423 research outputs found
Plasticity facilitates sustainable growth in the commons
In the commons, communities whose growth depends on public goods, individuals
often rely on surprisingly simple strategies, or heuristics, to decide whether
to contribute to the common good (at risk of exploitation by free-riders).
Although this appears a limitation, here we show how four heuristics lead to
sustainable growth by exploiting specific environmental constraints. The two
simplest ones --contribute permanently or switch stochastically between
contributing or not-- are first shown to bring sustainability when the public
good efficiently promotes growth. If efficiency declines and the commons is
structured in small groups, the most effective strategy resides in contributing
only when a majority of individuals are also contributors. In contrast, when
group size becomes large, the most effective behavior follows a minimal-effort
rule: contribute only when it is strictly necessary. Both plastic strategies
are observed in natural systems what presents them as fundamental social motifs
to successfully manage sustainability
Heterogeneity shapes groups growth in social online communities
Many complex systems are characterized by broad distributions capturing, for
example, the size of firms, the population of cities or the degree distribution
of complex networks. Typically this feature is explained by means of a
preferential growth mechanism. Although heterogeneity is expected to play a
role in the evolution it is usually not considered in the modeling probably due
to a lack of empirical evidence on how it is distributed. We characterize the
intrinsic heterogeneity of groups in an online community and then show that
together with a simple linear growth and an inhomogeneous birth rate it
explains the broad distribution of group members.Comment: 5 pages, 3 figure panel
Power Law of Customers' Expenditures in Convenience Stores
In a convenience store chain, a tail of the cumulative density function of
the expenditure of a person during a single shopping trip follows a power law
with an exponent of -2.5. The exponent is independent of the location of the
store, the shopper's age, the day of week, and the time of day.Comment: 9 pages, 5 figures. Accepted for publication in Journal of the
Physical Society of Japan Vol.77No.
Scaling Theory for Migration-Driven Aggregate Growth
We give a comprehensive rate equation description for the irreversible growth
of aggregates by migration from small to large aggregates. For a homogeneous
rate K(i;j) at which monomers migrate from aggregates of size i to those of
size j, that is, K(ai;aj) ~ a^{lambda} K(i,j), the mean aggregate size grows
with time as t^{1/(2-lambda)} for lambda<2. The aggregate size distribution
exhibits distinct regimes of behavior which are controlled by the scaling
properties of the migration rate from the smallest to the largest aggregates.
Our theory applies to diverse phenomena, such as the distribution of city
populations, late stage coarsening of non-symmetric binary systems, and models
for wealth exchange.Comment: 4 pages, 2-column revtex format. Revision to appear in PRL. Various
changes in response to referee comments. Figure from version 1 deleted but is
available at http://physics.bu.edu/~redne
Dynamics on expanding spaces: modeling the emergence of novelties
Novelties are part of our daily lives. We constantly adopt new technologies,
conceive new ideas, meet new people, experiment with new situations.
Occasionally, we as individuals, in a complicated cognitive and sometimes
fortuitous process, come up with something that is not only new to us, but to
our entire society so that what is a personal novelty can turn into an
innovation at a global level. Innovations occur throughout social, biological
and technological systems and, though we perceive them as a very natural
ingredient of our human experience, little is known about the processes
determining their emergence. Still the statistical occurrence of innovations
shows striking regularities that represent a starting point to get a deeper
insight in the whole phenomenology. This paper represents a small step in that
direction, focusing on reviewing the scientific attempts to effectively model
the emergence of the new and its regularities, with an emphasis on more recent
contributions: from the plain Simon's model tracing back to the 1950s, to the
newest model of Polya's urn with triggering of one novelty by another. What
seems to be key in the successful modelling schemes proposed so far is the idea
of looking at evolution as a path in a complex space, physical, conceptual,
biological, technological, whose structure and topology get continuously
reshaped and expanded by the occurrence of the new. Mathematically it is very
interesting to look at the consequences of the interplay between the "actual"
and the "possible" and this is the aim of this short review.Comment: 25 pages, 10 figure
On Hilberg's Law and Its Links with Guiraud's Law
Hilberg (1990) supposed that finite-order excess entropy of a random human
