3,464 research outputs found
Standing Swells Surveyed Showing Surprisingly Stable Solutions for the Lorenz '96 Model
The Lorenz '96 model is an adjustable dimension system of ODEs exhibiting
chaotic behavior representative of dynamics observed in the Earth's atmosphere.
In the present study, we characterize statistical properties of the chaotic
dynamics while varying the degrees of freedom and the forcing. Tuning the
dimensionality of the system, we find regions of parameter space with
surprising stability in the form of standing waves traveling amongst the slow
oscillators. The boundaries of these stable regions fluctuate regularly with
the number of slow oscillators. These results demonstrate hidden order in the
Lorenz '96 system, strengthening the evidence for its role as a hallmark
representative of nonlinear dynamical behavior.Comment: 10 pages, 8 figure
Small cities face greater impact from automation
The city has proven to be the most successful form of human agglomeration and
provides wide employment opportunities for its dwellers. As advances in
robotics and artificial intelligence revive concerns about the impact of
automation on jobs, a question looms: How will automation affect employment in
cities? Here, we provide a comparative picture of the impact of automation
across U.S. urban areas. Small cities will undertake greater adjustments, such
as worker displacement and job content substitutions. We demonstrate that large
cities exhibit increased occupational and skill specialization due to increased
abundance of managerial and technical professions. These occupations are not
easily automatable, and, thus, reduce the potential impact of automation in
large cities. Our results pass several robustness checks including potential
errors in the estimation of occupational automation and sub-sampling of
occupations. Our study provides the first empirical law connecting two societal
forces: urban agglomeration and automation's impact on employment
A common trajectory recapitulated by urban economies
Is there a general economic pathway recapitulated by individual cities over
and over? Identifying such evolution structure, if any, would inform models for
the assessment, maintenance, and forecasting of urban sustainability and
economic success as a quantitative baseline. This premise seems to contradict
the existing body of empirical evidences for path-dependent growth shaping the
unique history of individual cities. And yet, recent empirical evidences and
theoretical models have amounted to the universal patterns, mostly
size-dependent, thereby expressing many of urban quantities as a set of simple
scaling laws. Here, we provide a mathematical framework to integrate repeated
cross-sectional data, each of which freezes in time dimension, into a frame of
reference for longitudinal evolution of individual cities in time. Using data
of over 100 millions employment in thousand business categories between 1998
and 2013, we decompose each city's evolution into a pre-factor and relative
changes to eliminate national and global effects. In this way, we show the
longitudinal dynamics of individual cities recapitulate the observed
cross-sectional regularity. Larger cities are not only scaled-up versions of
their smaller peers but also of their past. In addition, our model shows that
both specialization and diversification are attributed to the distribution of
industry's scaling exponents, resulting a critical population of 1.2 million at
which a city makes an industrial transition into innovative economies
Happiness and the patterns of life: A study of geolocated tweets
The patterns of life exhibited by large populations have been described and modeled both as a basic science exercise and for a range of applied goals such as reducing automotive congestion, improving disaster response, and even predicting the location of individuals. However, these studies have had limited access to conversation content, rendering changes in expression as a function of movement invisible. In addition, they typically use the communication between a mobile phone and its nearest antenna tower to infer position, limiting the spatial resolution of the data to the geographical region serviced by each cellphone tower. We use a collection of 37 million geolocated tweets to characterize the movement patterns of 180,000 individuals, taking advantage of several orders of magnitude of increased spatial accuracy relative to previous work. Employing the recently developed sentiment analysis instrument known as the hedonometer, we characterize changes in word usage as a function of movement, and find that expressed happiness increases logarithmically with distance from an individual\u27s average location
An Evolutionary Algorithm Approach to Link Prediction in Dynamic Social Networks
Many real world, complex phenomena have underlying structures of evolving
networks where nodes and links are added and removed over time. A central
scientific challenge is the description and explanation of network dynamics,
with a key test being the prediction of short and long term changes. For the
problem of short-term link prediction, existing methods attempt to determine
neighborhood metrics that correlate with the appearance of a link in the next
observation period. Recent work has suggested that the incorporation of
topological features and node attributes can improve link prediction. We
provide an approach to predicting future links by applying the Covariance
Matrix Adaptation Evolution Strategy (CMA-ES) to optimize weights which are
used in a linear combination of sixteen neighborhood and node similarity
indices. We examine a large dynamic social network with over nodes
(Twitter reciprocal reply networks), both as a test of our general method and
as a problem of scientific interest in itself. Our method exhibits fast
convergence and high levels of precision for the top twenty predicted links.
