ScholarBank@NUShttps://scholarbank.nus.edu.sgThe DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.Fri, 27 Nov 2020 01:03:45 GMT2020-11-27T01:03:45Z50251- Network formation, statistical physics and social dynamicshttps://scholarbank.nus.edu.sg/handle/10635/168521Title: Network formation, statistical physics and social dynamics
Authors: GASTNER, MICHAEL THORSTEN
Abstract: The focus of this project was to understand the forma- tion and dynamics of complex networks with the methods of statistical physics. Some of our work was motivated by fundamental mathematical questions while others were driven by the analysis of real data.
Wed, 17 Feb 2016 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685212016-02-17T00:00:00Z
- The risk of marine bioinvasion caused by global shippinghttps://scholarbank.nus.edu.sg/handle/10635/168541Title: The risk of marine bioinvasion caused by global shipping
Authors: Seebens, H; Gastner, MT; Blasius, B
Abstract: The rate of biological invasions has strongly increased during the last decades, mostly due to the accelerated spread of species by increasing global trade and transport. Here, we combine the network of global cargo ship movements with port environmental conditions and biogeography to quantify the probability of new primary invasions through the release of ballast water. We find that invasion risks vary widely between coastal ecosystems and classify marine ecoregions according to their total invasion risk and the diversity of their invasion sources. Thereby, we identify high-risk invasion routes, hot spots of bioinvasion and major source regions from which bioinvasion is likely to occur. Our predictions agree with observations in the field and reveal that the invasion probability is highest for intermediate geographic distances between donor and recipient ports. Our findings suggest that network-based invasion models may serve as a basis for the development of effective, targeted bioinvasion management strategies. © 2013 John Wiley & Sons Ltd/CNRS.
Sat, 01 Jun 2013 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685412013-06-01T00:00:00Z
- The Ising chain constrained to an even or odd number of positive spinshttps://scholarbank.nus.edu.sg/handle/10635/168516Title: The Ising chain constrained to an even or odd number of positive spins
Authors: GASTNER, MICHAEL THORSTEN
Abstract: We investigate the statistical mechanics of the periodic one- dimensional Ising chain when the number of positive spins is constrained to be either an even or an odd number. We calculate the partition function using a generalization of the transfer matrix method. On this basis, we derive the exact magnetization, susceptibility, internal energy, heat capacity and correlation function. We show that in general the constraints substantially slow down convergence to the thermodynamic limit. By taking the thermodynamic limit together with the limit of zero temperature and zero magnetic field, the constraints lead to new scaling functions and different probability distributions for the magnetization. We demonstrate how these results solve a stochastic version of the one-dimensional voter model.
Tue, 03 Mar 2015 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685162015-03-03T00:00:00Z
- The impact of hypocrisy on opinion formation: A dynamic modelhttps://scholarbank.nus.edu.sg/handle/10635/168526Title: The impact of hypocrisy on opinion formation: A dynamic model
Authors: Gastner, Michael T; Takacs, Karoly; Gulyas, Mate; Szvetelszky, Zsuzsanna; Oborny, Beata
Abstract: © 2019 Gastner et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Humans have a demonstrated tendency to copy or imitate the behavior and attitude of others and actively influence each other’s opinions. In plenty of empirical contexts, publicly revealed opinions are not necessarily in line with internal opinions, causing complex social influence dynamics. We study to what extent hypocrisy is sustained during opinion formation and how hidden opinions change the convergence to consensus in a group. We build and analyze a modified version of the voter model with hypocrisy in a complete graph with a neutral competition between two alternatives. We compare the process from various initial conditions, varying the proportions between the two opinions in the external (revealed) and internal (hidden) layer. According to our results, hypocrisy always prolongs the time needed for reaching a consensus. In a complete graph, this time span increases linearly with group size. We find that the group-level opinion emerges in two steps: (1) a fast and directional process, during which the number of the two kinds of hypocrites equalizes; and (2) a slower, random drift of opinions. During stage (2), the ratio of opinions in the external layer is approximately equal to the ratio in the internal layer; that is, the hidden opinions do not differ significantly from the revealed ones at the group level. We furthermore find that the initial abundances of opinions, but not the initial prevalence of hypocrisy, predicts the mean consensus time and determines the opinions’ probabilities of winning. These insights highlight the unimportance of hypocrisy in consensus formation under neutral conditions. Our results have important societal implications in relation to hidden voter preferences in polls and improve our understanding of opinion formation in a more realistic setting than that of conventional voter models.
