The Disappearance of the HMS Umbra

A sailor aboard the HMS Umbra has a strange run-in with an ominous fog that won\u27t lift. As the days drag by, the weather is the least of his problems

NonLocal Systems of Balance Laws in Several Space Dimensions with Applications to Laser Technolog

For a class of systems of nonlinear and nonlocal balance laws in several space dimensions, we prove the local in time existence of solutions and their continuous dependence on the initial datum. The choice of this class is motivated by a new model devoted to the description of a metal plate being cut by a laser beam. Using realistic parameters, solutions to this model obtained through numerical integrations meet qualitative properties of real cuts. Moreover, the class of equations considered comprises a model describing the dynamics of solid particles along a conveyor belt

Rapid VHE variability in blazars

Active Galactic Nuclei (AGN) are known to show significant variability over a wide frequency range. We review observational results on the variability characteristics of blazars in the very high energy (VHE) domain, focusing on recent findings of rapid VHE variability and evidence for an underlying multiplicative driving process in PKS 2155-304. We explore a physical scenario where the variability is assumed to arise due to accretion disk fluctuations transmitted to the jet, and discuss its implications for the central powerhouse.Comment: 6 pages, 3 figures. Accepted to the Proceedings of the 25th Texas Symposium on Relativistic Astrophysics (Heidelberg, 2010

Sharedness as an innate basis for communication in the infant

From a cognitive perspective, intentional communication may be viewed as an agent's activity overtly aimed at modifying a partner's mental states. According to standard Gricean definitions, this requires each party to be able to ascribe mental states to the other, i.e., to entertain a so-called theory of mind. According to the relevant experimental literature, however, such capability does not appear before the third or fourth birthday; it would follow that children under that age should not be viewed as communicating agents. In order to solve the resulting dilemma, we propose that certain specific components of an agent's cognitive architecture (namely, a peculiar version of sharedness and communicative intention), are necessary and sufficient to explain infant communication in a mentalist framework. We also argue that these components are innate in the human species

Smooth and discontinuous junctions in the p-system

Consider the p-system describing the subsonic flow of a fluid in a pipe with section a = a(x). We prove that the resulting Cauchy problem generates a Lipschitz semigroup, provided the total variation of the initial datum and the oscillation of a are small. An explicit estimate on the bound of the total variation of a is provided, showing that at lower fluid speeds, higher total variations of a are acceptable. An example shows that the bound on TV(a) is mandatory, for otherwise the total variation of the solution may grow arbitrarily.Comment: 27 pages, 4 figure