249,664 research outputs found
Spatial flocking: Control by speed, distance, noise and delay
Fish, birds, insects and robots frequently swim or fly in groups. During
their 3 dimensional collective motion, these agents do not stop, they avoid
collisions by strong short-range repulsion, and achieve group cohesion by weak
long-range attraction. In a minimal model that is isotropic, and continuous in
both space and time, we demonstrate that (i) adjusting speed to a preferred
value, combined with (ii) radial repulsion and an (iii) effective long-range
attraction are sufficient for the stable ordering of autonomously moving agents
in space. Our results imply that beyond these three rules ordering in space
requires no further rules, for example, explicit velocity alignment, anisotropy
of the interactions or the frequent reversal of the direction of motion,
friction, elastic interactions, sticky surfaces, a viscous medium, or vertical
separation that prefers interactions within horizontal layers. Noise and delays
are inherent to the communication and decisions of all moving agents. Thus,
next we investigate their effects on ordering in the model. First, we find that
the amount of noise necessary for preventing the ordering of agents is not
sufficient for destroying order. In other words, for realistic noise amplitudes
the transition between order and disorder is rapid. Second, we demonstrate that
ordering is more sensitive to displacements caused by delayed interactions than
to uncorrelated noise (random errors). Third, we find that with changing
interaction delays the ordered state disappears at roughly the same rate,
whereas it emerges with different rates. In summary, we find that the model
discussed here is simple enough to allow a fair understanding of the modeled
phenomena, yet sufficiently detailed for the description and management of
large flocks with noisy and delayed interactions. Our code is available at
http://github.com/fij/flocComment: 12 pages, 7 figure
Hyperons in neutron-star cores and two-solar-mass pulsar
Recent measurement of mass of PSR J1614-2230 rules out most of existing
models of equation of state (EOS) of dense matter with high-density softening
due to hyperonization or a phase transition to quark matter or a boson
condensate.
We look for a solution of an apparent contradiction between the consequences
stemming from up-to-date hypernuclear data, indicating appearance of hyperons
at 3 nuclear densities and existence of a two-solar-mass neutron star.
We consider a non-linear relativistic mean field (RMF) model involving baryon
octet coupled to meson fields. An effective lagrangian includes quartic terms
in the meson fields. The values of the parameters of the model are obtained by
fitting semi-empirical parameters of nuclear matter at the saturation point, as
well as potential wells for hyperons in nuclear matter and the strength of the
Lambda-Lambda attraction in double-Lambda hypernuclei.
We propose a non-linear RMF model which is consistent with up-to-date
semiempirical nuclear and hypernuclear data and allows for neutron stars with
hyperon cores and M larger than 2 solar masses. The model involves
hidden-strangenes scalar and vector mesons, coupled to hyperons only, and
quartic terms involving vector meson fields.
Our EOS involving hyperons is stiffer than the corresponding nucleonic EOS
(with hyperons artificially suppressed) above five nuclear densities. Required
stiffening is generated by the quartic terms involving hidden-strangeness
vector meson.Comment: 7 pages, 5 figures. Main results of this paper were already presented
at the MODE-SNR-PWN Workshop in Bordeaux, France, November 15-17, 2010, and
in a poster at the CompStar 2011 Workshop in Catania, Italy, May 9-12, 2011.