Wed, 26 Jun 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685262019-06-26T00:00:00Z
- Transition from Connected to Fragmented Vegetation across an Environmental Gradient: Scaling Laws in Ecotone Geometryhttps://scholarbank.nus.edu.sg/handle/10635/168550Title: Transition from Connected to Fragmented Vegetation across an Environmental Gradient: Scaling Laws in Ecotone Geometry
Authors: Gastner, Michael T; Oborny, Beata; Zimmermann, DK; Pruessner, Gunnar
Abstract: A change in the environmental conditions across space-for example, altitude or latitude-can cause significant changes in the density of a vegetation type and, consequently, in spatial connectivity. We use spatially explicit simulations to study the transition from connected to fragmented vegetation. A static (gradient percolation) model is compared to dynamic (gradient contact process) models. Connectivity is characterized from the perspective of various species that use this vegetation type for habitat and differ in dispersal or migration range, that is, "step length" across the landscape. The boundary of connected vegetation delineated by a particular step length is termed the " hull edge." We found that for every step length and for every gradient, the hull edge is a fractal with dimension 7/ 4. The result is the same for different spatial models, suggesting that there are universal laws in ecotone geometry. To demonstrate that the model is applicable to real data, a hull edge of fractal dimension 7/4 is shown on a satellite image of a piñon-juniper woodland on a hillside. We propose to use the hull edge to define the boundary of a vegetation type unambiguously. This offers a new tool for detecting a shift of the boundary due to a climate change. © 2009 by The University of Chicago.
Wed, 01 Jul 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685502009-07-01T00:00:00Z
- The topology of large Open Connectome networks for the human brainhttps://scholarbank.nus.edu.sg/handle/10635/168554Title: The topology of large Open Connectome networks for the human brain
Authors: Gastner, Michael T; Odor, Geza
Abstract: The structural human connectome (i.e. the network of fiber connections in the brain) can be analyzed at ever finer spatial resolution thanks to advances in neuroimaging. Here we analyze several large data sets for the human brain network made available by the Open Connectome Project. We apply statistical model selection to characterize the degree distributions of graphs containing up to ≃ 106 nodes and ≃108 edges. A three-parameter generalized Weibull (also known as a stretched exponential) distribution is a good fit to most of the observed degree distributions. For almost all networks, simple power laws cannot fit the data, but in some cases there is statistical support for power laws with an exponential cutoff. We also calculate the topological (graph) dimension D and the small-world coefficient £m of these networks. While £m suggests a small-world topology, we found that D < 4 showing that long-distance connections provide only a small correction to the topology of the embedding three-dimensional space.
Tue, 07 Jun 2016 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685542016-06-07T00:00:00Z
- Voter model on networks partitioned into two cliques of arbitrary sizeshttps://scholarbank.nus.edu.sg/handle/10635/168553Title: Voter model on networks partitioned into two cliques of arbitrary sizes
Authors: Gastner, Michael T; Ishida, Kota
Abstract: © 2019 IOP Publishing Ltd. The voter model is an archetypal stochastic process that represents opinion dynamics. In each update, one agent is chosen uniformly at random. The selected agent then copies the current opinion of a randomly selected neighbour. We investigate the voter model on a network with an exogenous community structure: two cliques (i.e. complete subgraphs) randomly linked by X interclique edges. We show that, counterintuitively, the mean consensus time is typically not a monotonically decreasing function of X. Cliques of fixed proportions with opposite initial opinions reach a consensus, on average, most quickly if X scales as N 3/2, where N is the number of agents in the network. Hence, to accelerate a consensus between cliques, agents should connect to more members in the other clique as N increases but not to the extent that cliques lose their identity as distinct communities. We support our numerical results with an equation-based analysis. By interpolating between two asymptotic heterogeneous mean-field approximations, we obtain an equation for the mean consensus time that is in excellent agreement with simulations for all values of X.
