A scale-free network hidden in the collapsing polymer

We show that the collapsed globular phase of a polymer accommodates a scale-free incompatibility graph of its contacts. The degree distribution of this network is found to decay with the exponent $\gamma = 1/(2-c)$ up to a cut-off degree $d_c \propto L^{2-c}$, where $c$ is the loop exponent for dense polymers ($c=11/8$ in two dimensions) and $L$ is the length of the polymer. Our results exemplify how a scale-free network (SFN) can emerge from standard criticality.Comment: 4 pages, 3 figures, address correcte

What is the length of a knot in a polymer?

We give statistical definitions of the length, l, of a loose prime knot tied into a long, fluctuating ring macromolecule. Monte Carlo results for the equilibrium, good solvent regime show that ~ N^t, where N is the ring length and t ~ 0.75 is independent of the knot topology. In the collapsed regime below the theta temperature, length determinations based on the entropic competition of different knots within the same ring show delocalization (t~1).Comment: 9 pages, 5 Postscript figure

A functional RNase P protein subunit of bacterial origin in some eukaryotes

RNase P catalyzes 5′-maturation of tRNAs. While bacterial RNase P comprises an RNA catalyst and a protein cofactor, the eukaryotic (nuclear) variant contains an RNA and up to ten proteins, all unrelated to the bacterial protein. Unexpectedly, a nuclear-encoded bacterial RNase P protein (RPP) homolog is found in several prasinophyte algae including Ostreococcus tauri. We demonstrate that recombinant O. tauri RPP can functionally reconstitute with bacterial RNase P RNAs (RPRs) but not with O. tauri organellar RPRs, despite the latter’s presumed bacterial origins. We also show that O. tauri PRORP, a homolog of Arabidopsis PRORP-1, displays tRNA 5′-processing activity in vitro. We discuss the implications of the striking diversity of RNase P in O. tauri, the smallest known free-living eukaryote.Ministerio de Ciencia e Innovación European Regional Fund BFU2007-60651Junta de Andalucía P06-CVI-01692National Science Foundation MCB-0238233 MCB-0843543European Union ASSEMBLE 22779

Relativistic precession around rotating neutron stars: Effects due to frame-dragging and stellar oblateness

General relativity predicts that a rotating body produces a frame-dragging (or Lense-Thirring) effect: the orbital plane of a test particle in a non-equatorial orbit precesses about the body's symmetry axis. In this paper we compute the precession frequencies of circular orbits around rapidly rotating neutron stars for a variety of masses and equations of state. The precession frequencies computed are expressed as numerical functions of the orbital frequency observed at infinity. The post-Newtonian expansion of the exact precession formula is examined to identify the relative magnitudes of the precession caused by the Lense-Thirring effect, the usual Newtonian quadrupole effect and relativistic corrections. The first post-Newtonian correction to the Newtonian quadrupole precession is derived in the limit of slow rotation. We show that the post-Newtonian precession formula is a good approximation to the exact precession close to the neutron star in the slow rotation limit (up to \sim 400 Hz in the present context). The results are applied to recent RXTE observations of neutron star low-mass X-ray binaries, which display kHz quasi-periodic oscillations and, within the framework of beat frequency models, allow the measurement of both the neutron star spin frequency and the Keplerian frequency of the innermost ring of matter in the accretion disk around it. For a wide range of realistic equations of state, we find that the predicted precession frequency of this ring is close to one half of the low-frequency (\sim 20 - 35 Hz) quasi-periodic oscillations seen in several Atoll sources.Comment: 35 pages including 10 figures and 6 tables. To appear in the Astrophysical Journa

GaP betavoltaic cells as a power source

Maximum power output for the GaP cells of this study was found to be on the order of 1 microW. This resulted from exposure to 200 and 40 KeV electrons at a flux of 2 x 10(exp 9) electrons/sq cm/s, equivalent to a 54 mCurie source. The efficiencies of the cells ranged from 5 to 9 percent for 200 and 40 KeV electrons respectively. The lower efficiency at higher energy is due to a substantial fraction of energy deposition in the substrate, further than a diffusion length from the depletion region of the cell. Radiation damage was clearly observed in GaP after exposure to 200 KeV electrons at a fluence of 2 x 10(exp 12) electrons/sq cm. No discernable damage was observed after exposure to 40 KeV electrons at the same fluence. Analysis indicates that a GaP betavoltaic system would not be practical if limited to low energy beta sources. The power available would be too low even in the ideal case. By utilizing high activity beta sources, such as Sr-90/Y-90, it may be possible to achieve performance that could be suitable for some space power applications. However, to utilize such a source the problem of radiation damage in the beta cell material must be overcome

