1,247 research outputs found
On the micro mechanics of one-dimensional normal compression
Discrete-element modelling has been used to investigate the micro mechanics of one-dimensional compression. One-dimensional compression is modelled in three dimensions using an oedometer and a large number of particles, and without the use of agglomerates. The fracture of a particle is governed by the octahedral shear stress within the particle due to the multiple contacts and a Weibull distribution of strengths. Different fracture mechanisms are considered, and the influence of the distribution of fragments produced for each fracture on the global particle size distribution and the slope of the normal compression line is investigated. Using the discrete-element method, compression is related to the evolution of a fractal distribution of particles. The compression index is found to be solely a function of the strengths of the particles as a function of size
Aspiring to the fittest and promotion of cooperation in the prisoner's dilemma game
Strategy changes are an essential part of evolutionary games. Here we
introduce a simple rule that, depending on the value of a single parameter ,
influences the selection of players that are considered as potential sources of
the new strategy. For positive players with high payoffs will be considered
more likely, while for negative the opposite holds. Setting equal to
zero returns the frequently adopted random selection of the opponent. We find
that increasing the probability of adopting the strategy from the fittest
player within reach, i.e. setting positive, promotes the evolution of
cooperation. The robustness of this observation is tested against different
levels of uncertainty in the strategy adoption process and for different
interaction network. Since the evolution to widespread defection is tightly
associated with cooperators having a lower fitness than defectors, the fact
that positive values of facilitate cooperation is quite surprising. We show
that the results can be explained by means of a negative feedback effect that
increases the vulnerability of defectors although initially increasing their
survivability. Moreover, we demonstrate that the introduction of
effectively alters the interaction network and thus also the impact of
uncertainty by strategy adoptions on the evolution of cooperation.Comment: 7 two-column pages, 5 figures; accepted for publication in Physical
Review
Restricted connections among distinguished players support cooperation
We study the evolution of cooperation within the spatial prisoner's dilemma
game on a square lattice where a fraction of players can spread their
strategy more easily than the rest due to a predetermined larger teaching
capability. In addition, players characterized with the larger teaching
capability are allowed to temporarily link with distant opponents of the same
kind with probability , thus introducing shortcut connections among the
distinguished. We show that these additional temporary connections are able to
sustain cooperation throughout the whole range of the temptation to defect.
Remarkably, we observe that as the temptation to defect increases the optimal
decreases, and moreover, only minute values of warrant the best
promotion of cooperation. Our study thus indicates that influential individuals
must be few and sparsely connected in order for cooperation to thrive in a
defection prone environment.Comment: 6 two-column pages, 6 figures; accepted for publication in Physical
Review
On Phase Transitions to Cooperation in the Prisoner's Dilemma
Game theory formalizes certain interactions between physical particles or
between living beings in biology, sociology, and economics, and quantifies the
outcomes by payoffs. The prisoner's dilemma (PD) describes situations in which
it is profitable if everybody cooperates rather than defects (free-rides or
cheats), but as cooperation is risky and defection is tempting, the expected
outcome is defection. Nevertheless, some biological and social mechanisms can
support cooperation by effectively transforming the payoffs. Here, we study the
related phase transitions, which can be of first order (discontinous) or of
second order (continuous), implying a variety of different routes to
cooperation. After classifying the transitions into cases of equilibrium
displacement, equilibrium selection, and equilibrium creation, we show that a
transition to cooperation may take place even if the stationary states and the
eigenvalues of the replicator equation for the PD stay unchanged. Our example
is based on adaptive group pressure, which makes the payoffs dependent on the
endogeneous dynamics in the population. The resulting bistability can invert
the expected outcome in favor of cooperation.Comment: For related work see http://www.soms.ethz.ch
Impact of aging on the evolution of cooperation in the spatial prisoner's dilemma game
Aging is always present, tailoring our interactions with others and
postulating a finite lifespan during which we are able to exercise them. We
consider the prisoner's dilemma game on a square lattice, and examine how
quenched age distributions and different aging protocols influence the
evolution of cooperation when taking the life experience and knowledge
accumulation into account as time passes. In agreement with previous studies,
we find that a quenched assignment of age to players, introducing heterogeneity
to the game, substantially promotes cooperative behavior. Introduction of aging
and subsequent death as a coevolutionary process may act detrimental on
cooperation but enhances it efficiently if the offspring of individuals that
have successfully passed their strategy is considered newborn. We study
resulting age distributions of players, and show that the heterogeneity is
vital yet insufficient for explaining the observed differences in cooperator
abundance on the spatial grid. The unexpected increment of cooperation levels
can be explained by a dynamical effect that has a highly selective impact on
the propagation of cooperator and defector states.Comment: 7 two-column pages, 5 figures; accepted for publication in Physical
Review
Chemical fracture and distribution of extreme values
When a corrosive solution reaches the limits of a solid sample, a chemical
fracture occurs. An analytical theory for the probability of this chemical
fracture is proposed and confirmed by extensive numerical experiments on a two
dimensional model. This theory follows from the general probability theory of
extreme events given by Gumbel. The analytic law differs from the Weibull law
commonly used to describe mechanical failures for brittle materials. However a
three parameters fit with the Weibull law gives good results, confirming the
empirical value of this kind of analysis.Comment: 7 pages, 5 figures, to appear in Europhysics Letter
Statistical mechanics of systems with heterogeneous agents: Minority Games
We study analytically a simple game theoretical model of heterogeneous
interacting agents. We show that the stationary state of the system is
described by the ground state of a disordered spin model which is exactly
solvable within the simple replica symmetric ansatz. Such a stationary state
differs from the Nash equilibrium where each agent maximizes her own utility.
