Calculation of percolation thresholds in high dimensions for fcc, bcc, and diamond lattices

In a recent article, Galam and Mauger proposed an invariant for site and bond percolation thresholds, based on known values for twenty lattices (Eur. Phys. J. B 1 (1998) 255-258). Here we give a larger list of values for more than forty lattices in two to six dimensions. In this list are new results for fcc, bcc, and diamond lattices in 4, 5, and 6 dimensions. The list contains examples of lattices with equal site percolation thresholds, but different bond percolation thresholds. These and other examples show that there are deviations from the proposed invariant of up to 12% in two dimensions, increasing to 69% in higher dimensions.Comment: 12 pages, 3 figures (EPS), LaTe

The Tate-Hochschild cohomology ring of a group algebra

We show that the Tate-Hochschild cohomology ring $HH^*(RG,RG)$ of a finite group algebra $RG$ is isomorphic to a direct sum of the Tate cohomology rings of the centralizers of conjugacy class representatives of $G$. Moreover, our main result provides an explicit formula for the cup product in $HH^*(RG,RG)$ with respect to this decomposition. As an example, this formula helps us to compute the Tate-Hochschild cohomology ring of the symmetric group $S_3$ with coefficients in a field of characteristic 3.Comment: 15 page

Finite generation of Tate cohomology of symmetric Hopf algebras

Let $A$ be a finite dimensional symmetric Hopf algebra over a field $k$. We show that there are $A$-modules whose Tate cohomology is not finitely generated over the Tate cohomology ring of $A$. However, we also construct $A$-modules which have finitely generated Tate cohomology. It turns out that if a module in a connected component of the stable Auslander-Reiten quiver associated to $A$ has finitely generated Tate cohomology, then so does every module in that component. We apply some of these finite generation results on Tate cohomology to an algebra defined by Radford and to the restricted universal enveloping algebra of $sl_2(k)$.Comment: 12 pages, comments and suggestions are welcome. arXiv admin note: substantial text overlap with arXiv:0804.4246 by other author

Oscillatory superfluid Ekman pumping in Helium II and neutron stars

The linear response of a superfluid, rotating uniformly in a cylindrical container and threaded with a large number of vortex lines, to an impulsive increase in the angular velocity of the container is investigated. At zero temperature and with perfect pinning of vortices to the top and bottom of the container, we demonstrate that the system oscillates persistently with a frequency proportional to the vortex line tension parameter to the quarter power. This low-frequency mode is generated by a secondary flow analogous to classical Ekman pumping that is periodically reversed by the vortex tension in the boundary layers. We compare analytic solutions to the two-fluid equations of Chandler & Baym (1986) with the spin-up experiments of Tsakadze & Tsakadze (1980) in helium II and find the frequency agrees within a factor of four, although the experiment is not perfectly suited to the application of the linear theory. We argue that this oscillatory Ekman pumping mode, and not Tkachenko modes provide a natural explanation for the observed oscillation. In neutron stars, the oscillation period depends on the pinning interaction between neutron vortices and flux tubes in the outer core. Using a simplified pinning model, we demonstrate that strong pinning can accommodate modes with periods of days to years, which are only weakly damped by mutual friction over longer timescales.Comment: 25 pages, 6 figures, submitted to Journal of Fluid Mechanic

Subsidized Jobs for Unemployed Workers in Slovakia

This paper uses an administrative dataset from the Slovak Republic on durations of individual unemployment spells. The focus of the analysis is on the effect of the duration of temporary subsidized jobs on the job finding rate of unemployed workers. It appears that the duration of the temporary jobs is an important determinant of the speed by which unemployed workers find regular jobs. In this sense shorter temporary jobs are more effective than long temporary jobs. The main reason for this is probably that temporary jobs with a long duration induce workers in the first period on the temporary job to search less intensive for a regular job than temporary jobs with a short duration do.http://deepblue.lib.umich.edu/bitstream/2027.42/39695/3/wp311.pd

Aspects of the motivation for voluntary disclosures: evidence from the publication of value added statements in an emerging economy

This paper investigates the motivation for the voluntary disclosure of financial information by companies in their annual financial statements, by examining aspects of the usefulness of the value added statement. The value added statement is published voluntarily with the annual financial statements and is currently experiencing high levels of publication in South Africa, which is evidently brought about by the high political costs and significant legitimacy threats that companies operating in South Africa are facing. It was found from the literature and from a survey among management that the value added statement was primarily aimed at the employees. Employees have also been regarded as users of financial information in the literature. However, a survey among trade unions in South Africa found that almost no use is made of the value added statement even though the unions make use of other financial information. This indicates that voluntary disclosures do not necessarily satisfy the information needs of their intended audience. The research also indicates that the trade unions might not use the value added statement because they suspect that the statement is being used to reduce political costs and legitimacy threats, and is therefore not reliable. This is a major shortcoming of voluntary disclosures