Stanilov-Tsankov-Videv Theory

We survey some recent results concerning Stanilov-Tsankov-Videv theory, conformal Osserman geometry, and Walker geometry which relate algebraic properties of the curvature operator to the underlying geometry of the manifold.Comment: This is a contribution to the Proceedings of the 2007 Midwest Geometry Conference in honor of Thomas P. Branson, published in SIGMA (Symmetry, Integrability and Geometry: Methods and Applications) at http://www.emis.de/journals/SIGMA

Time scale competition leading to fragmentation and recombination transitions in the coevolution of network and states

We study the co-evolution of network structure and node states in a model of multiple state interacting agents. The system displays two transitions, network recombination and fragmentation, governed by time scales that emerge from the dynamics. The recombination transition separates a frozen configuration, composed by disconnected network components whose agents share the same state, from an active configuration, with a fraction of links that are continuously being rewired. The nature of this transition is explained analytically as the maximum of a characteristic time. The fragmentation transition, that appears between two absorbing frozen phases, is an anomalous order-disorder transition, governed by a crossover between the time scales that control the structure and state dynamics.Comment: 5 pages, 5 figures, figures 2 and 4 changed, tile changed, to be published in PR

Giant magnetoimpedance: new electrochemical option to monitor surface effects?

Magnetoimpedance, MI, change due to surface modification of the sensitive element caused by biofluids was studied with the aim of creating a robust sensor capable of separating the chemical surface modification from the sensing process. A MI sensor prototype with an as-quenched FeCoSiB amorphous ribbon sensitive element was designed and calibrated for a frequency range of 0.5 to 10 MHz at an intensity of the current of 60 mA. Measurements as a function of the exposure time were made, first, in a regime where chemical surface modification and sensing were separated and then, in a regime where they were not separated (in a bath for fluids). The MI variation was explained by the change of the surface magnetic anisotropy. It was shown that the magnetoimpedance effect can be successfully employed as a new electrochemical option to probe the electric features of surface-modified magnetic electrodes when the biofluid, the material of the sensitive element, and the detection conditions are properly selected and synergetically adjusted.Comment: 22 pages, 6 figure

Divergent Time Scale in Axelrod Model Dynamics

We study the evolution of the Axelrod model for cultural diversity. We consider a simple version of the model in which each individual is characterized by two features, each of which can assume q possibilities. Within a mean-field description, we find a transition at a critical value q_c between an active state of diversity and a frozen state. For q just below q_c, the density of active links between interaction partners is non-monotonic in time and the asymptotic approach to the steady state is controlled by a time scale that diverges as (q-q_c)^{-1/2}.Comment: 4 pages, 5 figures, 2-column revtex4 forma

Analytical Solution of the Voter Model on Disordered Networks

We present a mathematical description of the voter model dynamics on heterogeneous networks. When the average degree of the graph is $\mu \leq 2$ the system reaches complete order exponentially fast. For $\mu >2$, a finite system falls, before it fully orders, in a quasistationary state in which the average density of active links (links between opposite-state nodes) in surviving runs is constant and equal to $\frac{(\mu-2)}{3(\mu-1)}$, while an infinite large system stays ad infinitum in a partially ordered stationary active state. The mean life time of the quasistationary state is proportional to the mean time to reach the fully ordered state $T$, which scales as $T \sim \frac{(\mu-1) \mu^2 N}{(\mu-2) \mu_2}$, where $N$ is the number of nodes of the network, and $\mu_2$ is the second moment of the degree distribution. We find good agreement between these analytical results and numerical simulations on random networks with various degree distributions.Comment: 20 pages, 8 figure

Biologically active thiosemicarbazone Fe chelators and their reactions with ferrioxamine B and ferric EDTA; a kinetic study

The Fe abstraction from Fe /DFO and Fe /EDTA complex systems by thiosemicarbazone ligands derived from 2-acetylpyridine has been studied from a kinetico-mechanistic perspective at relevant pH conditions and at varying temperatures and buffer solutions. The reactions have been found to be extremely dependent on the dominant E/Z isomeric form of the TSC ligands present in the reaction medium. Consequently the isomerisation processes occurring on the free ligands have also been monitored under equivalent conditions. The isomerisation process is found to be acid dependent, despite the absence of protonation under the conditions used, and presumably proceeds via an azo-type tautomer of the ligand. In all cases the existence of outer-sphere interaction processes has been established, both promoting the reactions and producing dead-end complexes. The better oriented forms of the ligands (EZ thiolate) have been found to react faster with the [Fe(HDFO)] complex, although for mono-N substituted thiosemicarbazones the process is retarded by the formation of a dead-end outer-sphere complex. A comparison with the abstraction of Fe from [Fe(EDTA)(H O)] has also been conducted with significant differences in the kinetic features that implicate keystone outer-sphere interactions which dominate reactivity, even with isomeric forms that are not the best suited for direct complexation

Los obstáculos para el desarrollo profesional de profesoras de enseñanza secundaria en ciencias experimentales

Este trabajo es un estudio de casos de profesoras de ciencias experimentales, con el objetivo de determinar los obstáculos que surgen de la interacción entre la reflexión y la práctica de aula, así como su influencia en el desarrollo profesional. El estudio se efectúa en dos planos diferentes, una investigación de orientación cualitativa y otra de naturaleza crítica, orientado hacia el desarrollo profesional. Los instrumentos de análisis fueron de diversa índole, entre ellos el horizonte de la integración. Los resultados mostraron que las profesoras analizadas se encontraban, a rasgos generales, en proceso de evolucionar de forma más compleja su práctica de aula, mostrando un grado de integración notable con los procesos reflexivos. A un nivel más particular, la naturaleza de los obstáculos es diferente para cada profesora

Preferential attachment in the protein network evolution

The Saccharomyces cerevisiae protein-protein interaction map, as well as many natural and man-made networks, shares the scale-free topology. The preferential attachment model was suggested as a generic network evolution model that yields this universal topology. However, it is not clear that the model assumptions hold for the protein interaction network. Using a cross genome comparison we show that (a) the older a protein, the better connected it is, and (b) The number of interactions a protein gains during its evolution is proportional to its connectivity. Therefore, preferential attachment governs the protein network evolution. The evolutionary mechanism leading to such preference and some implications are discussed.Comment: Minor changes per referees requests; to appear in PR