The latter turns out to be characterized by a replica symmetry broken
structure. Numerical results fully agree with our analytic findings.Comment: 4 pages, 1 Postscript figure. Revised versio
Conditional strategies and the evolution of cooperation in spatial public goods games
The fact that individuals will most likely behave differently in different
situations begets the introduction of conditional strategies. Inspired by this,
we study the evolution of cooperation in the spatial public goods game, where
besides unconditional cooperators and defectors, also different types of
conditional cooperators compete for space. Conditional cooperators will
contribute to the public good only if other players within the group are likely
to cooperate as well, but will withhold their contribution otherwise. Depending
on the number of other cooperators that are required to elicit cooperation of a
conditional cooperator, the latter can be classified in as many types as there
are players within each group. We find that the most cautious cooperators, such
that require all other players within a group to be conditional cooperators,
are the undisputed victors of the evolutionary process, even at very low
synergy factors. We show that the remarkable promotion of cooperation is due
primarily to the spontaneous emergence of quarantining of defectors, which
become surrounded by conditional cooperators and are forced into isolated
convex "bubbles" from where they are unable to exploit the public good. This
phenomenon can be observed only in structured populations, thus adding to the
relevance of pattern formation for the successful evolution of cooperation.Comment: 7 two-column pages, 7 figures; accepted for publication in Physical
Review
New methods for unmixing sediment grain size data
Grain size distribution (GSD) data are widely used in Earth sciences and although large data sets are regularly generated, detailed numerical analyses are not routine. Unmixing GSDs into components can help understand sediment provenance and depositional regimes/processes. End-member analysis (EMA), which fits one set of end-members to a given data set, is a powerful way to unmix GSDs into geologically meaningful parts. EMA estimates end-members based on covariability within a data set and can be considered as a nonparametric approach. Available EMA algorithms, however, either produce suboptimal solutions or are time consuming. We introduce unmixing algorithms inspired by hyperspectral image analysis that can be applied to GSD data and which provide an improvement over current techniques. Nonparametric EMA is often unable to identify unimodal grain size subpopulations that correspond to single sediment sources. An alternative approach is single-specimen unmixing (SSU), which unmixes individual GSDs into unimodal parametric distributions (e.g., lognormal). We demonstrate that the inherent nonuniqueness of SSU solutions renders this approach unviable for estimating underlying mixing processes. To overcome this, we develop a new algorithm to perform parametric EMA, whereby an entire data set can be unmixed into unimodal parametric end-members (e.g., Weibull distributions). This makes it easier to identify individual grain size subpopulations in highly mixed data sets. To aid investigators in applying these methods, all of the new algorithms are available in AnalySize, which is GUI software for processing and unmixing grain size data
Discrete element modelling of rock communition in a cone crusher using a bonded particle model
It is known that discrete element method modelling (DEM) of rock size reduction can be achieved by two approaches: the population balance model (PBM) and the bonded particle model (BPM). However, only PBM has been successfully used in DEM modelling cone crusher in the literature. The aim of this paper is to explore the feasibility of using the BPM to represent the size reduction of rock experienced within the cone crusher chamber. The feed rock particles were represented by isotropic dense random packing agglomerates. The simulation results were compared with the PBM simulation results, and it was shown that the BPM cone crusher model was able to satisfactorily replicate the performance of a cone crusher as well and it can provide more accurate prediction of the percentage of the fine products. In addition, the novel contribution here is that the rock feed material comprises particles of realistic shapes which break into more realistically shaped fragments compared with the fragments with defined shapes in the PBM model
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