Price Competition on Network

We present a model of imperfect price competition where not all firms can sell to all consumers. A network structure models the local interaction of firms and consumers. We find that aggregate surplus is maximized with a fully connected network, which corresponds to perfect competition, and decreases monotonically as the network becomes less connected until firms become local monopolists. When we study which networks are likely to form in equilibrium, we find that stable networks are not fully connected but are connected enough to rule out local monopolists. Our results extend to oligopolistic competition when consumers can either buy from a single firm or from all firms.Network markets, price competition, oligopoly competition, Bertrand competition.

¿Qué y cómo evaluar el desempeño docente? Una propuesta basada en los factores que favorecen el aprendizaje

A proposal is presented to assess teacher’s performance based on the aspects that educational research has found as fundamental to achieve students’ learning. The proposal is aimed mainly at teachers and people responsible for teacher’s assessment programs. After criticizing the traditional way of assessing teacher’s performance, it is shown a different way of doing it considering four essential dimensions of teaching: Knowledge of what is taught, management of didactic aspects, teacher’s didactic thinking and teacher motivation, responsibility and self-efficacy. It is described and justified what and how to assess, suggesting to do it by observing the educational practice. Therefore, guidelines are provided to assess it, pointing out the didactic aspects that are considered essential for students to acquire knowledge. It ends pointing out its advantages and giving some recommendations to apply it.Se presenta una propuesta para evaluar el desempeño docente basada en los aspectos que la investigación educativa ha encontrado como fundamentales para lograr el aprendi-zaje de los alumnos. La propuesta está dirigida principalmente a los docentes y responsa-bles de programas de evaluación docente. Luego de criticar la manera tradicional de valo-rar el desempeño docente, se muestra una forma diferente para realizarlo considerando cuatro dimensiones esenciales de la enseñanza: Dominio de lo enseñado, manejo de los aspectos didácticos, el pensamiento didáctico del maestro y la motivación, responsabilidad y autoeficacia docente. Se describe y justifica el qué evaluar y el cómo hacerlo, lo cual se sugiere hacerlo mediante la observación de la práctica docente; por ello se dan lineamientos para valorarla, resaltando los aspectos didácticos por considerarlos cruciales para que los alumnos adquieran el conocimiento. Se termina resaltando sus ventajas y algunas sugerencias para poder aplicarla

Polyradical character and spin frustration in fullerene molecules: An ab initio non-collinear Hartree--Fock study

Most {\em ab initio} calculations on fullerene molecules have been carried out based on the paradigm of the H\"uckel model. This is consistent with the restricted nature of the independent-particle model underlying such calculations, even in single-reference-based correlated approaches. On the other hand, previous works on some of these molecules using model Hamiltonians have clearly indicated the importance of short-range inter-atomic spin-spin correlations. In this work, we consider {\em ab initio} non-collinear Hartree--Fock (HF) solutions for representative fullerene systems: the bowl, cage, ring, and pentagon isomers of C$_{20}$, and the larger C$_{30}$, C$_{36}$, C$_{60}$, C$_{70}$, and C$_{84}$ fullerene cages. In all cases but the ring we find that the HF minimum corresponds to a truly non-collinear solution with a torsional spin density wave. Optimized geometries at the generalized HF (GHF) level lead to fully symmetric structures, even in those cases where Jahn-Teller distortions have been previously considered. The nature of the GHF solutions is consistent with the $\pi$-electron space becoming polyradical in nature: each $p$-orbital remains effectively singly occupied. The spin frustration, induced by the pentagon rings in an otherwise anti-ferromagnetic background, is minimized at the HF level by aligning the spins in non-collinear arrangements. The long-range magnetic ordering observed is reminiscent of the character of broken symmetry HF solutions in polyacene systems.Comment: 16 figure

¿Qué y cómo evaluar el desempeño docente? Una propuesta basada en los factores que favorecen el aprendizaje

