An extended Agassi model: algebraic structure, phase diagram, and large size limit

The Agassi model is a schematic two-level model that involves pairing and monopole-monopole interactions. It is, therefore, an extension of the well known Lipkin-Meshkov-Glick (LMG) model. In this paper we review the algebraic formulation of an extension of the Agassi model as well as its bosonic realization through the Schwinger representation. Moreover, a mean-field approximation for the model is presented and its phase diagram discussed. Finally, a $1/j$ analysis, with $j$ proportional to the degeneracy of each level, is worked out to obtain the thermodynamic limit of the ground state energy and some order parameters from the exact Hamiltonian diagonalization for finite$-j$.Comment: Accepted in Physica Scripta. Focus on SSNET 201

Phase diagram of an extended Agassi model

Background: The Agassi model is an extension of the Lipkin-Meshkov-Glick model that incorporates the pairing interaction. It is a schematic model that describes the interplay between particle-hole and pair correlations. It was proposed in the 1960's by D. Agassi as a model to simulate the properties of the quadrupole plus pairing model. Purpose: The aim of this work is to extend a previous study by Davis and Heiss generalizing the Agassi model and analyze in detail the phase diagram of the model as well as the different regions with coexistence of several phases. Method: We solve the model Hamiltonian through the Hartree-Fock-Bogoliubov (HFB) approximation, introducing two variational parameters that play the role of order parameters. We also compare the HFB calculations with the exact ones. Results: We obtain the phase diagram of the model and classify the order of the different quantum phase transitions appearing in the diagram. The phase diagram presents broad regions where several phases, up to three, coexist. Moreover, there is also a line and a point where four and five phases are degenerated, respectively. Conclusions: The phase diagram of the extended Agassi model presents a rich variety of phases. Phase coexistence is present in extended areas of the parameter space. The model could be an important tool for benchmarking novel many-body approximations.Comment: Accepted for publication in PR

On a branch-and-bound approach for a Huff-like Stackelberg location problem

Modelling the location decision of two competing firms that intend to build a new facility in a planar market can be done by a Huff-like Stackelberg location problem. In a Huff-like model, the market share captured by a firm is given by a gravity model determined by distance calculations to facilities. In a Stackelberg model, the leader is the firm that locates first and takes into account the actions of the competing chain (follower) locating a new facility after the leader. The follower problem is known to be a hard global optimisation problem. The leader problem is even harder, since the leader has to decide on location given the optimal action of the follower. So far, in literature only heuristic approaches have been tested to solve the leader problem. Our research question is to solve the leader problem rigorously in the sense of having a guarantee on the reached accuracy. To answer this question, we develop a branch-and-bound approach. Essentially, the bounding is based on the zero sum concept: what is gain for one chain is loss for the other. We also discuss several ways of creating bounds for the underlying (follower) sub-problems, and show their performance for numerical cases

Fiscal data revisions in Europe

Public deficit figures are subject to revisions, as most macroeconomic aggregates are. Nevertheless, in the case of Europe, the latter could be particularly worrisome given the role of fiscal data in the functioning of EU’s multilateral surveillance rules. Adherence to such rules is judged upon initial releases of data, in the framework of the so-called Excessive Deficit Procedure (EDP) Notifications. In addition, the lack of reliability of fiscal data may hinder the credibility of fiscal consolidation plans. In this paper we document the empirical properties of revisions to annual government deficit figures in Europe by exploiting the information contained in a pool of real-time vintages of data pertaining to fifteen EU countries over the period 1995-2008. We build up such real-time dataset from official publications. Our main findings are as follows: (i) preliminary deficit data releases are biased and non-efficient predictors of subsequent releases, with later vintages of data tending to show larger deficits on average; (ii) such systematic bias in deficit revisions is a general feature of the sample, and cannot solely be attributed to the behaviour of a small number of countries, even though the Greek case is clearly an outlier; (iii) Methodological improvements and clarifications stemming from Eurostat’s decisions that may lead to data revisions explain a significant share of the bias, providing some evidence of window dressing on the side of individual countries; (iv) expected real GDP growth, political cycles and the strength of fiscal rules also contribute to explain revision patterns; (v) nevertheless, if the systematic bias is excluded, revisions can be considered rational after two years. JEL Classification: E01, E21, E24, E31, E5, H600data revisions, fiscal statistics, news and noise, Rationality, real-time data