text is proportional to the square root of the text length. Assuming that
Hilberg's hypothesis is true, we derive Guiraud's law, which states that the
number of word types in a text is greater than proportional to the square root
of the text length. Our derivation is based on some mathematical conjecture in
coding theory and on several experiments suggesting that words can be defined
approximately as the nonterminals of the shortest context-free grammar for the
text. Such operational definition of words can be applied even to texts
deprived of spaces, which do not allow for Mandelbrot's ``intermittent
silence'' explanation of Zipf's and Guiraud's laws. In contrast to
Mandelbrot's, our model assumes some probabilistic long-memory effects in human
narration and might be capable of explaining Menzerath's law.Comment: To appear in Journal of Quantitative Linguistic
Positive words carry less information than negative words
We show that the frequency of word use is not only determined by the word
length \cite{Zipf1935} and the average information content
\cite{Piantadosi2011}, but also by its emotional content. We have analyzed
three established lexica of affective word usage in English, German, and
Spanish, to verify that these lexica have a neutral, unbiased, emotional
content. Taking into account the frequency of word usage, we find that words
with a positive emotional content are more frequently used. This lends support
to Pollyanna hypothesis \cite{Boucher1969} that there should be a positive bias
in human expression. We also find that negative words contain more information
than positive words, as the informativeness of a word increases uniformly with
its valence decrease. Our findings support earlier conjectures about (i) the
relation between word frequency and information content, and (ii) the impact of
positive emotions on communication and social links.Comment: 16 pages, 3 figures, 3 table
Imaging Spectroscopy for Extrasolar Planet Detection
Coronagraphic imaging in combination with moderate to high spectral
resolution may prove more effective in both detecting extrasolar planets and
characterizing them than a standard coronagraphic imaging approach. We envisage
an integral-field spectrograph coupled to a coronagraph to produce a 3D
datacube. For the idealised case where the spectrum of the star is well-known
and unchanging across the field, we discuss the utility of cross-correlation to
seek the extrasolar planet signal, and describe a mathematical approach to
completely eliminate stray light from the host star (although not its Poisson
noise). For the case where the PSF is dominated by diffraction and scattering
effects, and comprises a multitude of speckles within an Airy pattern typical
of a space-based observation, we turn the wavelength dependence of the PSF to
advantage and present a general way to eliminate the contribution from the star
while preserving both the flux and spectrum of the extrasolar planet. We call
this method `spectral deconvolution'. We illustrate the dramatic gains by
showing an idealized simulation that results in a 20-sigma detection of a
Jovian planet at 2 pc with a 2-m coronagraphic space telescope, even though the
planet's peak flux is only 1% that of the PSF wings of the host star. This
scales to detection of a terrestrial extrasolar planet at 2 pc with an 8-m
coronagraphic Terrestrial Planet Finder (TPF) in ~7 hr (or less with
appropriate spatial filtering). Data on the spectral characteristics of the
extrasolar planet and hence on its atmospheric constituents and possible
biomarkers are obtained naturally as part of this process.Comment: 62 pages 27 figures accepted for publication in Ap
New stopping criteria for segmenting DNA sequences
We propose a solution on the stopping criterion in segmenting inhomogeneous
DNA sequences with complex statistical patterns. This new stopping criterion is
based on Bayesian Information Criterion (BIC) in the model selection framework.
When this stopping criterion is applied to a left telomere sequence of yeast
Saccharomyces cerevisiae and the complete genome sequence of bacterium
Escherichia coli, borders of biologically meaningful units were identified
(e.g. subtelomeric units, replication origin, and replication terminus), and a
more reasonable number of domains was obtained. We also introduce a measure
called segmentation strength which can be used to control the delineation of
large domains. The relationship between the average domain size and the
threshold of segmentation strength is determined for several genome sequences.Comment: 4 pages, 4 figures, Physical Review Letters, to appea
Electron impact excitation cross sections for allowed transitions in atoms
We present a semiempirical Gaunt factor for widely used Van Regemorter
formula [Astrophys. J. 136, 906 (1962)] for the case of allowed transitions in
atoms with the LS coupling scheme. Cross sections calculated using this Gaunt
factor agree with measured cross sections to within the experimental error.Comment: RevTeX, 3 pages, 10 PS figures, 2 PS tables, submitted to Phys. Rev.
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