2,677 research outputs found
Imperfect Imitation Can Enhance Cooperation
The promotion of cooperation on spatial lattices is an important issue in
evolutionary game theory. This effect clearly depends on the update rule: it
diminishes with stochastic imitative rules whereas it increases with
unconditional imitation. To study the transition between both regimes, we
propose a new evolutionary rule, which stochastically combines unconditional
imitation with another imitative rule. We find that, surprinsingly, in many
social dilemmas this rule yields higher cooperative levels than any of the two
original ones. This nontrivial effect occurs because the basic rules induce a
separation of timescales in the microscopic processes at cluster interfaces.
The result is robust in the space of 2x2 symmetric games, on regular lattices
and on scale-free networks.Comment: 4 pages, 4 figure
Alianzas y oposiciones entre indios y militares durante el siglo XX. Un modelo social ecuatoriano
Catastrophic regime shifts in model ecological communities are true phase transitions
Ecosystems often undergo abrupt regime shifts in response to gradual external
changes. These shifts are theoretically understood as a regime switch between
alternative stable states of the ecosystem dynamical response to smooth changes
in external conditions. Usual models introduce nonlinearities in the
macroscopic dynamics of the ecosystem that lead to different stable attractors
among which the shift takes place. Here we propose an alternative explanation
of catastrophic regime shifts based on a recent model that pictures ecological
communities as systems in continuous fluctuation, according to certain
transition probabilities, between different micro-states in the phase space of
viable communities. We introduce a spontaneous extinction rate that accounts
for gradual changes in external conditions, and upon variations on this control
parameter the system undergoes a regime shift with similar features to those
previously reported. Under our microscopic viewpoint we recover the main
results obtained in previous theoretical and empirical work (anomalous
variance, hysteresis cycles, trophic cascades). The model predicts a gradual
loss of species in trophic levels from bottom to top near the transition. But
more importantly, the spectral analysis of the transition probability matrix
allows us to rigorously establish that we are observing the fingerprints, in a
finite size system, of a true phase transition driven by background
extinctions.Comment: 19 pages, 11 figures, revised versio
Cluster density functional theory for lattice models based on the theory of Mobius functions
Rosenfeld's fundamental measure theory for lattice models is given a rigorous
formulation in terms of the theory of Mobius functions of partially ordered
sets. The free-energy density functional is expressed as an expansion in a
finite set of lattice clusters. This set is endowed a partial order, so that
the coefficients of the cluster expansion are connected to its Mobius function.
Because of this, it is rigorously proven that a unique such expansion exists
for any lattice model. The low-density analysis of the free-energy functional
motivates a redefinition of the basic clusters (zero-dimensional cavities)
which guarantees a correct zero-density limit of the pair and triplet direct
correlation functions. This new definition extends Rosenfeld's theory to
lattice model with any kind of short-range interaction (repulsive or
attractive, hard or soft, one- or multi-component...). Finally, a proof is
given that these functionals have a consistent dimensional reduction, i.e. the
functional for dimension d' can be obtained from that for dimension d (d'<d) if
the latter is evaluated at a density profile confined to a d'-dimensional
subset.Comment: 21 pages, 2 figures, uses iopart.cls, as well as diagrams.sty
(included
Fluid-fluid phase separation in hard spheres with a bimodal size distribution
The effect of polydispersity on the phase behaviour of hard spheres is
examined using a moment projection method. It is found that the
Boublik-Mansoori-Carnahan-Starling-Leland equation of state shows a spinodal
instability for a bimodal distribution if the large spheres are sufficiently
polydisperse, and if there is sufficient disparity in mean size between the
small and large spheres. The spinodal instability direction points to the
appearance of a very dense phase of large spheres.Comment: 7 pages, 3 figures, moderately REVISED following referees' comments
(original was 4 pages, 3 postscript figures
Proterozoic amphibolites from Central Sector of Ossa Morena Zone: geochemistry and geodynamics implications
[Resumen] Las anfibolitas objeto de este estudio aparecen en la Sucesión Montemolín, correspondiente
al Proterozoico Superior del sector central de la Ossa-Morena. Esta
Sucesión está constituida por una alternancia de esquistos y cuarzo-esquistos biotíticos
con pasadas anfibolíticas que se hacen más abundantes hacia el techo. En
conjunto, presenta un metamorfismo regional progresivo que varía desde el grado
bajo-muy bajo hasta muy alto en el núcleo migmático de Monesterio y está caracterizado
por gradientes de altas temperaturas y bajas presiones.
Petrográficamente las anfibolitas se pueden dividir en varios grupos, tanto desde
el punto de vista textural como desde el mineralógico, variando desde anfibolitas
con actinolita-clorita a anfibolitas con hornblenda marrón y piroxeno, desapareciendo
progresivamente los rasgos texturales de rocas ígneas.
