We have studied the energy spectrum of a system formed by two concentric, coupled, semiconductor quantum rings. We have investigated the effect of a uniform magnetic field applied along the rings axis, on the energy spectrum of the system. We found that the spectrum of the coupled rings with similar confinement length corresponds roughly to the superposition of the spectra of two separate rings, although modified by the anticrossings produced among different states of the individual rings with same angular momentum. The Aharonov-Bohm oscillations do not have a definite period.

An IEEE 802.11 analytical performance evaluation model for ad-hoc WLAN's comprising terminals with different traffic source characteristics is presented. Although some publications address this issue, most of them propose to modify the original standard by some means that will affect the probability of transmission of a device when the network reaches congestion. The approach of this publication is to be able to establish a set of equations such that an intelligent choice of configuration parameters of standard home devices may improve the performance of the wireless network. Actually, two models are presented and compared, a simple one based on stationary behavior of the network assuming collisions have a negligible effect on network performance, and a second model based on a stationary stochastic model of a network, where devices have a packet ready for transmission at all times.

For functions f whose Taylor coefficients at the origin form a Hausdorff moment sequence we study the behaviour of w(y) := |f(γ + iy)| for y > 0 (γ ≤ 1 fixed)

The nth cyclic function is defined by jn (z)=C . n=0 znn (nn)! (z ¥ C, 2 [ n ¥ N). We prove that if k is an integer with 1 [ k [ n−1, then R (n − k)! j(k) n (x) xn−k S a < jn (x) < R (n − k)! j(k) n (x) xn−k S b holds for all positive real numbers x with the best possible constants a=1 and b=R2n − k n S .

A search is presented for the pair-production of heavy vector-like quarks in the lepton+jets final state using 140 fb^{-1} of proton-proton collisions at \sqrt{s}= 13 TeV collected with the ATLAS detector. The search is optimised for vector-like top-quarks (T) that decay into a W boson and a b-quark, with one W boson decaying leptonically and the other hadronically. Other vector-like quark flavours and decay modes are also considered. Events are selected with one high transverse-momentum electron or muon, large missing transverse momentum, a large-radius jet identified as a W boson, and multiple small-radius jets, at least one of which is b-tagged. Vector-like T-quarks with 100% branching ratio to Wb are excluded at 95% CL for masses below 1700 GeV. These limits are also applied to vector-like Y-quarks, which decay exclusively into a W boson and a b-quark. Isospin singlets with {\cal B}(T \to Wb:Ht:Zt)={1/2}:{1/4}:{1/4} are excluded for masses below 1360 GeV.

Bajo la influencia del federalismo fiscal y las teorías de la descentralización gubernamental, una parte importante de los sistemas de salud de todo el mundo enfrentaron la pandemia de COVID-19 después de haber sido moldeados o reformados por procesos de transferencia de poderes de los gobiernos centrales a los locales. La adquisición de suministros clave es uno de los componentes que opera de manera descentralizada, lo que obliga a los gobiernos locales a competir entre sí. Este fue el origen de lo que se ha llamado las “guerras de licitaciones” entre gobiernos subnacionales al comienzo de la respuesta a la pandemia. Estas guerras llevaron a políticas de centralización en Estados Unidos, Reino Unido y la Unión Europea. Sin embargo, se sabe menos sobre los casos del sur global. Al analizar la adquisición de Equipos de Protección Personal (EPP) en 320 municipios chilenos encargados de la atención primaria de salud, esta investigación proporciona evidencia de los impactos de la competencia gubernamental horizontal en la capacidad de adquirir suministros clave. En Chile, durante la respuesta a la pandemia de 2020, los municipios más ricos pudieron adquirir más mascarillas por habitante, generándose con ello economías de escala que les permitieron acceder a precios más bajos, a diferencia de aquellos municipios con menores recursos. Los autores apoyan la noción teórica de “concurrencia” como un concepto que añade matices al debate sobre centralización-descentralización. En Chile, por ejemplo, si bien el testeo y la trazabilidad de casos SARS-CoV-2 requerían la descentralización de dichas funciones, las compras de EPP probablemente podrían haberse beneficiado de la centralización de los procesos de adquisición para evitar la reproducción de desigualdades territoriales.

We propose a solution for the long standing puzzle of a too steeply falling fragmentation function for a quark fragmenting into a pion, calculated by Berger [E.L. Berger, Phys. Lett. B 89 (1980) 241] in the Born approximation. Contrary to the simple anticipation that gluon resummation worsens the problem, we find good agreement with data. Higher quark Fock states slow down the quark, an effect which we call jet lag. It can be also expressed in terms of vacuum energy loss. As a result, the space–time development of the jet shrinks and the z-dependence becomes flatter than in the Born approximation. The space–time pattern is also of great importance for in-medium hadronization.

This research aims to quantify the importance of a country's entrepreneurship level in terms of its competitiveness rates. Our hypothesis is that those countries entrepreneurship growth rates increase their competitiveness indicators and that this entrepreneurial improvement could be a key factor in reaching the next stage of development. We establish this relationship using a longitudinal database of Latin American countries that participated in the Global Entrepreneurship Monitor (GEM) and the Global Competitiveness Reports of the World Economic Forum (WEF) from 2001 to 2006. GEM and WEF construct aggregated indexes using several variables to rate each country's entrepreneurship activity and competitiveness development. We use a discriminant analysis to identify various countries' competitiveness subgroups and show how each country's entrepreneurship rates have weight in different stages of competitiveness, placing a special emphasis on Latin America. Our results suggest that Latin American countries need to gain entrepreneurial dynamics and economic (and competitiveness) development by transforming their typical self-employment or low value-added new ventures for local markets into strong, innovative networked firms competing globally. Some management and policy implications are also discussed