90,112 research outputs found

    SPoT: Representing the Social, Spatial, and Temporal Dimensions of Human Mobility with a Unifying Framework

    Get PDF
    Modeling human mobility is crucial in the analysis and simulation of opportunistic networks, where contacts are exploited as opportunities for peer-topeer message forwarding. The current approach with human mobility modeling has been based on continuously modifying models, trying to embed in them the mobility properties (e.g., visiting patterns to locations or specific distributions of inter-contact times) as they came up from trace analysis. As a consequence, with these models it is difficult, if not impossible, to modify the features of mobility or to control the exact shape of mobility metrics (e.g., modifying the distribution of inter-contact times). For these reasons, in this paper we propose a mobility framework rather than a mobility model, with the explicit goal of providing a exible and controllable tool for modeling mathematically and generating simulatively different possible features of human mobility. Our framework, named SPoT, is able to incorporate the three dimensions - spatial, social, and temporal - of human mobility. The way SPoT does it is by mapping the different social communities of the network into different locations, whose members visit with a configurable temporal pattern. In order to characterize the temporal patterns of user visits to locations and the relative positioning of locations based on their shared users, we analyze the traces of real user movements extracted from three location-based online social networks (Gowalla, Foursquare, and Altergeo). We observe that a Bernoulli process effectively approximates user visits to locations in the majority of cases and that locations that share many common users visiting them frequently tend to be located close to each other. In addition, we use these traces to test the exibility of the framework, and we show that SPoT is able to accurately reproduce the mobility behavior observed in traces. Finally, relying on the Bernoulli assumption for arrival processes, we provide a throughout mathematical analysis of the controllability of the framework, deriving the conditions under which heavy-tailed and exponentially-tailed aggregate inter-contact times (often observed in real traces) emerge

    Extinctions of aculeate pollinators in Britain and the role of large-scale agricultural changes

    Get PDF
    Pollinators are fundamental to maintaining both biodiversity and agricultural productivity, but habitat destruction, loss of flower resources, and increased use of pesticides are causing declines in their abundance and diversity. Using historical records we assessed the rate of extinction of bee and flower-visiting wasp species in Britain, from the mid 19th century to the present. The most rapid phase of extinction appears to be related to changes in agricultural policy and practice beginning in the 1920s, before the agricultural intensification prompted by the Second World War, often cited as the most important driver of biodiversity loss in Britain. Slowing of the extinction rate from the 1960s onwards may be due to prior loss of the most sensitive species and/or effective conservation programs

    Minimum Rates of Approximate Sufficient Statistics

    Full text link
    Given a sufficient statistic for a parametric family of distributions, one can estimate the parameter without access to the data. However, the memory or code size for storing the sufficient statistic may nonetheless still be prohibitive. Indeed, for nn independent samples drawn from a kk-nomial distribution with d=k−1d=k-1 degrees of freedom, the length of the code scales as dlog⁡n+O(1)d\log n+O(1). In many applications, we may not have a useful notion of sufficient statistics (e.g., when the parametric family is not an exponential family) and we also may not need to reconstruct the generating distribution exactly. By adopting a Shannon-theoretic approach in which we allow a small error in estimating the generating distribution, we construct various {\em approximate sufficient statistics} and show that the code length can be reduced to d2log⁡n+O(1)\frac{d}{2}\log n+O(1). We consider errors measured according to the relative entropy and variational distance criteria. For the code constructions, we leverage Rissanen's minimum description length principle, which yields a non-vanishing error measured according to the relative entropy. For the converse parts, we use Clarke and Barron's formula for the relative entropy of a parametrized distribution and the corresponding mixture distribution. However, this method only yields a weak converse for the variational distance. We develop new techniques to achieve vanishing errors and we also prove strong converses. The latter means that even if the code is allowed to have a non-vanishing error, its length must still be at least d2log⁡n\frac{d}{2}\log n.Comment: To appear in the IEEE Transactions on Information Theor

    Do Selection Criteria Make a Difference? Visa Category and the Labour Force Status of Australian Immigrants.

    Get PDF
    This paper assesses the role of selection criteria in the immigrant settlement process by analysing the labour force status of immigrants entering Australia under different immigration programs. In particular, do immigrants selected on the basis of labour market skills rather than family relationships have higher participation and employment rates immediately after migration? To what extent does this represent a head start as opposed to long-term labour market advantage? Information fron the Longitudinal Survey of Immigrants to Australia (LSIA) are used address these questions.IMMIGRATION ; LABOUR MARKET ; AUSTRALIA

    Reliability function of general classical-quantum channel

    Get PDF
    In information theory the reliability function and its bounds, describing the exponential behavior of the error probability, are the most important quantitative characteristics of the channel performance. From a general point of view, these bounds provide certain measures of distinguishability of a given set of states. In an earlier paper we introduced quantum analogs of the random coding and the expurgation lower bounds for the case of pure signal states. Here we discuss the general case, in particular, we prove the previously conjectured expurgation bound and find the quantum cutoff rate in the case of arbitrary mixed signal states.Comment: 15 pages, partially reported at the Workshop on Complexity, Computation and Physics of Information, Cambridge, July 5-23, 1999; to appear in IEEE Trans. on Information Theory. Several corrections suggested by the referees are include
    • 

    corecore