Fri, 13 Dec 2019 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685532019-12-13T00:00:00Z
- The geometry of percolation fronts in two-dimensional lattices with spatially varying densitieshttps://scholarbank.nus.edu.sg/handle/10635/168559Title: The geometry of percolation fronts in two-dimensional lattices with spatially varying densities
Authors: Gastner, Michael T; Oborny, Beata
Abstract: Percolation theory is usually applied to lattices with a uniform probability p that a site is occupied or that a bond is closed. The more general case, where p is a function of the position x, has received less attention. Previous studies with long-range spatial variations in p(x) have only investigated cases where p has a finite, non-zero gradient at the critical point p c. Here we extend the theory to two-dimensional cases in which the gradient can change from zero to infinity. We present scaling laws for the width and length of the hull (i.e. the boundary of the spanning cluster). We show that the scaling exponents for the width and the length depend on the shape of p(x), but they always have a constant ratio 4/3 so that the hull's fractal dimension D = 7/4 is invariant. On this basis, we derive and verify numerically an asymptotic expression for the probability h(x) that a site at a given distance x from p c is on the hull. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Mon, 15 Oct 2012 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685592012-10-15T00:00:00Z
- Price of anarchy in transportation networks: Efficiency and optimality controlhttps://scholarbank.nus.edu.sg/handle/10635/168551Title: Price of anarchy in transportation networks: Efficiency and optimality control
Authors: Youn, Hyejin; Gastner, Michael T; Jeong, Hawoong
Abstract: Uncoordinated individuals in human society pursuing their personally optimal strategies do not always achieve the social optimum, the most beneficial state to the society as a whole. Instead, strategies form Nash equilibria which are often socially suboptimal. Society, therefore, has to pay a price of anarchy for the lack of coordination among its members. Here we assess this price of anarchy by analyzing the travel times in road networks of several major cities. Our simulation shows that uncoordinated drivers possibly waste a considerable amount of their travel time. Counterintuitively, simply blocking certain streets can partially improve the traffic conditions. We analyze various complex networks and discuss the possibility of similar paradoxes in physics. © 2008 The American Physical Society.
Fri, 19 Sep 2008 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685512008-09-19T00:00:00Z
- Maps and cartograms of the 2004 US presidential election resultshttps://scholarbank.nus.edu.sg/handle/10635/168552Title: Maps and cartograms of the 2004 US presidential election results
Authors: Gastner, MT; Shalizi, CR; Newman, MEJ
Abstract: Conventional maps of election results can give a misleading picture of the popular support that candidates have because population is highly non-uniform and equal areas on a map may not correspond to equal numbers of voters. Taking the example of the 2004 United States presidential election, we show how this problem can be corrected using a cartogram - a map in which the sizes of regions such as states are rescaled according to population or some other variable of interest. © World Scientific Publishing Company.
Tue, 01 Mar 2005 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685522005-03-01T00:00:00Z
- The spatial structure of networkshttps://scholarbank.nus.edu.sg/handle/10635/168556Title: The spatial structure of networks
Authors: Gastner, MT; Newman, MEJ
Abstract: We study networks that connect points in geographic space, such as transportation networks and the Internet. We find that there are strong signatures in these networks of topography and use patterns, giving the networks shapes that are quite distinct from one another and from non-geographic networks. We offer an explanation of these differences in terms of the costs and benefits of transportation and communication, and give a simple model based on the Monte Carlo optimization of these costs and benefits that reproduces well the qualitative features of the networks studied.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685562006-01-01T00:00:00Z
- Price of Anarchy in Transportation Networks: Efficiency and Optimality Control (vol 101, art no 128701, 2008)https://scholarbank.nus.edu.sg/handle/10635/168683Title: Price of Anarchy in Transportation Networks: Efficiency and Optimality Control (vol 101, art no 128701, 2008)
Authors: Youn, Hyejin; Gastner, Michael T; Jeong, Hawoong
Fri, 30 Jan 2009 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1686832009-01-30T00:00:00Z
- The geography and carbon footprint of mobile phone use in Cote d'Ivoirehttps://scholarbank.nus.edu.sg/handle/10635/168542Title: The geography and carbon footprint of mobile phone use in Cote d'Ivoire
Authors: Salnikov, Vsevolod; Schien, Daniel; Youn, Hyejin; Lambiotte, Renaud; Gastner, Michael T
Abstract: © 2014 Salnikov et al.; licensee Springer. The newly released Orange D4D mobile phone data base provides new insights into the use of mobile technology in a developing country. Here we perform a series of spatial data analyses that reveal important geographic aspects of mobile phone use in Cote d’Ivoire. We first map the locations of base stations with respect to the population distribution and the number and duration of calls at each base station. On this basis, we estimate the energy consumed by the mobile phone network. Finally, we perform an analysis of inter-city mobility, and identify high-traffic roads in the country.