A proposal is presented to assess teacher’s performance based on the aspects that educational research has found as fundamental to achieve students’ learning. The proposal is aimed mainly at teachers and people responsible for teacher’s assessment programs. After criticizing the traditional way of assessing teacher’s performance, it is shown a different way of doing it considering four essential dimensions of teaching: Knowledge of what is taught, management of didactic aspects, teacher’s didactic thinking and teacher motivation, responsibility and self-efficacy. It is described and justified what and how to assess, suggesting to do it by observing the educational practice. Therefore, guidelines are provided to assess it, pointing out the didactic aspects that are considered essential for students to acquire knowledge. It ends pointing out its advantages and giving some recommendations to apply it.Se presenta una propuesta para evaluar el desempeño docente basada en los aspectos que la investigación educativa ha encontrado como fundamentales para lograr el aprendi-zaje de los alumnos. La propuesta está dirigida principalmente a los docentes y responsa-bles de programas de evaluación docente. Luego de criticar la manera tradicional de valo-rar el desempeño docente, se muestra una forma diferente para realizarlo considerando cuatro dimensiones esenciales de la enseñanza: Dominio de lo enseñado, manejo de los aspectos didácticos, el pensamiento didáctico del maestro y la motivación, responsabilidad y autoeficacia docente. Se describe y justifica el qué evaluar y el cómo hacerlo, lo cual se sugiere hacerlo mediante la observación de la práctica docente; por ello se dan lineamientos para valorarla, resaltando los aspectos didácticos por considerarlos cruciales para que los alumnos adquieran el conocimiento. Se termina resaltando sus ventajas y algunas sugerencias para poder aplicarla

Multi-reference symmetry-projected variational approximation for the ground state of the doped one-dimensional Hubbard model

A multi-reference configuration mixing scheme is used to describe the ground state, characterized by well defined spin and space group symmetry quantum numbers as well as doping fractions $N_{e}/N_{sites}$, of one dimensional Hubbard lattices with nearest-neighbor hopping and periodic boundary conditions. Within this scheme, each ground state is expanded in a given number of nonorthogonal and variationally determined symmetry-projected configurations. The results obtained for the ground state and correlation energies of half-filled and doped lattices with 30, 34 and 50 sites, compare well with the exact Lieb-Wu solutions as well as with the ones obtained with other state-of-the-art approximations. The structure of the intrinsic symmetry-broken determinants resulting from the variational procedure is interpreted in terms of solitons whose translational and breathing motions can be regarded as basic units of quantum fluctuations. It is also shown that in the case of doped 1D lattices, a part of such fluctuations can also be interpreted in terms of polarons. In addition to momentum distributions, both spin-spin and density-density correlation functions are studied as functions of doping. The spectral functions and density of states, computed with an ansatz whose quality can be well-controlled by the number of symmetry-projected configurations used to approximate the $N_{e} \pm 1$ electron systems, display features beyond a simple quasiparticle distribution, as well as spin-charge separation trends.Comment: 16 pages, 11 figure

Análisis del acuerdo de cielos abiertos entre Colombia y Estados Unidos

El presente ensayo argumentativo busca explorar el mecanismo de participación económica internacional denominado `acuerdo de Cielos Abiertos, en referencia a la actual incursión que tiene Colombia en dicho acuerdo, suscribiendo en la actualidad su primer convenio en este tema, firmando el pacto con el país de los Estados Unidos el 10 de mayo del 2011. Propendiendo ensayísticamente de esta forma, realizar énfasis en el marco normativo del mismo acuerdo ofrecido por la Organización de Aviación Civil Internacional (OACI), los alcances jurídicos en nuestro país para el mismo y las posibilidades de participación comercial de Colombia frente al más grande contribuyente del sector aéreo mundial como lo es la nación norteamericana. Diseñando estrategias para un mayor aprovechamiento del acuerdo de Cielos Abiertos por parte de Colombia, garantizando mayores porcentajes de beneficios en los diferentes escenarios referentes, como los son: el sector turístico, económico, aéreo, desarrollo de infraestructura en general