Based on our findings, we suggest possible factors which may be driving the
evolution of Twitter reciprocal reply networks.Comment: 17 pages, 12 figures, 4 tables, Submitted to the Journal of
Computational Scienc
Electric Polarizability of Neutral Hadrons from Lattice QCD
By simulating a uniform electric field on a lattice and measuring the change
in the rest mass, we calculate the electric polarizability of neutral mesons
and baryons using the methods of quenched lattice QCD. Specifically, we measure
the electric polarizability coefficient from the quadratic response to the
electric field for 10 particles: the vector mesons and ; the
octet baryons n, , , , and ;
and the decouplet baryons , , and .
Independent calculations using two fermion actions were done for consistency
and comparison purposes. One calculation uses Wilson fermions with a lattice
spacing of fm. The other uses tadpole improved L\"usher-Weiss gauge
fields and clover quark action with a lattice spacing fm. Our results
for neutron electric polarizability are compared to experiment.Comment: 25 pages, 20 figure
Universal resilience patterns in labor markets
Cities are the innovation centers of the US economy, but technological disruptions can exclude workers and inhibit a middle class. Therefore, urban policy must promote the jobs and skills that increase worker pay, create employment, and foster economic resilience. In this paper, we model labor market resilience with an ecologically-inspired job network constructed from the similarity of occupations' skill requirements. This framework reveals that the economic resilience of cities is universally and uniquely determined by the connectivity within a city's job network. US cities with greater job connectivity experienced lower unemployment during the Great Recession. Further, cities that increase their job connectivity see increasing wage bills, and workers of embedded occupations enjoy higher wages than their peers elsewhere. Finally, we show how job connectivity may clarify the augmenting and deleterious impact of automation in US cities. Policies that promote labor connectivity may grow labor markets and promote economic resilience.This work was supported by the Massachusetts Institute of Technology (MIT) and the Ministerio de Economia y Competividad (Spain) through Project FIS2016-78904-C3-3-P and PID2019-106811GB-C32
The Geography of Happiness: Connecting Twitter Sentiment and Expression, Demographics, and Objective Characteristics of Place
We conduct a detailed investigation of correlations between real-time expressions of individuals made across the United States and a wide range of emotional, geographic, demographic, and health characteristics. We do so by combining (1) a massive, geo-tagged data set comprising over 80 million words generated in 2011 on the social network service Twitter and (2) annually-surveyed characteristics of all 50 states and close to 400 urban populations. Among many results, we generate taxonomies of states and cities based on their similarities in word use; estimate the happiness levels of states and cities; correlate highly-resolved demographic characteristics with happiness levels; and connect word choice and message length with urban characteristics such as education levels and obesity rates. Our results show how social media may potentially be used to estimate real-time levels and changes in population-scale measures such as obesity rates. © 2013 Mitchell et al
Optophysiological characterisation of inner retina responses with high-resolution optical coherence tomography
Low coherence laser interferometry has revolutionised quantitative biomedical
imaging of optically transparent structures at cellular resolutions. We report the first
optical recording of neuronal excitation at cellular resolution in the inner retina by
quantifying optically recorded stimulus-evoked responses from the retinal ganglion
cell layer and comparing them with an electrophysiological standard. We imaged
anaesthetised paralysed tree shrews, gated image acquisition, and used numerical
filters to eliminate noise arising from retinal movements during respiratory and
cardiac cycles. We observed increases in contrast variability in the retinal ganglion
cell layer and nerve fibre layer with flash stimuli and gratings. Regions of interest
were subdivided into three-dimensional patches (up to 5-15μm in diameter) based on
response similarity. We hypothesise that these patches correspond to individual
cells, or segments of blood vessels within the inner retina. We observed a close
correlation between the patch optical responses and mean electrical activity of
afferent visual neurons. While our data suggest that optical imaging of retinal activity
is possible with high resolution OCT, the technical challenges are not trivial
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