826 research outputs found
Scarcity may promote cooperation in populations of simple agents
In the study of the evolution of cooperation, resource limitations are
usually assumed just to provide a finite population size. Recently, however, it
has been pointed out that resource limitation may also generate dynamical
payoffs able to modify the original structure of the games. Here we study
analytically a phase transition from a homogeneous population of defectors when
resources are abundant to the survival of unconditional cooperators when
resources reduce below a threshold. To this end, we introduce a model of simple
agents, with no memory or ability of recognition, interacting in well-mixed
populations. The result might shed light on the role played by resource
constraints on the origin of multicellularity.Comment: 5 pages, 2 figure
Analytical models for well-mixed populations of cooperators and defectors under limiting resources
In the study of the evolution of cooperation, resource limitations are
usually assumed just to provide a finite population size. Recently, however,
agent-based models have pointed out that resource limitation may modify the
original structure of the interactions and allow for the survival of
unconditional cooperators in well-mixed populations. Here, we present
analytical simplified versions of two types of agent-based models recently
published: one in which the limiting resource constrains the ability of
reproduction of individuals but not their survival, and a second one where the
limiting resource is necessary for both reproduction and survival. One finds
that the analytical models display, with a few differences, the same
qualitative behavior of the more complex agent-based models. In addition, the
analytical models allow us to expand the study and identify the dimensionless
parameters governing the final fate of the system, such as coexistence of
cooperators and defectors, or dominance of defectors or of cooperators. We
provide a detailed analysis of the occurring phase transitions as these
parameters are varied.Comment: 7 pages, 8 figure
Analytical models for well-mixed populations of cooperators and defectors under limiting resources
Stability and robustness analysis of cooperation cycles driven by destructive agents in finite populations
The emergence and promotion of cooperation are two of the main issues in evolutionary game theory, as cooperation is amenable to exploitation by defectors, which take advantage of cooperative individuals at no cost, dooming them to extinction. It has been recently shown that the existence of purely destructive agents (termed jokers) acting on the common enterprises (public goods games) can induce stable limit cycles among cooperation, defection, and destruction when infinite populations are considered. These cycles allow for time lapses in which cooperators represent a relevant fraction of the population, providing a mechanism for the emergence of cooperative states in nature and human societies. Here we study analytically and through agent-based simulations the dynamics generated by jokers in finite populations for several selection rules. Cycles appear in all cases studied, thus showing that the joker dynamics generically yields a robust cyclic behavior not restricted to infinite populations. We also compute the average time in which the population consists mostly of just one strategy and compare the results with numerical simulations.Financial support from the Ministerio de Ciencia y Tecnología (Spain) under Projects No. FIS2009-13730-C02-02 (A.A.) and No. FIS2009-13370-C02-01 (J.C. and R.J.R.),
MOSAICO, PRODIEVO, and Complexity-NET RESINEE (J.A.C.); from the Barcelona Graduate School of Economics and the government of Catalonia (A.A.); from the Generalitat de Catalunya under Projects No. 2009SGR0838 (A.A.) and
No. 2009SGR0164 (J.C. and R.J.R.); and from Comunidad de Madrid under Project MODELICO-CM (J.A.C.) is appreciated.