Mon, 01 Dec 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685422014-12-01T00:00:00Z
- Fast flow-based algorithm for creating density-equalizing map projectionshttps://scholarbank.nus.edu.sg/handle/10635/169012Title: Fast flow-based algorithm for creating density-equalizing map projections
Authors: Gastner, Michael T; Seguy, Vivien; More, Pratyush
Abstract: © 2018 National Academy of Sciences. All Rights Reserved. Cartograms are maps that rescale geographic regions (e.g., countries, districts) such that their areas are proportional to quantitative demographic data (e.g., population size, gross domestic product). Unlike conventional bar or pie charts, cartograms can represent correctly which regions share common borders, resulting in insightful visualizations that can be the basis for further spatial statistical analysis. Computer programs can assist data scientists in preparing cartograms, but developing an algorithm that can quickly transform every coordinate on the map (including points that are not exactly on a border) while generating recognizable images has remained a challenge. Methods that translate the cartographic deformations into physics-inspired equations of motion have become popular, but solving these equations with sufficient accuracy can still take several minutes on current hardware. Here we introduce a flow-based algorithm whose equations of motion are numerically easier to solve compared with previous methods. The equations allow straightforward parallelization so that the calculation takes only a few seconds even for complex and detailed input. Despite the speedup, the proposed algorithm still keeps the advantages of previous techniques: With comparable quantitative measures of shape distortion, it accurately scales all areas, correctly fits the regions together, and generates a map projection for every point. We demonstrate the use of our algorithm with applications to the 2016 US election results, the gross domestic products of Indian states and Chinese provinces, and the spatial distribution of deaths in the London borough of Kensington and Chelsea between 2011 and 2014.
Tue, 06 Mar 2018 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1690122018-03-06T00:00:00Z
- The complex network of global cargo ship movementshttps://scholarbank.nus.edu.sg/handle/10635/169013Title: The complex network of global cargo ship movements
Authors: Kaluza, Pablo; Koelzsch, Andrea; Gastner, Michael T; Blasius, Bernd
Abstract: Transportation networks play a crucial role in human mobility, the exchange of goods and the spread of invasive species. With 90 per cent of world trade carried by sea, the global network of merchant ships provides one of the most important modes of transportation. Here, we use information about the itineraries of 16 363 cargo ships during the year 2007 to construct a network of links between ports. We show that the network has several features that set it apart from other transportation networks. In particular, most ships can be classified into three categories: bulk dry carriers, container ships and oil tankers. These three categories do not only differ in the ships' physical characteristics, but also in their mobility patterns and networks. Container ships follow regularly repeating paths whereas bulk dry carriers and oil tankers move less predictably between ports. The network of all ship movements possesses a heavy-tailed distribution for the connectivity of ports and for the loads transported on the links with systematic differences between ship types. The data analysed in this paper improve current assumptions based on gravity models of ship movements, an important step towards understanding patterns of global trade and bioinvasion. © 2010 The Royal Society.
Tue, 06 Jul 2010 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1690132010-07-06T00:00:00Z
- Scaling and entropy in p-median facility location along a linehttps://scholarbank.nus.edu.sg/handle/10635/168992Title: Scaling and entropy in p-median facility location along a line
Authors: Gastner, Michael T
Abstract: The p-median problem is a common model for optimal facility location. The task is to place p facilities (e.g., warehouses or schools) in a heterogeneously populated space such that the average distance from a person's home to the nearest facility is minimized. Here we study the special case where the population lives along a line (e.g., a road or a river). If facilities are optimally placed, the length of the line segment served by a facility is inversely proportional to the square root of the population density. This scaling law is derived analytically and confirmed for concrete numerical examples of three US interstate highways and the Mississippi River. If facility locations are permitted to deviate from the optimum, the number of possible solutions increases dramatically. Using Monte Carlo simulations, we compute how scaling is affected by an increase in the average distance to the nearest facility. We find that the scaling exponents change and are most sensitive near the optimum facility distribution. © 2011 American Physical Society.
