168 research outputs found
Nash equilibria in quantum games with generalized two-parameter strategies
In the Eisert protocol for 2 X 2 quantum games [Phys. Rev. Lett. 83, 3077], a
number of authors have investigated the features arising from making the
strategic space a two-parameter subset of single qubit unitary operators. We
argue that the new Nash equilibria and the classical-quantum transitions that
occur are simply an artifact of the particular strategy space chosen. By
choosing a different, but equally plausible, two-parameter strategic space we
show that different Nash equilibria with different classical-quantum
transitions can arise. We generalize the two-parameter strategies and also
consider these strategies in a multiplayer setting.Comment: 19 pages, 2 eps figure
Crossing scales, crossing disciplines: collective motion and collective action in the Global Commonsâ
Two conflicting tendencies can be seen throughout the biological world: individuality and collective behaviour. Natural selection operates on differences among individuals, rewarding those who perform better. Nonetheless, even within this milieu, cooperation arises, and the repeated emergence of multicellularity is the most striking example. The same tendencies are played out at higher levels, as individuals cooperate in groups, which compete with other such groups. Many of our environmental and other global problems can be traced to such conflicts, and to the unwillingness of individual agents to take account of the greater good. One of the great challenges in achieving sustainability will be in understanding the basis of cooperation, and in taking multicellularity to yet a higher level, finding the pathways to the level of cooperation that is the only hope for the preservation of the planet
The MACHO Project 9 Million Star Color-Magnitude Diagram of the Large Magellanic Cloud
We present a 9 million star color-magnitude diagram (9M CMD) of the LMC bar.
The 9M CMD reveals a complex superposition of different age and metallicity
stellar populations, with important stellar evolutionary phases occurring over
3 orders of magnitude in number density. First, we count the non-variable
supergiants, the associated Cepheids, and measure the effective temperatures
defining the instability strip. Lifetime predictions of stellar evolution
theory are tested, with implications for the origin of low-luminosity Cepheids.
The highly-evolved AGB stars have a bimodal distribution in brightness, which
we interpret as discrete old populations (>1 Gyr). The faint AGB may be
metal-poor and very old. We identify the clusters NGC 411 and M3 as templates
for the admixture of old stellar populations. However, there are indications
that the old and metal-poor field population has a red HB morphology: the RR
Lyraes lie on the red edge of the instability strip, the AGB-bump is very red,
and the ratio of AGB-bump stars to RR Lyraes is quite large. If the HB second
parameter is age, the old and metal-poor field population likely formed after
the oldest clusters. Lifetime predictions of stellar evolution theory lead us
to associate a significant fraction of the red HB clump giants with the same
old and metal-poor population producing the RR Lyraes and the AGB-bump. In this
case, compared to the age-dependent luminosity predictions of stellar evolution
theory, the red HB clump is too bright relative to the RR Lyraes and AGB-bump.
Last, the surface density profile of RR Lyraes is fit by an exponential,
favoring a disk-like rather than spheroidal distribution. We conclude that the
age of the LMC disk is probably similar to the age of the Galactic disk.
(ABRIDGED)Comment: to appear in the Astronomical Journal, 49 pages, 12 figures,
aaspp4.st
Mass-Losing Semiregular Variable Stars in Baade's Windows
By cross-correlating the results of two recent large-scale surveys, the
general properties of a well defined sample of semi-regular variable stars have
been determined. ISOGAL mid-infrared photometry and MACHO lightcurves are
assembled for approximately 300 stars in the Baade's Windows of low extinction
towards the Galactic bulge. These stars are mainly giants of late M spectral
type, evolving along the asymptotic giant branch (AGB). They are found to
possess a wide and continuous distribution of pulsation periods and to obey an
approximate log~period -- bolometric magnitude relation or set of such
relations.
Approximate mass-loss rates in the range of 1e-8 to 5e-7 M_sun per year are
derived from ISOGAL mid-infrared photometry and models of stellar spectra
adjusted for the presence of optically-thin circumstellar silicate dust.
Mass-loss rates depend on luminosity and pulsation period. Some stars lose mass
as rapidly as short-period Miras but do not show Mira-like amplitudes. A period
of 70 days or longer is a necessary but not a sufficient condition for mass
loss to occur.
For AGB stars in the mass-loss ranges that we observe, the functional
dependence of mass-loss rate on temperature and luminosity is found to be in
agreement with recent theoretical predictions. If we include our mass-loss
rates with a sample of extreme mass-losing AGB stars in the Large Magellanic
Cloud, we get the general result for AGB stars that mass-loss rate is
proportional to luminosity^{2.7}, valid for AGB stars with 10^{-8} to 10^{-4}
M_sun per year (Abridged).Comment: to appear in The Astrophysical Journal, 51 pages, 9 figures, 3
tables; table 1 will be available in machine-readable format at the
electronic Ap
What Constitutes an Explanation in Biology?
One of biology's fundamental aims is to generate understanding of the living world aroundâand withinâus. In this chapter, I aim to provide a relatively nonpartisan discussion of the nature of explanation in biology, grounded in widely shared philosophical views about scientific explanation. But this discussion also reflects what I think is important for philosophers and biologists alike to appreciate about successful scientific explanations, so some points will be controversial, at least among philosophers. I make three main points: (1) causal relationships and broad patterns have often been granted importance to scientific explanations, and they are in fact both important; (2) some explanations in biology cite the components of or processes in systems that account for the systemsâ features, whereas other explanations feature large-scale or structural causes that influence a system; and (3) there can be multiple different explanations of a given biological phenomenon, explanations that respond to different research aims and can thus be compatible with one another even when they may seem to disagree
The Problem of Experience in the Study of Organizations
This paper deals with the fact that we cannot experience large organizations directly, in the same way as we can experience individuals or small groups, and that this non-experientiability has certain implications for our scientific theories of organizations. Whereas a science is animated by a constructive interplay of theory concepts and experience concepts, the study of organizations has been confined to theory concepts alone. Implications of this analysis for developing a science of organizations are considered.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68303/2/10.1177_017084069301400102.pd
The Linkage Between Neighborhood and Voluntary Association Patterns: a Comparison of Black and White Urban Populations
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68314/2/10.1177_089976407400300201.pd
Evolution of cooperation in stochastic games
Social dilemmas occur when incentives for individuals are misaligned with group interests 1-7 . According to the 'tragedy of the commons', these misalignments can lead to overexploitation and collapse of public resources. The resulting behaviours can be analysed with the tools of game theory 8 . The theory of direct reciprocity 9-15 suggests that repeated interactions can alleviate such dilemmas, but previous work has assumed that the public resource remains constant over time. Here we introduce the idea that the public resource is instead changeable and depends on the strategic choices of individuals. An intuitive scenario is that cooperation increases the public resource, whereas defection decreases it. Thus, cooperation allows the possibility of playing a more valuable game with higher payoffs, whereas defection leads to a less valuable game. We analyse this idea using the theory of stochastic games 16-19 and evolutionary game theory. We find that the dependence of the public resource on previous interactions can greatly enhance the propensity for cooperation. For these results, the interaction between reciprocity and payoff feedback is crucial: neither repeated interactions in a constant environment nor single interactions in a changing environment yield similar cooperation rates. Our framework shows which feedbacks between exploitation and environment - either naturally occurring or designed - help to overcome social dilemmas
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