M such that M k9 Us (Us- open subsets) /U e , o U s -IR, d O. Equivalently, d ^ .q for some closed 1-form . L.c.s. manifolds can be seen as generalized phase spaces of Hamiltonian dynamical systems since the form of the Hamilton equations is, in fact, preserved by homothetlc canonical transformations. The paper discusses first Hamiltonian vector fields, and infinitesimal automorphisms (i.a.) on l.c.s, manifolds. If (M,) has an i.a. X such that (X) 0, we say that M is of the first kind and assumes the particular form de ^ e Such an M is a 2-contact manifold with the structure forms (,e), and it has a vertical 2-dlmensional foliation V. If V is regular, we can give a flbration theorem which shows that M is a T2-principal bundle over a symplectlc manifold. Particularly, 9 is regular for some homogeneous l.c.s, manifolds, and this leads to a general con- struction of compact homogeneous l.c.s, manifolds. Various related geometric results, including reductivity theorems for Lie algebras of i.a. are also given. Most of the proofs are adaptations of corresponding proofs in symplectlc and contact geometry. The paper ends with an Appendix which states an analogous fibratlon theorem in Riemannlan

On locally conformal almost Kähler manifolds

In the first section of this note, we discuss locally conformal symplectic manifolds, which are differentiable manifoldsV2n endowed with a nondegenerate 2-form Ω such thatdΩ=θ ∧ Ω for some closed form θ. Examples and several geometric properties are obtained, especially for the case whendΩ ≠ 0 at every point. In the second section, we discuss the case when Ω above is the fundamental form of an (almost) Hermitian manifold, i.e. the case of the locally conformal (almost) Kähler manifolds. Characterizations of such manifolds are given. Particularly, the locally conformal Kähler manifolds are almost Hermitian manifolds for which some canonically associated connection (called the Weyl connection) is almost complex. Examples of locally conformal (almost) Kähler manifolds which are not globally conformal (almost) Kähler are given. One such example is provided by the well-known Hopf manifolds.

On the geometric quantization of Poisson manifolds

In a paper by Huebschmann [J. Reine Angew. Math. 408, 57 (1990)], the geometric quantization of Poisson manifolds appears as a particular case of the quantization of Poisson algebras. Here, this quantization is presented straightforwardly. The results include a geometric prequantization integrality condition and its discussion in particular cases such as Dirac brackets, an adaptation of the notion of a polarization and a construction of a quantum Hilbert space, and a computational example. In the last section of the paper the general prequantization representations in the sense of Urwin [Adv. Math. 50, 126 (1983)] are described for the Poisson and Jacobi manifolds.

Some curvature properties of complex surfaces

SummaryIn this paper, we are investigating curvature properties of complex two-dimensional Hermitian manifolds, particularly in the compact case. To do this, we start with the remark that the fundamental form of such a manifold is integrable, and we use the analogy with the locally conformal KÄhler manifolds, which follows from this remark. Among the obtained results, we have the following: a compact Hermitian surface for which either the Riemannian curvature tensor satisfies the KÄhler symmetries or the Hermitian curvature tensor satisfies the Riemannian Bianchi identity is KÄhler; a compact Hermitian surface of constant sectional curvature is a flat KÄhler surface; a compact Hermitian surface M with nonnegative nonidentical zero holomorphie Hermitian bisectional curvature has vanishing plurigenera, c1(M) ⩾ 0, and no exceptional curves; a compact Hermitian surface with distinguished metric, and positive integral Riemannian scalar curvature has vanishing plurigenera, etc.

Symplectic curvature tensors

In the paper, one establishes the decomposition of the space of tensors which have the symmetries of the curvature of a torsionless symplectic connection into Sp (n)-irreducible components. This leads to three interesting classes of symplectic connections: flat, Ricci flat, and similar to the Levi-Civita connections of Kähler manifolds with constant holomorphic sectional curvature (we call them connections with reducible curvature). A symplectic manifold with two transversal polarizations has a canonical symplectic connection, and we study properties that are encountered if this canonical connection belongs to the classes mentioned above. For instance, in the reducible case we can compute the Pontrjagin classes, and these will be zero if the polarizations are real, etc. If the polarizations are real and there exist points where they are either singular or nontransversal, one has residues in the sense ofLehmann [L], which should be expected to play an interesting role in symplectic geometry.

Transitive Courant algebroids

We express any Courant algebroid bracket by means of a metric connection, and construct a Courant algebroid structure on any orthogonal, Whitney sum E⊕C where E is a given Courant algebroid and C is a flat, pseudo-Euclidean vector bundle. Then, we establish the general expression of the bracket of a transitive Courant algebroid, that is, a Courant algebroid with a surjective anchor, and describe a class of transitive Courant algebroids which are Whitney sums of a Courant subalgebroid with neutral metric and Courant-like bracket and a pseudo-Euclidean vector bundle with a flat, metric connection. In particular, this class contains all the transitive Courant algebroids of minimal rank; for these, the flat term mentioned above is zero. The results extend to regular Courant algebroids, that is, Courant algebroids with a constant rank anchor. The paper ends with miscellaneous remarks and an appendix on Dirac linear spaces.

COUPLING POISSON AND JACOBI STRUCTURES ON FOLIATED MANIFOLDS

Let M be a differentiable manifold endowed with a foliation ℱ. A Poisson structure P on M is ℱ-coupling if ♯P(ann(Tℱ)) is a normal bundle of the foliation. This notion extends Sternbergs coupling symplectic form of a particle in a Yang–Mills field [11]. In the present paper we extend Vorobievs theory of coupling Poisson structures [16] from fiber bundles to foliated manifolds and give simpler proofs of Vorobievs existence and equivalence theorems of coupling Poisson structures on duals of kernels of transitive Lie algebroids over symplectic manifolds. We then discuss the extension of the coupling condition to Jacobi structures on foliated manifolds.

Towards a double field theory on para-Hermitian manifolds

In a previous paper, we have shown that the geometry of double field theory has a natural interpretation on flat para-Kahler manifolds. In this paper, we show that the same geometric constructions can be made on any para-Hermitian manifold. The field is interpreted as a compatible (pseudo-)Riemannian metric. The tangent bundle of the manifold has a natural, metric-compatible bracket that extends the C-bracket of double field theory. In the para-Kahler case, this bracket is equal to the sum of the Courant brackets of the two Lagrangian foliations of the manifold. Then, we define a canonical connection and an action of the field that correspond to similar objects of double field theory. Another section is devoted to the Marsden-Weinstein reduction in double field theory on para-Hermitian manifolds. Finally, we give examples of fields on some well-known para-Hermitian manifolds.

Lagrange geometry on tangent manifolds

Lagrange geometry is the geometry of the tensor field defined by the fiberwise Hessian of a nondegenerate Lagrangian function on the total space of a tangent bundle. Finsler geometry is the geometrically most interesting case of Lagrange geometry. In this paper, we study a generalization which consists of replacing the tangent bundle by a general tangent manifold, and the Lagrangian by a family of compatible, local, Lagrangian functions. We give several examples and find the cohomological obstructions to globalization. Then, we extend the connections used in Finsler and Lagrange geometry, while giving an index-free presentation of these connections.