Tue, 20 Sep 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1689922011-09-20T00:00:00Z
- Optimal design of spatial distribution networkshttps://scholarbank.nus.edu.sg/handle/10635/169010Title: Optimal design of spatial distribution networks
Authors: Gastner, Michael T; Newman, MEJ
Abstract: We consider the problem of constructing facilities such as hospitals, airports, or malls in a country with a nonuniform population density, such that the average distance from a person's home to the nearest facility is minimized. We review some previous approximate treatments of this problem that indicate that the optimal distribution of facilities should have a density that increases with population density, but does so slower than linearly, as the two-thirds power. We confirm this result numerically for the particular case of the United States with recent population data using two independent methods, one a straightforward regression analysis, the other based on density-dependent map projections. We also consider strategies for linking the facilities to form a spatial network, such as a network of flights between airports, so that the combined cost of maintenance of and travel on the network is minimized. We show specific examples of such optimal networks for the case of the United States. © 2006 The American Physical Society.
Sat, 01 Jul 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1690102006-07-01T00:00:00Z
- Changes in the Gradient Percolation Transition Caused by an Allee Effecthttps://scholarbank.nus.edu.sg/handle/10635/169017Title: Changes in the Gradient Percolation Transition Caused by an Allee Effect
Authors: Gastner, Michael T; Oborny, Beata; Ryabov, Alexey B; Blasius, Bernd
Abstract: The establishment and spreading of biological populations depends crucially on population growth at low densities. The Allee effect is a problem in those populations where the per capita growth rate at low densities is reduced. We examine stochastic spatial models in which the reproduction rate changes across a gradient g so that the population undergoes a 2D-percolation transition. Without the Allee effect, the transition is continuous and the width w of the hull scales as in conventional (i.e., uncorrelated) gradient percolation, w∝g-0.57. However, with a strong Allee effect the transition is first order and w∝g-0.26. © 2011 American Physical Society.
Wed, 23 Mar 2011 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1690172011-03-23T00:00:00Z
- Shape and efficiency in spatial distribution networkshttps://scholarbank.nus.edu.sg/handle/10635/169512Title: Shape and efficiency in spatial distribution networks
Authors: Gastner, MT; Newman, MEJ
Abstract: We study spatial networks that are designed to distribute or collect a commodity, such as gas pipelines or train tracks. We focus on the cost of a network, as represented by the total length of all its edges, and its efficiency in terms of the directness of routes from point to point. Using data for several real-world examples, we find that distribution networks appear remarkably close to optimal where both these properties are concerned. We propose two models of network growth that offer explanations of how this situation might arise. © IOP Publishing Ltd.
Sun, 01 Jan 2006 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1695122006-01-01T00:00:00Z
- How heavy-tailed is the distribution of global cargo ship traffic?https://scholarbank.nus.edu.sg/handle/10635/169551Title: How heavy-tailed is the distribution of global cargo ship traffic?
Authors: Gastner, Michael T; Ducruet, Cesar
Abstract: © 2014 IEEE. Power laws, once believed to be a universal feature of degree distributions in complex networks, have come under attack in recent years. More sophisticated statistical analysis has often revealed other heavy-tailed distributions as more adequate descriptions of real-world data. Here we study degree and strength distributions of the network of worldwide cargo ship movements-the main transport network for world trade-from 14 different years between 1890 and 2008. We compare the Akaike information criterion of various common probabilistic models. In almost all cases, the Akaike weights identify a stretched exponential distribution as the most likely among the investigated models. Simple or truncated power laws, by contrast, do not capture the observations equally well. Cargo ship traffic is thus heavy-tailed with some ports being significantly busier than the average, but the distribution is not scale-free. The maximum-likelihood estimators indicate that the normalized distribution became increasingly shorter-tailed for one century. However, since the start of this millennium this trend appears to be reversing.