Una Nueva especie de Flammulina P. Karsten, F. cephalariae (Agaricales) encontrada en España

Based on morphological and molecular studies, we propose Flammulina cephalariae as a new species to the scientific community. It grows in limestone soil on Cephalaria leucantha debris and its distribution seems to be limited to Losa Valley, in the provinc e of Burgos (Northem Spain). We offer a description, its iconography and a comparison with similar taxa, and we include also a key for European species of genus Flammulina P . Karsten.Bas{mdonos en estudios morfológicos y moleculares, proponemos Flammulina cephalariae como especie nueva para la ciencia. Fructifica en terreno calcttreo, sobre restos de la planta Cephalaria leucantha y su distribución parece quedar limitada a una pequeña zona del Vall e de Losa en la provincia de Burgos (norte de España). Se describe, iconografia y compara con taxones afines, al tiempo que se incluye una clave de las especies europeas del género Flammulina P. Karste

Modificación de las actitudes del alumnado de secundaria hacia la práctica de actividad

La orientación de las actitudes hacia la práctica física se presenta como un aspecto importante para la adquisición y mantenimiento de hábitos físico-deportivos en la adolescencia durante su tiempo de ocio. En este trabajo se resentan los resultados logrados a través de un programa de intervención en Educación Física orientada a la salud en alumnos de 4o de E.S.O., caracterizado por desarrollar conocimiento aplicado a la práctica, espíritu crítico respecto a reencias sociales sobre salud inadecuadas y disfrute en la práctica, utilizando para ello, entre otros recursos, foros de debate en Internet.The orientation of attitudes towards physical practice is shown to be an important aspect for teenagers to acquire and keep up sports and physical habits during their free time. In this paper we set out the results achieved through a health-oriented Physical Education intervention programme in pupils in the fourth year of Secondary Education, characterised by developing knowledge applied to practice and a critical spirit regarding unsuitable social beliefs about health and enjoyment in practice, using, among other resources, Internet forums. The results of the investigation show, at a statistical level, a very remarkable increase both in the attitudes towards the process and towards the product in the intervention group (or Physical-Health Education group) unlike the control group (or Physical Education group). Therefore, as a whole, we can conclude by highlighting the suitability of the intervention aimed at modifying attitudes towards health-oriented physical practic

Trees, homology, and automorphism groups of right-angled Artin groups

We study the homology of an explicit finite-index subgroup of the automorphism group of a partially commutative group, in the case when its defining graph is a tree. More concretely, we give a lower bound on the first Betti number of this subgroup, based on the number and degree of a certain type of vertices, which we call deep. We then use combinatorial methods to analyze the average value of this Betti number, in terms of the size of the defining tree

The Level of NMDA Receptor in the Membrane Modulates Amyloid-β Association and Perforation

Alzheimer’s disease is a neurodegenerative disorder that affects mostly the elderly. The main histopathological markers are the senile plaques formed by amyloid-β peptide (Aβ) aggregates that can perforate the plasma membrane of cells, increasing the intracellular calcium levels and releasing synaptic vesicles that finally lead to a delayed synaptic failure. Several membrane proteins and lipids interact with Aβ affecting its toxicity in neurons. Here, we focus on NMDA receptors (NMDARs) as proteins that could be modulating the association and neurotoxic perforation induced by Aβ on the plasma membrane. In fact, our results showed that decreasing NMDARs, using enzymatic or siRNA approaches, increased the association of Aβ to the neurons. Furthermore, overexpression of NMDARs also resulted in an enhanced association between NMDA and Aβ. Functionally, the reduction in membrane NMDARs augmented the process of membrane perforation. On the other hand, overexpressing NMDARs had a protective effect because Aβ was now unable to cause membrane perforation, suggesting a complex relationship between Aβ and NMDARs. Because previous studies have recognized that Aβ oligomers are able to increase membrane permeability and produce amyloid pores, the present study supports the conclusion that NMDARs play a critical protective role on Aβ actions in hippocampal neurons. These results could explain the lack of correlation between brain Aβ burden and clinically observed dementia