On the singularity of Green's tensor for a perfectly conducting semi-infinite cone

This paper is intended to clarify a misunderstanding concerningthe source singularity of the electric Green's tensor for aperfectly conducting semi-infinite cone of circular cross-section.Tai's series expansion of the Green's tensor is known to lacka singular term at the source region. Jones has reconstructedthe solution to this problem and has pointed out the differencebetween his result and that of Tai. The aim of our paper isto demonstrate that, although Jones's closed-form solution iscorrect, there is a mistake in his comparison with Tai's seriessolution. We conclude that one of the two additional singularterms that Jones claims as missing from Tai's formula must beomitted. Besides, we compare Jones's closed-form solution withSmyshlyaev's solution to the very same problem. We concludethat the magnetic field expressions given by Jones and Smyshlyaevcoincide, but a singular term is missing from Smyshlyaev's expressionfor the electric field.     

Polar decomposition of regularly varying time series in star-shaped metric spaces

There exist two ways of defining regular variation of a time series in a star-shaped metric space: either by the distributions of finite stretches of the series or by viewing the whole series as a single random element in a sequence space. The two definitions are shown to be equivalent. The introduction of a norm-like function, called modulus, yields a polar decomposition similar to the one in Euclidean spaces. The angular component of the time series, called angular or spectral tail process, captures all aspects of extremal dependence. The stationarity of the underlying series induces a transformation formula of the spectral tail process under time shifts.     

Evidence for positron trapping in magnetism from dopplerbroadening measurements

Positron annihilation Dopplerbroadening measurements, display for the first time the effect of positron trapping in thermally induced vacancies in magnesium. The vacancy formation enthalpy is derived and is 0.85 eV.     

The influence of geometry on the resolution of a positron annihilation lifetime spectrometer

A detailed study of the prompt resolution curve of a positron annihilation lifetime spectrometer shows that the geometry of the source-detector set-up has great influence on its shape.     

Analysis of several subcycling schemes in partitioned simulations of a strongly coupled fluid-structure interaction

Fluid-structure interaction (FSI) simulations are used extensively to calculate the vibration of structures subjected to an internal or external flow. In the case of partitioned FSI simulations, separate flow and structure solvers are used, which requires some kind of coupling between both. The time step in both solvers is typically taken the same, but this unnecessarily leads to long calculation times when the time step is small due to stability reasons in one of the two solvers. Subcycling, the procedure where the time step of one solver is chosen smaller than the time step used in the other solver, may reduce the computational cost of the FSI simulation. The subcycling procedure can be either explicit or implicit, the latter implying the use of coupling iterations in each time step. Contrary to explicit subcycling, no stability analyses of implicit subcycling schemes are found in the literature. In this paper, the temporal stability of the implicit subcycling procedure is investigated. The one-dimensional flow in an elastic cylindrical tube is studied analytically. The results of this analysis are subsequently compared to a partitioned two-dimensional axisymmetric FSI calculation with implicit coupling between the flow and structure solvers.     

Trapping of positronium in a size-dependent spherical square well potential

The real trapping potential energy U(r) of positronium in vacancy-type holes and self-sustaining bubbles in liquids is replaced by a spherical square well potential of finite energy depth U_m and the scaling procedure proposed by Yu et al. is applied. Available data for the ortho-positronium lifetime in molecular crystals and in liquids are re-examined and a unique relation is found between the potential energy depth U_m and the calculated radius R of the trapping site. The general form of a surface-enhanced real potential energy U(r) which is compatible with the behaviour of U_m is proposed.     

Positron annihilation study of defects in the cyclically transformed martensite phase in a Cu-Zn-Al Alloy

Positron annihilation lineshape and lifetime measurements showed that by thermal cycling through the martensitic phase transformation, defects are introduced in Cu-Zn-Al alloys. It was concluded that mobile and immobile defects were introduced, most probably vacancies and dislocations.     

Regularly varying multivariate time series

Extreme values of a stationary, multivariate time series may exhibit dependence across coordinates and over time. The aim of this paper is to offer a new and potentially useful tool called tail process to describe and model such extremes. The key property is the following fact: existence of the tail process is equivalent to multivariate regular variation of finite cuts of the original process. Certain remarkable properties of the tail process are exploited to shed new light on known results on certain point processes of extremes. The theory is shown to be applicable with great ease to stationary solutions of stochastic autoregressive processes with random coefficient matrices, an interesting special case being a recently proposed factor GARCH model. In this class of models, the distribution of the tail process is calculated by a combination of analytical methods and a novel sampling algorithm.     

On the poles of topological zeta functions

We study the topological zeta function associated to a polynomial with complex coefficients. This is a rational function in one variable, and we want to determine the numbers that can occur as a pole of some topological zeta function; by definition these poles are negative rational numbers. We deal with this question in any dimension. Denote has a pole in . We show that is a subset of ; for and , the last two authors proved before that these are exactly the poles less than . As the main result we prove that each rational number in the interval is contained in .