Wed, 01 Jan 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1695512014-01-01T00:00:00Z
- Consensus time in a voter model with concealed and publicly expressed opinionshttps://scholarbank.nus.edu.sg/handle/10635/169672Title: Consensus time in a voter model with concealed and publicly expressed opinions
Authors: Gastner, Michael T; Oborny, Beata; Gulyas, Mate
Abstract: © 2018 The Author(s). Published by IOP Publishing Ltd on behalf of SISSA Medialab srl. The voter model is a simple agent-based model to mimic opinion dynamics in social networks: a randomly chosen agent adopts the opinion of a randomly chosen neighbour. This process is repeated until a consensus emerges. Although the basic voter model is theoretically intriguing, it misses an important feature of real opinion dynamics: it does not distinguish between an agent's publicly expressed opinion and her inner conviction. A person may not feel comfortable declaring her conviction if her social circle appears to hold an opposing view. Here we introduce the Concealed Voter Model where we add a second, concealed layer of opinions to the public layer. If an agent's public and concealed opinions disagree, she can reconcile them by either publicly disclosing her previously secret point of view or by accepting her public opinion as inner conviction. We study a complete graph of agents who can choose from two opinions. We define a martingale M that determines the probability of all agents eventually agreeing on a particular opinion. By analyzing the evolution of M in the limit of a large number of agents, we derive the leading-order terms for the mean and standard deviation of the consensus time (i.e. the time needed until all opinions are identical). We thereby give a precise prediction by how much concealed opinions slow down a consensus.
Fri, 01 Jun 2018 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1696722018-06-01T00:00:00Z
- Diffusion-based method for producing density-equalizing mapshttps://scholarbank.nus.edu.sg/handle/10635/169670Title: Diffusion-based method for producing density-equalizing maps
Authors: Gastner, MT; Newman, MEJ
Abstract: Map makers have for many years searched for a way to construct cartograms, maps in which the sizes of geographic regions such as countries or provinces appear in proportion to their population or some other analogous property. Such maps are invaluable for the representation of census results, election returns, disease incidence, and many other kinds of human data. Unfortunately, to scale regions and still have them fit together, one is normally forced to distort the regions' shapes, potentially resulting in maps that are difficult to read. Many methods for making cartograms have been proposed, some of them are extremely complex, but all suffer either from this lack of readability or from other pathologies, like overlapping regions or strong dependence on the choice of coordinate axes. Here, we present a technique based on ideas borrowed from elementary physics that suffers none of these drawbacks. Our method is conceptually simple and produces useful, elegant, and easily readable maps. We illustrate the method with applications to the results of the 2000 U.S. presidential election, lung cancer cases in the State of New York, and the geographical distribution of stories appearing in the news.
Tue, 18 May 2004 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1696702004-05-18T00:00:00Z
- The distribution functions of vessel calls and port connectivity in the global cargo ship networkhttps://scholarbank.nus.edu.sg/handle/10635/168519Title: The distribution functions of vessel calls and port connectivity in the global cargo ship network
Authors: GASTNER, MICHAEL THORSTEN; Ducruet, Cesar
Editors: Ducruet, C
Abstract: Characterizing the connectivity of nodes in economic and technological networks is of key interest to assess their role and function. Here we study the distributions of vessel calls and the ports’ degrees (i.e. the number of other ports that a port is directly linked with) in the network of worldwide cargo ship movements – the main transport network for world trade – for twenty different years between 1890 and 2008. We compare the Akaike information criterion and goodness- of-fit statistics of various common probabilistic models. Simple power laws, once believed to be a universal feature of degree distributions in complex networks, are inadequate to fit the data. Other subexponential distributions, such as lognormal or Weibull distributions, perform consistently better. Cargo ship traffic has thus for the entire study period been heavy-tailed with some ports being significantly busier than the average, but the distribution is not scale-free. Vuong’s likelihood ratio test confirms that since 1975 a Weibull distribution can be regarded as a plausible null hypothesis. Lognormal distributions perform well for most years in Kolmogorov-Smirnov and Anderson-Darling tests for the call distribution. The Gini coefficient of the distribution has slightly, but statistically significantly, decreased over the study period, highlighting a tendency towards a more polycentric distribution in port traffic.
Mon, 05 Oct 2015 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1685192015-10-05T00:00:00Z
- Opinion formation models on a gradienthttps://scholarbank.nus.edu.sg/handle/10635/142576Title: Opinion formation models on a gradient
Authors: Gastner M.T.; Markou N.; Pruessner G.; Draief M.
Wed, 01 Jan 2014 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1425762014-01-01T00:00:00Z
- Motivating Good Practices for the Creation of Contiguous Area Cartogramshttps://scholarbank.nus.edu.sg/handle/10635/183764Title: Motivating Good Practices for the Creation of Contiguous Area Cartograms
Authors: Gastner, Michael Thorsten; Duncan, Ian K; Chang, Yen-Ning; Tingsheng, Shi
Wed, 01 Jan 2020 00:00:00 GMThttps://scholarbank.nus.edu.sg/handle/10635/1837642020-01-01T00:00:00Z