The paper is being submitted to Astronomy & Astrophysic
A Generalized Sznajd Model
In the last decade the Sznajd Model has been successfully employed in
modeling some properties and scale features of both proportional and majority
elections. We propose a new version of the Sznajd model with a generalized
bounded confidence rule - a rule that limits the convincing capability of
agents and that is essential to allow coexistence of opinions in the stationary
state. With an appropriate choice of parameters it can be reduced to previous
models. We solved this new model both in a mean-field approach (for an
arbitrary number of opinions) and numerically in a Barabasi-Albert network (for
three and four opinions), studying the transient and the possible stationary
states. We built the phase portrait for the special cases of three and four
opinions, defining the attractors and their basins of attraction. Through this
analysis, we were able to understand and explain discrepancies between
mean-field and simulation results obtained in previous works for the usual
Sznajd Model with bounded confidence and three opinions. Both the dynamical
system approach and our generalized bounded confidence rule are quite general
and we think it can be useful to the understanding of other similar models.Comment: 19 pages with 8 figures. Submitted to Physical Review
Modularity and Optimality in Social Choice
Marengo and the second author have developed in the last years a geometric
model of social choice when this takes place among bundles of interdependent
elements, showing that by bundling and unbundling the same set of constituent
elements an authority has the power of determining the social outcome. In this
paper we will tie the model above to tournament theory, solving some of the
mathematical problems arising in their work and opening new questions which are
interesting not only from a mathematical and a social choice point of view, but
also from an economic and a genetic one. In particular, we will introduce the
notion of u-local optima and we will study it from both a theoretical and a
numerical/probabilistic point of view; we will also describe an algorithm that
computes the universal basin of attraction of a social outcome in O(M^3 logM)
time (where M is the number of social outcomes).Comment: 42 pages, 4 figures, 8 tables, 1 algorithm
Adaptive hybrid optimization strategy for calibration and parameter estimation of physical models
A new adaptive hybrid optimization strategy, entitled squads, is proposed for
complex inverse analysis of computationally intensive physical models. The new
strategy is designed to be computationally efficient and robust in
identification of the global optimum (e.g. maximum or minimum value of an
objective function). It integrates a global Adaptive Particle Swarm
Optimization (APSO) strategy with a local Levenberg-Marquardt (LM) optimization
strategy using adaptive rules based on runtime performance. The global strategy
optimizes the location of a set of solutions (particles) in the parameter
space. The LM strategy is applied only to a subset of the particles at
different stages of the optimization based on the adaptive rules. After the LM
adjustment of the subset of particle positions, the updated particles are
returned to the APSO strategy. The advantages of coupling APSO and LM in the
manner implemented in squads is demonstrated by comparisons of squads
performance against Levenberg-Marquardt (LM), Particle Swarm Optimization
(PSO), Adaptive Particle Swarm Optimization (APSO; the TRIBES strategy), and an
existing hybrid optimization strategy (hPSO). All the strategies are tested on
2D, 5D and 10D Rosenbrock and Griewank polynomial test functions and a
synthetic hydrogeologic application to identify the source of a contaminant
plume in an aquifer. Tests are performed using a series of runs with random
initial guesses for the estimated (function/model) parameters. Squads is
observed to have the best performance when both robustness and efficiency are
taken into consideration than the other strategies for all test functions and
the hydrogeologic application
Quantum Probabilities as Behavioral Probabilities
We demonstrate that behavioral probabilities of human decision makers share
many common features with quantum probabilities. This does not imply that
humans are some quantum objects, but just shows that the mathematics of quantum
theory is applicable to the description of human decision making. The
applicability of quantum rules for describing decision making is connected with
the nontrivial process of making decisions in the case of composite prospects
under uncertainty. Such a process involves deliberations of a decision maker
when making a choice. In addition to the evaluation of the utilities of
considered prospects, real decision makers also appreciate their respective
attractiveness. Therefore, human choice is not based solely on the utility of
prospects, but includes the necessity of resolving the utility-attraction
duality. In order to justify that human consciousness really functions
similarly to the rules of quantum theory, we develop an approach defining human
behavioral probabilities as the probabilities determined by quantum rules. We
show that quantum behavioral probabilities of humans not merely explain
qualitatively how human decisions are made, but they predict quantitative
values of the behavioral probabilities. Analyzing a large set of empirical
data, we find good quantitative agreement between theoretical predictions and
observed experimental data.Comment: Latex file, 32 page
Experience-weighted Attraction Learning in Normal Form Games
In ‘experience-weighted attraction’ (EWA) learning, strategies have attractions that reflect initial predispositions, are updated based on payoff experience, and determine choice probabilities according to some rule (e.g., logit). A key feature is a parameter δ that weights the strength of hypothetical reinforcement of strategies that were not chosen according to the payoff they would have yielded, relative to reinforcement of chosen strategies according to received payoffs. The other key features are two discount rates, φ and ρ, which separately discount previous attractions, and an experience weight. EWA includes reinforcement learning and weighted fictitious play (belief learning) as special cases, and hybridizes their key elements. When δ= 0 and ρ= 0, cumulative choice reinforcement results. When δ= 1 and ρ=φ, levels of reinforcement of strategies are exactly the same as expected payoffs given weighted fictitious play beliefs. Using three sets of experimental data, parameter estimates of the model were calibrated on part of the data and used to predict a holdout sample. Estimates of δ are generally around .50, φ around .8 − 1, and ρ varies from 0 to φ. Reinforcement and belief-learning special cases are generally rejected in favor of EWA, though belief models do better in some constant-sum games. EWA is able to combine the best features of previous approaches, allowing attractions to begin and grow flexibly as choice reinforcement does, but reinforcing unchosen strategies substantially as belief-based models implicitly do
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