In this article, we consider the almost Hermitian structure on TM induced by a pair of a metric and an affine connection on M . We find the conditions under which TM admits almost Kähler structures, Kähler structures and Einstein metrics, respectively. Moreover, we give two examples of Kähler-Einstein structures on TM . 2000 Mathematics Subject Classification: 53C55, 53C15, 53C25.

We consider the effect of inhomogeneities on the expansion of the Einstein-de Sitter universe. We find that the backreaction of linear scalar metric perturbations results in apparent dark energy with a mixture of equations of state between 0 and –4/3. We discuss the possibility that backreaction could account for present-day acceleration.

The structure of a solvable Lie groups admitting an Einstein left-invariant metric is, in a sense, completely determined by the nilradical of its Lie algebra. We give an easy-to-check necessary and sufficient condition for a nilpotent algebra to be an Einstein nilradical whose Einstein derivation has simple eigenvalues. As an application, we classify filiform Einstein nilradicals (modulo known ...

TheMinkowski sum of two setsA,B ∈ IR, denotedA⊕B, is defined as {a+ b | a ∈ A, b ∈ B}. We describe an efficient and robust implementation for the construction of Minkowski sums of polygons in IR using the convolution of the polygon boundaries. This method allows for faster computation of the sum of non-convex polygons in comparison to the widely-used methods for Minkowski-sum computation that d...

Suppose M a compact manifold which admits an Einstein metric g which is Kähler with respect to some complex structure J . Is every other Einstein metric h on M also Kähler-Einstein? If the complex dimension of (M,J) is ≥ 3, the answer is generally no; for example, CP3 admits both the FubiniStudy metric, which is Kähler-Einstein, and a non-Kähler Einstein metric [2] obtained by appropriately squ...

A theorem of Følner asserts that for any set A ⊂ Z of positive upper density there is a Bohr neigbourhood B of 0 such that B \ (A − A) has zero density. We use this result to deduce some consequences about the structure of difference sets of sets of integers having a positive upper density.

We study the existence of projectable G-invariant Einstein metrics on the total space of G-equivariant fibrations M = G/L → G/K, for a compact connected semisimple Lie group G. We obtain necessary conditions for the existence of such Einstein metrics in terms of appropriate Casimir operators, which is a generalization of the result by Wang and Ziller about Einstein normal metrics. We describe b...

A number of recent surveys for gravitational lenses have found examples of double Einstein rings. Here, we investigate analytically the occurence of multiple Einstein rings. We prove, under very general assumptions, that at most one Einstein ring can arise from a mass distribution in a single plane lensing a single background source. Two or more Einstein rings can therefore only occur in multi-...

The subject of left-invariant Ricci soliton metrics on nilpotent Lie groups has enjoyed quite a bit of attention in the past several years. These metrics are intimately related to left-invariant Einstein metrics on non-unimodular solvable Lie groups. In fact, a classification of one is equivalent to a classification of the other. In this note, we focus our attention on nilpotent Lie groups and ...

Submitted for the MAR08 Meeting of The American Physical Society Interference between atomic Bardeen-Cooper-Schrieffer gases TUN WANG, Austrian Academy of Science, SUSANNE YELIN, Univ. of Connecticut — We study the interference between two atomic Bardeen-CooperSchrieffer (BCS) gases using noise correlations. Fringes as seen in the interference between two Bose-Einstein Condensates (BECs) do not...