Desde el punto de vista de su geoquímica presentan una afinidad oceánica, la mayoría de ellas similares a basaltos de tipo MORB, que se asemejaría a los N-MORE según la distribución de tierras raras. Estas anfibolitas podrían corresponder a toleítas de arco de islas. Se presentan datos de otras anfibolitas de zonas próximas a fin de comparar tanto su quimismo como su posible ambiente geodinámico.[Abstract] The amphibolites studied herein crop out in the Montemolín Series from Upper Proterozoic of the Ossa-Morena. This succesion is made of alternating biotitic schists and quartz-schists with interbedded amphibolites more common toward the top. The ensemble shows a regional, progressive metamorphism which varies from very low -low-grade up to high-grade conditions at the Monesterio migmatic core and is characterized by high - T and Low- P gradients. From a petrographic point of view amphibolites may be divided into sorne groups on the basis of both textural or mineralogical criteria. Actinolite-chloriteamphibolites are observed to grade into brown hornblende and pyroxenebearing amphibolites, the primary textural features of protoliths progressively disappearing.
Geochemical affinities of these rocks are oceanic, most of the MORB basalts being similar to N-MORB-types on the basis ofREE distributions. They could be ascribed to island-arc tholeiites. Geochemical data from other amphibolites at neighbouring areas are also presented with the aim of comparing their compositions as well as their geodynamic setting
Bardeen-Petterson effect and the disk structure of the Seyfert galaxy NGC 1068
VLBA high spatial resolution observations of the disk structure of the active
galactic nucleus NGC 1068 has recently revealed that the kinematics and
geometry of this AGN is well characterized by an outer disk of H2O maser
emission having a compact milliarcsecond (parsec) scale structure, which is
encircling a thin rotating inner disk surrounding a ~10^7 M_\sun compact
mass, likely a black hole. A curious feature in this source is the occurrence
of a misalignment between the inner and outer parts of the disk, with the
galaxy's radio jet being orthogonal to the inner disk. We interpret this
peculiar configuration as due to the Bardeen-Petterson effect, a general
relativistic effect that warps an initially inclined (to the black hole
equator) viscous disk, and drives the angular momentum vector of its inner part
into alignment with the rotating black hole spin. We estimate the time-scale
for both angular momenta to get aligned as a function the spin parameter of the
Kerr black hole. We also reproduce the shape of the parsec and kiloparsec scale
jets, assuming a model in which the jet is precessing with a period and
aperture angle that decrease exponentially with time, as expected from the
Bardeen-Petterson effect.Comment: 12 pages, 3 figures, accepted for publication in The Astrophysical
Journa
Non-scattering Metasurface-bound Cavities for Field Localization, Enhancement, and Suppression
We propose and analyse metasurface-bound invisible (non-scattering) partially
open cavities where the inside field distribution can be engineered. It is
demonstrated both theoretically and experimentally that the cavities exhibit
unidirectional invisibility at the operating frequency with enhanced or
suppressed field at different positions inside the cavity volume. Several
examples of applications of the designed cavities are proposed and analyzed, in
particular, cloaking sensors and obstacles, enhancement of emission, and
"invisible waveguides". The non-scattering mode excited in the proposed cavity
is driven by the incident wave and resembles an ideal bound state in the
continuum of electromagnetic frequency spectrum. In contrast to known bound
states in the continuum, the mode can stay localized in the cavity infinitely
long, provided that the incident wave illuminates the cavity
Addressing unresolved tensions to build effective partnerships: Lessons from an Aboriginal cancer support network
Background: Cancer is the second leading cause of death among Aboriginal and Torres Strait Islander people and their survival once diagnosed with cancer is lower compared to that of other Australians. This highlights the need to improve cancer-related health services for Indigenous Australians although how to achieve this remains unclear. Cancer support groups provide emotional and practical support, foster a sense of community and belonging and can improve health outcomes. However, despite evidence on their positive effects on people affected by cancer, there is scarce information on the function and effectiveness of Indigenous-specific cancer peer-support programs in Australia. Using qualitative data from an evaluation study, this paper explores different understandings of how a cancer support group should operate and the impact of unresolved tensions following the establishment of an Indigenous women cancer peer-support network in a regional town in Western Australia. Methods: Data were collected through semi-structured interviews with 24 participants purposively selected among Indigenous and mainstream healthcare service providers, and group members and clients. Interviews were audiotaped and transcribed verbatim. Transcripts were subjected to inductive thematic analysis. NVivo was used to manage the data and assist in the data analysis. Rigour was enhanced through team member checking, coding validation and peer debriefing. Results: Flexibility and a resistance to formal structuring were at the core of how the group operated. It was acknowledged that the network partly owned its success to its fluid approach; however, most mainstream healthcare service providers believed that a more structured approach was needed for the group to be sustainable. This was seen as acting in opposition to the flexible, organic approach considered necessary to adequately respond to Indigenous women's needs. At the core of these tensions were opposing perspectives on the constructs of 'structure' and 'flexibility' between Indigenous and non-Indigenous participants. Conclusions: Despite the group's achievements, unresolved tensions between opposing perspectives on how a support group should operate negatively impacted on the working relationship between the group and mainstream service providers, and posed a threat to the Network's sustainability. Our results support the need to acknowledge and address different perspectives and world views in order to build strong, effective partnerships between service providers and Indigenous communities
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