1,361 research outputs found
Correlation of Positive and Negative Reciprocity Fails to Confer an Evolutionary Advantage: Phase Transitions to Elementary Strategies
Economic experiments reveal that humans value cooperation and fairness. Punishing unfair behavior is therefore common, and according to the theory of strong reciprocity, it is also directly related to rewarding cooperative behavior. However, empirical data fail to confirm that positive and negative reciprocity are correlated. Inspired by this disagreement, we determine whether the combined application of reward and punishment is evolutionarily advantageous. We study a spatial public goods game, where in addition to the three elementary strategies of defection, rewarding, and punishment, a fourth strategy that combines the latter two competes for space. We find rich dynamical behavior that gives rise to intricate phase diagrams where continuous and discontinuous phase transitions occur in succession. Indirect territorial competition, spontaneous emergence of cyclic dominance, as well as divergent fluctuations of oscillations that terminate in an absorbing phase are observed. Yet, despite the high complexity of solutions, the combined strategy can survive only in very narrow and unrealistic parameter regions. Elementary strategies, either in pure or mixed phases, are much more common and likely to prevail. Our results highlight the importance of patterns and structure in human cooperation, which should be considered in future experiments
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
Self-Organized Ordering of Nanostructures Produced by Ion-Beam Sputtering
We study the self-organized ordering of nanostructures produced by ion-beam
sputtering (IBS) of targets amorphizing under irradiation. By introducing a
model akin to models of pattern formation in aeolian sand dunes, we extend
consistently the current continuum theory of erosion by IBS. We obtain new
non-linear effects responsible for the in-plane ordering of the structures,
whose strength correlates with the degree of ordering found in experiments. Our
results highlight the importance of redeposition and surface viscous flow to
this nanopattern formation process.Comment: 4 pages, 2 figure
Dynamical charge and spin density wave scattering in cuprate superconductor
We show that a variety of spectral features in high-T_c cuprates can be
understood from the coupling of charge carriers to some kind of dynamical order
which we exemplify in terms of fluctuating charge and spin density waves. Two
theoretical models are investigated which capture different aspects of such
dynamical scattering. The first approach leaves the ground state in the
disordered phase but couples the electrons to bosonic degrees of freedom,
corresponding to the quasi singular scattering associated with the closeness to
an ordered phase. The second, more phenomological approach starts from the
construction of a frequency dependent order parameter which vanishes for small
energies. Both theories capture scanning tunneling microscopy and angle-resoved
photoemission experiments which suggest the protection of quasiparticles close
to the Fermi energy but the manifestation of long-range order at higher
frequencies.Comment: 27 pages, 13 figures, to appear in New J. Phy
A Fluid Dynamics Calculation of Sputtering from a Cylindrical Thermal Spike
The sputtering yield, Y, from a cylindrical thermal spike is calculated using
a two dimensional fluid dynamics model which includes the transport of energy,
momentum and mass. The results show that the high pressure built-up within the
spike causes the hot core to perform a rapid expansion both laterally and
upwards. This expansion appears to play a significant role in the sputtering
process. It is responsible for the ejection of mass from the surface and causes
fast cooling of the cascade. The competition between these effects accounts for
the nearly linear dependence of with the deposited energy per unit depth
that was observed in recent Molecular Dynamics simulations. Based on this we
describe the conditions for attaining a linear yield at high excitation
densities and give a simple model for this yield.Comment: 10 pages, 9 pages (including 9 figures), submitted to PR
A Two-Player Game of Life
We present a new extension of Conway's game of life for two players, which we
call p2life. P2life allows one of two types of token, black or white, to
inhabit a cell, and adds competitive elements into the birth and survival rules
of the original game. We solve the mean-field equation for p2life and determine
by simulation that the asymptotic density of p2life approaches 0.0362.Comment: 7 pages, 3 figure
Unrestricted slave-boson mean-field approximation for the two-dimensional Hubbard model
The Kotliar-Ruckenstein slave-boson scheme is used to allow for an
unrestricted variation of the bosonic and fermionic fields on the saddle-point
level. Various inhomogeneous solutions, such as spin polarons and domain walls
are discussed within the two-dimensional Hubbard model and compared with
results of unrestricted Hartree-Fock (HF) calculations. We find that the
present approach drastically reduces the polarization of these states and leads
to increased delocalized wave functions as compared to the HF model. The
interaction between two spin polarons turns out to be attractive over a wide
range of the on-site repulsion U. In addition we obtain the crossover from
vertical to diagonal domain walls at a higher value of U than predicted by HF.Comment: 6 pages, 8 figures, Accepted for publication in Phys. Rev.
Renormalization Group Analysis of a Noisy Kuramoto-Sivashinsky Equation
We have analyzed the Kuramoto-Sivashinsky equation with a stochastic noise
term through a dynamic renormalization group calculation. For a system in which
the lattice spacing is smaller than the typical wavelength of the linear
instability occurring in the system, the large-distance and long-time behavior
of this equation is the same as for the Kardar-Parisi-Zhang equation in one and
two spatial dimensions. For the case the agreement is only qualitative.
On the other hand, when coarse-graining on larger scales the asymptotic flow
depends on the initial values of the parameters.Comment: 8 pages, 5 figures, revte
Does strong heterogeneity promote cooperation by group interactions?
Previous research has highlighted the importance of strong heterogeneity for
the successful evolution of cooperation in games governed by pairwise
interactions. Here we determine to what extent this is true for games governed
by group interactions. We therefore study the evolution of cooperation in the
public goods game on the square lattice, the triangular lattice and the random
regular graph, whereby the payoffs are distributed either uniformly or
exponentially amongst the players by assigning to them individual scaling
factors that determine the share of the public good they will receive. We find
that uniformly distributed public goods are more successful in maintaining high
levels of cooperation than exponentially distributed public goods. This is not
in agreement with previous results on games governed by pairwise interactions,
indicating that group interactions may be less susceptible to the promotion of
cooperation by means of strong heterogeneity as originally assumed, and that
the role of strongly heterogeneous states should be reexamined for other types
of games.Comment: 12 pages, 4 figures; accepted for publication in New Journal of
Physics [related work available at http://arxiv.org/abs/0708.1746 and
http://www.matjazperc.com/
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