Petrov type D pure radiation fields of Kundt's class

We present all Petrov type D pure radiation space-times, with or without cosmological constant, with a shear-free, non-diverging geodesic principal null congruence

Are All Directors Treated Equally? Evidence from Director Turnover Following Opportunistic Insider Selling

This study investigates the likelihood of director turnover following opportunistic insider selling. Given that opportunistic insider selling may be costly to a firm due to potential legal risk and firm legitimacy concerns, we hypothesize that directors engaging in this type of transactions have a higher likelihood of subsequently leaving the board. Using archival data of 11,409 directors in 2280 US firms from 2005 to 2014, univariate comparisons show that directors engaging in opportunistic insider selling are about 8% more likely to exit their firms’ board compared to directors not engaging in this behavior. Furthermore, multivariate results show that the likelihood of director departure following opportunistic insider selling is higher for some directors but not all. Specifically, directors who are especially valuable to the board or costly to replace do not seem to experience elevated levels of turnover. Interestingly, this difference in director turnover is only observed in smaller firms. We find that in larger firms, the likelihood of director turnover following opportunistic insider selling does not depend on director characteristics. As such, results seem to suggest that boards do not homogeneously self-regulate in this context as some directors seem to be shielded from turnover following unethical behavior

Oral health in Late Pleistocene and Holocene North West Africa

Archaeological sites in North West Africa have yielded a rich record of human occupation, including well dated human burials from the Late Pleistocene Iberomaurusian and early Holocene Capsian periods. The transition broadly coincides with climatic amelioration at the end of the Holocene enabling expansion into slighter dryer inland areas. Here we investigate possible changes in oral health and subsistence behaviours during the transition between the Iberomaurusian (n = 109 individuals) and the Capsian (n = 19 individuals), based on the study of dental remains. Frequencies in oral pathologies (caries, abscesses, periodontal disease and antemortem tooth loss) were studied to assess possible differences between the late Pleistocene and early Holocene. The Late Pleistocene Iberomaurusians were characterised by high caries frequencies (60% of observed teeth). The Capsians displayed very similar patterns in oral pathologies but a slightly lower percentage of carious teeth (49%). The similarity in oral health in Iberomaurusian and Capsian populations is consistent with similarities in diet and oral hygiene. The implication of cultural and biological continuity between the Iberomaurusian and Capsian periods is supported by indicators, such as exploitation of wild plants and snails, tooth evulsion and craniofacial and dental morphology. © 201

Purely radiative perfect fluids with degenerate shear tensor

We consider non-rotating geodesic perfect fluid spacetimes which are purely radiative in the sense that the gravitational field satisfies the covariant transverse conditions div H = div E = 0. We show that when the shear tensor S is degenerate, H, E and S necessarily commute and hence the resulting spacetimes are hypersurface homogeneous of Bianchi class A (modulo some purely electric exceptions).Comment: 8 pages, references added, typos corrected, simplified some algebraic manipulation

Concurrent constraint programming with process mobility

We propose an extension of concurrent constraint programming with primitives for process migration within a hierarchical network, and we study its semantics. To this purpose, we first investigate a "pure " paradigm for process migration, namely a paradigm where the only actions are those dealing with transmissions of processes. Our goal is to give a structural definition of the semantics of migration; namely, we want to describe the behaviour of the system, during the transmission of a process, in terms of the behaviour of the components. We achieve this goal by using a labeled transition system where the effects of sending a process, and requesting a process, are modeled by symmetric rules (similar to handshaking-rules for synchronous communication) between the two partner nodes in the network. Next, we extend our paradigm with the primitives of concurrent constraint programming, and we show how to enrich the semantics to cope with the notions of environment and constraint store. Finally, we show how the operational semantics can be used to define an interpreter for the basic calculus.