A BPS Skyrme model

Within the set of generalized Skyrme models, we identify a submodel which has both infinitely many symmetries and a Bogomolny bound which is saturated by infinitely many exact soliton solutions. Concretely, the submodel consists of the square of the baryon current and a potential term only. Further, already on the classical level, this BPS Skyrme model reproduces some features of the liquid drop model of nuclei. Here, we review the properties of the model and we discuss the semiclassical quantization of the simplest Skyrmion (the nucleon).Comment: Conference Proceedings of the 28th International Colloquium On Group Theoretical Methods In Physics (GROUP 28), July 2010, Northumbria, England. 10pages, 1 figure. Version 2: Publication information adde

A Skyrme-type proposal for baryonic matter

The Skyrme model is a low-energy effective field theory for QCD, where the baryons emerge as soliton solutions. It is, however, not so easy within the standard Skyrme model to reproduce the almost exact linear growth of the nuclear masses with the baryon number (topological charge), due to the lack of Bogomolny solutions in this model, which has also hindered analytical progress. Here we identify a submodel within the Skyrme-type low energy effective action which does have a Bogomolny bound and exact Bogomolny solutions, and therefore, at least at the classical level, reproduces the nuclear masses by construction. Due to its high symmetry, this model qualitatively reproduces the main features of the liquid droplet model of nuclei. Finally, we discuss under which circumstances the proposed sextic term, which is of an essentially geometric and topological nature, can be expected to give a reasonable description of properties of nuclei.Comment: 11 pages, 2 figures, latex. v3: Extended and revised version, some clarifications added. Some references and 2 figures added. v4: matches published versio

Pricing Discretely Monitored Barrier Options and Credit Default Swaps under Lévy Processes

We introduce a new, fast and accurate method to calculate prices and sensitivities of European vanilla and digital options under the Variance Gamma model. For near at-the-money options of short maturity, our method is much faster than those based on discretization and truncation of the inverse Fourier transform integral (iFT method). We show that the results calculated with our method agree with those obtained with the iFT algorithm using very long and fine grids. Taking the results of our method as a benchmark, we show that the parabolic modification of the iFT method (Boyarchenko and Levendorskiĭ, 2012) is much more efficient than the standard (flat) version. Based on this conclusion, we consider an approach which uses a combination of backward induction and parabolic iFT to price discretely monitored barrier options, as well as credit default swaps, under wide classes of Lévy models. At each step of backward induction, we use piece-wise polynomial interpolation and parabolic iFT, which allows for efficient error control. We derive accurate recommendations for the choice of parameters of the numerical scheme, and produce numerical examples showing that oversimplified prescriptions in other methods can result in large errors

Skyrmions on the 2-sphere.

We study static solutions of the Skyrme model on the 2-sphere of radius L, for various choices of potential. The high-density Skyrmion phase corresponds to the ratio β = L/(size of Skyrmion) being small, whereas the low-density phase corresponds to β being large. The transition between these two phases, and in particular the behaviour of a relevant order parameter, is examined

Folding in the Skyrme Model

There are only three stable singularities of a differentiable map between three-dimensional manifolds, namely folds, cusps and swallowtails. A Skyrme configuration is a map from space to SU(2), and its singularities correspond to the points where the baryon density vanishes. In this paper we consider the singularity structure of Skyrme configurations. The Skyrme model can only be solved numerically. However, there are good analytic ansaetze. The simplest of these, the rational map ansatz, has a non-generic singularity structure. This leads us to introduce a non-holomorphic ansatz as a generalization. For baryon number two, three and four, the approximate solutions derived from this ansatz are closer in energy to the true solutions than any other ansatz solution. We find that there is a tiny amount of negative baryon density for baryon number three, but none for two or four. We comment briefly on the relationship to Bogomolny-Prasad-Sommerfield monopoles

Genetic structure of Barbus spp. populations in the Marches region of central Italy and its relevance to conservation actions

A genetic survey of Barbus spp. populations in the Marches Region (Adriatic River basins), central Italy, was carried out using mitochondrial and nuclear markers (partial D-loop, cyt b sequences and microsatellite loci) in order to ascertain their systematic position and to address their genetic structure which is key to conservation action planning. Analyses were conducted on sequences obtained from 91 individuals collected from eight sampling sites in five different rivers, from two specimens provided by the Ichthyological Centre of Rome and mitochondrial sequences of Barbus spp. retrieved from GenBank. Presumptive classification based on external morphological characters was not confirmed by genetic analysis, by means of which all specimens collected in the Marches Region were ascribed to Barbus plebejus. Genetic diversity values (h and π) of sampling groups were all different from 0 except the one sample collected from the upper reaches of the River Tenna, above a hydroelectric dam. Population connectivity and colonization patterns of the studied area were inferred from an analysis of molecular variance distribution and evolutionary relationships among haplotypes. The results point to different levels of isolation among sampling groups due to ecological and anthropogenic factors and the effect of an artificial barrier on genetic variability and conservation status of the population. Finally, this study confirms the uncertainty associated with systematic classification of Barbus spp. based on morphological characters due to the phenotypic plasticity of the species

Tratamiento de la litiasis coledociana mediante cirugía laparoscópica, endoscopía o combinado

Introducción. La litiasis biliar es una patología muy común en nuestro medio, con una prevalencia del 21,9%. Entre las complicaciones más frecuentes, se presenta asociada a una coledocolitiasis en aproximadamente 3% a 10% de los casos. Su tratamiento puede realizarse en forma mini-invasiva mediante laparoscopía, endoscopía o una combinación de ambos. En la actualidad no existe consenso acerca de cuál es la mejor estrategia para el tratamiento. Nuestro objetivo fue realizar una revisión simple con la mayor calidad de evidencia disponible, comparando dos estrategias terapéuticas para la litiasis coledociana. La litiasis coledociana se divide en aquella que se presenta en pacientes ya colecistectomizados y la que se presenta asociada a una litiasis vesicular. En el primer grupo encontamos tres subgrupos: a) pacientes en los que se dejó una litiasis biliar en el intraoperatorio, b) litiasis coledociana con un drenaje biliar, y c) litiasis coledociana sin un drenaje biliar. En el primer caso se puede realizar un tratamiento laparoscópico en uno o dos tiempos; en el segundo, un abordaje transfistular; y en el tercero, un tratamiento endoscópico que, en caso de fracasar, puede ser seguido de un abordaje percutáneo o una reoperación laparoscópica. El tratamiento de la litiasis vesicular asociada a una litiasis coledociana puede realizarse por laparoscopía, endoscopía o una combinación de ambos. Concluimos que la innovación de las técnicas quirúrgicas mínimamente invasivas ha revolucionado el manejo de la litiasis de la vía biliar. El tratamiento en un tiempo es posible en pacientes con riesgo operatorio adecuado y en manos de cirujanos entrenados. Es esencial el abordaje multidisciplinario para resolver de este modo cuál es el método diagnóstico y terapéutico más acorde al paciente en cuestión, dependiendo de los medios con los que cuente cada institución