R.J.R. acknowledges the financial support of the Universitat Autònoma de Barcelona and the Spanish government
Cationic exchange in nanosized ZnFe2O4 spinel revealed by experimental and simulated near-edge absorption structure
The non-equilibrium cation site occupancy in nanosized zinc ferrites (6-13
nm) with different degree of inversion (0.2 to 0.4) was investigated using Fe
and Zn K-edge x-ray absorption spectroscopy XANES and EXAFS, and magnetic
measurements. The very good agreement between experimental and ab-initio
calculations on the Zn K-edge XANES region clearly show the large
Zn2+(A)--Zn2+[B] transference that takes place in addition to the
well-identified Fe3+[B]--Fe3+(A) one, without altering the long-range
structural order. XANES spectra features as a function of the spinel inversion
were shown to depend on the configuration of the ligand shells surrounding the
absorbing atom. This XANES approach provides a direct way to sense cationic
inversion in these spinel compounds. We also demonstrated that a mechanical
crystallization takes place on nanocrystalline spinel that causes an increase
of both grain and magnetic sizes and, simultaneously, generates a significant
augment of the inversion.Comment: 5 pages, 5 eps figures, uses revtex4, corrected table
Structure of Extremely Nanosized and Confined In-O Species in Ordered Porous Materials
Perturbed-angular correlation, x-ray absorption, and small-angle x-ray
scattering spectroscopies were suitably combined to elucidate the local
structure of highly diluted and dispersed InOx species confined in porous of
ZSM5 zeolite. These novel approach allow us to determined the structure of
extremely nanosized In-O species exchanged inside the 10-atom-ring channel of
the zeolite, and to quantify the amount of In2O3 crystallites deposited onto
the external zeolite surface.Comment: 4 pages, 5 postscript figures, REVTEX4, published in Physical Review
Letter
Thinking outside the box: effects of modes larger than the survey on matter power spectrum covariance
Accurate power spectrum (or correlation function) covariance matrices are a crucial requirement for cosmological parameter estimation from large scale structure surveys. In order to minimize reliance on computationally expensive mock catalogs, it is important to have a solid analytic understanding of the different components that make up a covariance matrix. Considering the matter power spectrum covariance matrix, it has recently been found that there is a potentially dominant effect on mildly non-linear scales due to power in modes of size equal to and larger than the survey volume. This beat coupling effect has been derived analytically in perturbation theory and while it has been tested with simulations, some questions remain unanswered. Moreover, there is an additional effect of these large modes, which has so far not been included in analytic studies, namely the effect on the estimated average density which enters the power spectrum estimate. In this article, we work out analytic, perturbation theory based expressions including both the beat coupling and this local average effect and we show that while, when isolated, beat coupling indeed causes large excess covariance in agreement with the literature, in a realistic scenario this is compensated almost entirely by the local average effect, leaving only similar to 10% of the excess. We test our analytic expressions by comparison to a suite of large N-body simulations, using both full simulation boxes and subboxes thereof to study cases without beat coupling, with beat coupling and with both beat coupling and the local average effect. For the variances, we find excellent agreement with the analytic expressions for k < 0.2 hMpc(-1) at z = 0.5, while the correlation coefficients agree to beyond k = 0.4 hMpc(-1). As expected, the range of agreement increases towards higher redshift and decreases slightly towards z = 0. We finish by including the large-mode effects in a full covariance matrix description for arbitrary survey geometry and confirming its validity using simulations. This may be useful as a stepping stone towards building an actual galaxy (or other tracer's) power spectrum covariance matrix
Producción forrajera de Bouteloua gracilis 1986/87 (H.B.K.) Lag. ex Steud durante una temporada de crecimiento
In a plot of Bouteloua gradlis (a warm introduced grass) to the NW of Carro Quemado town, the monthly production of aereal phytomass during the growing period and its answer to nitrogenous fertilization were determined. The highest production was of '22.7,6gMS.m-2 in May /87. The percentaje of its partidpation in the total of aereal fitomass from 80 to 97% during the period analized. The response to the fertilization was good, the best results were obtained with the dose of 30 Kg N/ha in de first cut (vegetative period). In the second cut (end growing period) the best dose was the one of 90 Kg N/ha.En una parcela de Bouteloua gracilis (gramínea introducida estival), al NW de Carro Quemado, se determinó la producción mensual de fitomasa aérea durante un periodo de crecimiento y su respuesta a la fertilización nitrogenada. La máxima producción fue de '22.7,6gMS.m-2 en Mayo/87. El porcentaje de su participación en el total de fitomasa aérea fue del 80 al 97 % durante el periodo analizado. La respuesta a la fertilización fue buena, obteniendo los mejores resultados con la dosis de 30 Kg de N/ha en el primer corte (etapa vegetativa). En el segundo corte (final de la temporada de crecimiento) la mejor dosis fue la de 90 Kg de N/ha.Trabajo final de graduación. Cátedra de Ecología Vegetal. Facultad de Agronomía. UNLPa
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