5,767 research outputs found
Formal Derivation of Concurrent Garbage Collectors
Concurrent garbage collectors are notoriously difficult to implement
correctly. Previous approaches to the issue of producing correct collectors
have mainly been based on posit-and-prove verification or on the application of
domain-specific templates and transformations. We show how to derive the upper
reaches of a family of concurrent garbage collectors by refinement from a
formal specification, emphasizing the application of domain-independent design
theories and transformations. A key contribution is an extension to the
classical lattice-theoretic fixpoint theorems to account for the dynamics of
concurrent mutation and collection.Comment: 38 pages, 21 figures. The short version of this paper appeared in the
Proceedings of MPC 201
Transmission Losses in Left-handed Materials
We numerically analyze the origin of the transmission losses in left-handed
structures. Our data confirms that left handed structures can have very good
transmission properties, in spite of the expectable dispersion of their
effective permeability and refraction index. The large permittivity of the
metallic components improves the transmission. High losses, observed in recent
experiments, could be explained by the absorption of the dielectric board
Refraction of Electromagnetic Energy for Wave Packets Incident on a Negative Index Medium is Always Negative
We analyze refraction of electromagnetic wave packets on passing from an
isotropic positive to an isotropic negative refractive index medium. We
definitively show that in all cases the energy is always refracted negatively.
For localized wave packets, the group refraction is also always negative.Comment: 5 pages, 3 figure
Inventory of ammonia emissions from UK agriculture 2009
The National Ammonia Reduction Strategy Evaluation System (NARSES) model (spreadsheet version) was used to estimate ammonia (NH3) emissions from UK agriculture for the year 2009. Year-specific livestock numbers and fertiliser N use were added for 2009 and revised for previous years. The estimate for 2009 was 231.8 kt NH3, representing a 2.3 kt increase from the previously submitted estimate for 2008. Backward and forward projections using the 2009 model structure gave estimates of 317, 245 and 244 kt NH3 for the years 1990, 2010 and 2020, respectively. This inventory reports emission from livestock agriculture and from nitrogen fertilisers applied to agricultural land. There are a number of other minor sources reported as ‘agriculture’ in the total UK emission inventory, including horses not kept on agricultural holdings, emissions from composting and domestic fertiliser use
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Radar studies of the vertical distribution of insects migrating over southern Britain: the influence of temperature inversions on nocturnal layer concentrations
Insects migrating over two sites in southern UK (Malvern in Worcestershire, and Harpenden in Hertfordshire) have been monitored continuously with nutating vertical-looking radars (VLRs) equipped with powerful control and analysis software. These observations make possible, for the first time, a systematic investigation of the vertical distribution of insect aerial density in the atmosphere, over temporal scales ranging from the short (instantaneous vertical profiles updated every 15 min) to the very long (profiles aggregated over whole seasons or even years). In the present paper, an outline is given of some general features of insect stratification as revealed by the radars, followed by a description of occasions during warm nights in the summer months when intense insect layers developed. Some of these nocturnal layers were due to the insects flying preferentially at the top of strong surface temperature inversions, and in other cases, layering was associated with higher-altitude temperature maxima, such as those due to subsidence inversions. The layers were formed from insects of a great variety of sizes, but peaks in the mass distributions pointed to a preponderance of medium-sized noctuid moths on certain occasions
Transmission Studies of Left-handed Materials
Left-handed materials are studied numerically using an improved version of
the transfer-matrix method. The transmission, reflection, the phase of the
reflection and the absorption are calculated and compared with experiments for
both single split-ring resonators (SRR) with negative permeability and
left-handed materials (LHMs) which have both the permittivity and permeability
negative. Our results suggest ways of positively identifying materials that
have both permittivity and permeability negative, from materials that have
either permeability or permittivity negative
A review of size and geometrical factors influencing resonant frequencies in metamaterials
Although metamaterials and so-called left-handed media have originated from theoretical considerations, it is only by their practical fabrication and the measurement of their properties that they have gained credibility and can fulfil the potential of their predicted properties. In this review we consider some of the more generally applicable fabrication methods and changes in geometry as they have progressed, exhibiting resonant frequencies ranging from radio waves to the visible optical region
Guided Modes in Negative Refractive Index Waveguides
We study linear guided waves propagating in a slab waveguide made of a
negative-refraction- index material, the so-called left-handed waveguide. We
reveal that the guided waves in left-handed waveguides possess a number of
peculiar properties, such as the absence of the fundamental modes, mode double
degeneracy, and sign-varying energy ux. In particular, we predict the existence
of novel types of guided waves with a dipole-vortex structure of the Pointing
vector.Comment: 4 pages, 4 figure
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