The Unusual Universality of Branching Interfaces in Random Media

We study the criticality of a Potts interface by introducing a {\it froth} model which, unlike its SOS Ising counterpart, incorporates bubbles of different phases. The interface is fractal at the phase transition of a pure system. However, a position space approximation suggests that the probability of loop formation vanishes marginally at a transition dominated by {\it strong random bond disorder}. This implies a linear critical interface, and provides a mechanism for the conjectured equivalence of critical random Potts and Ising models.Comment: REVTEX, 13 pages, 3 Postscript figures appended using uufile

Steady-state analysis of shortest expected delay routing

We consider a queueing system consisting of two non-identical exponential servers, where each server has its own dedicated queue and serves the customers in that queue FCFS. Customers arrive according to a Poisson process and join the queue promising the shortest expected delay, which is a natural and near-optimal policy for systems with non-identical servers. This system can be modeled as an inhomogeneous random walk in the quadrant. By stretching the boundaries of the compensation approach we prove that the equilibrium distribution of this random walk can be expressed as a series of product-forms that can be determined recursively. The resulting series expression is directly amenable for numerical calculations and it also provides insight in the asymptotic behavior of the equilibrium probabilities as one of the state coordinates tends to infinity.Comment: 41 pages, 13 figure

Olhar sistêmico sobre as dificuldades de aprendizagem: Projeto Rondon como espaço de transformação

Trabalho apresentado no II Congresso Nacional do PROJETO RONDON, realizado em Florianópolis, SC, no período de 23 a 25 de setembro de 2015 - Universidade Federal de Santa Catarina.INTRODUÇÃO: A dificuldade de aprendizagem pode ser compreendida como um fenômeno de natureza complexa em que a criança não corresponde às expectativas sobre ela. A criança pode estar a representar uma condição de vulnerabilidade psicossocial, propiciando sentimentos de baixa autoestima e inferioridade. Essas dificuldades, na aprendizagem escolar, frequentemente são acompanhadas de déficits em habilidades sociais e problemas emocionais ou de comportamento, sob a óptica de uma visão sistêmica do sujeito. Pensar e agir numa perspectiva sistêmica é manter um diálogo criativo entre o todo e as partes, considerando a produção do conhecimento como algo dinâmico e que se refaz tanto nas relações internas do próprio sujeito quanto nas suas relações sociais. Nesse cenário em construção, o Projeto Rondon viabiliza espaços para que possam ser desenvolvidas ações alternativas que contribuam para o desenvolvimento sustentável da comunidade, ampliando o bem-estar da população, favorecendo a formação do universitário como cidadão. OBJETIVO: Realizar uma oficina com a finalidade de explorar recursos educacionais com os professores, de forma a abarcar um contexto sob uma visão sistêmica e, assim, poder ressignificar as dificuldades que surgem no ambiente escolar. MATERIAL E MÉTODO: A oficina utilizará recursos audiovisuais para a contextualização das dificuldades de aprendizagem, estimulando os professores a trazerem os desafios encontrados em sala de aula. Para a finalização, será proposta uma roda de conversa, na qual serão apresentados recursos lúdicos para implementar as ações pedagógicas já realizadas. RESULTADOS ESPERADOS: Espera-se ampliar a compreensão dos professores sobre as dificuldades de aprendizagem, abrindo espaços para discussões e orientações colaborativas, compreendendo a necessidade de cada criança e o sistema no qual está inserida, resgatando recursos facilitadores para o seu desenvolvimento saudável e seus entornos. CONCLUSÃO: O olhar ampliado para a questão da dificuldade de aprendizagem possibilita que se formem novas construções, que redefinem os lugares da família, escola, comunidade, meio social, fortalecendo uma verdadeira rede relacional, permitindo a construção de narrativas mais includentes criadas em torno das competências e da resiliência da criança, família, escola e sociedade. Desse modo, é possível compreender uma dificuldade escolar como uma sugestão de mudança nas interrelações. Assim, a criação de novos significados para este desafio é o que justifica um trabalho orientado pelo pensamento sistêmico