1,721,058 research outputs found
The rise and fall of a networked society: A formal model
No full textStudies the dynamics of a networked societ
Financial instability from local market measures
No full textWe study the emergence of instabilities in a stylized model of a financial market, when different market actors calculate prices according to different (local) market measures. We derive typical properties for ensembles of large random markets using techniques borrowed from statistical mechanics of disordered systems. We show that, depending on the number of financial instruments available and on the heterogeneity of local measures, the market moves from an arbitrage-free phase to an unstable one, where the complexity of the market—as measured by the diversity of financial instruments—increases, and arbitrage opportunities arise. A sharp transition separates the two phases. Focusing on two different classes of local measures inspired by real market strategies, we are able to analytically compute the critical lines, corroborating our findings with numerical simulations
Criticality of mostly informative samples: A Bayesian model selection approach
Full text linkWe discuss a Bayesian model selection approach to high-dimensional data in the deep under-sampling regime. The data is based on a representation of the possible discrete states s, as defined by the observer, and it consists of M observations of the state. This approach shows that, for a given sample size M, not all states observed in the sample can be distinguished. Rather, only a partition of the sampled states s can be resolved. Such a partition defines an emergent classification qs of the states that becomes finer and finer as the sample size increases, through a process of symmetry breaking between states. This allows us to distinguish between the resolution of a given representation of the observer defined states s, which is given by the entropy of s, and its relevance, which is defined by the entropy of the partition qs. Relevance has a nonmonotonic dependence on resolution, for a given sample size. In addition, we characterise most relevant samples and we show that they exhibit power law frequency distributions, generally taken as signatures of criticality. This suggests that criticality reflects the relevance of a given representation of the states of a complex system, and does not necessarily require a specific mechanism of self-organisation to a critical point.Fil: Haimovici, Ariel. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Marsili, Matteo. The Abdus Salam; Itali
Assessing the relevance of node features for network structure
No full textNetworks describe a variety of interacting complex systems in social science, biology, and information technology. Usually the nodes of real networks are identified not only by their connections but also by some other characteristics. Examples of characteristics of nodes can be age, gender, or nationality of a person in a social network, the abundance of proteins in the cell taking part in protein-interaction networks, or the geographical
position of airports that are connected by directed flights. Integrating the information on the connections of each node with the information about its characteristics is crucial to discriminating between the essential and negligible characteristics of nodes for the structure of the network. In this paper we propose a general indicator Θ, based on entropy measures, to quantify the dependence of a network’s structure on a given set of features.
We apply this method to social networks of friendships in U.S. schools, to the protein-interaction network of Saccharomyces cerevisiae and to the U.S. airport network, showing that the proposed measure provides information that complements other known measures
Comment on the run time statistics in models of growth in disordered media
No full textWe point out an error in the equations for the evolution of the run time statistics given in a paper by Marsili and give the correct expression. Moreover, we discuss the annealing approximation which is implicit in the transformation
Observed choices and underlying opportunities
No full textOur societies are heterogeneous in many dimensions such as census, education, religion, ethnic and cultural composition. The links between individuals – e.g. by friendship, marriage or collaboration – are not evenly distributed, but rather tend to be concentrated within the same group. This phenomenon, called imbreeding homophily, has been related to either (social) preference for links with own–type individuals (choice–based homophily) or to the prevalence of individuals of her same type in the choice set of an individual (opportunity–based homophily). Choices determine the network of relations we observe whereas opportunities pertain to the composition of the
(unobservable) social network individuals are embedded in and out of which their network of relations is drawn. In this view, we propose a method that, in the presence of multiple data, allows one to distinguish between opportunity and choice based homophily. The main intuition is that, with unbiased opportunities, the effect of choice–based homophily gets weaker and weaker as the size of the minority shrinks, because individuals of the minority rarely meet and have the chance to establish links together. The occurrence of homophily in the limit of very small minorities is therefore an indicator of opportunity bias. We test this idea across the dimensions of race and education on data on US marriages, and across race on friendships in US schools
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Collaboration in social networks
No full textThe very notion of social network implies that linked individuals
interact repeatedly with each other. This notion allows them not
only to learn successful strategies and adapt to them, but also to
condition their own behavior on the behavior of others, in a strategic
forward looking manner. Game theory of repeated games
shows that these circumstances are conducive to the emergence of
collaboration in simple games of two players. We investigate the
extension of this concept to the case where players are engaged in
a local contribution game and show that rationality and credibility
of threats identify a class of Nash equilibria—that we call “collaborative
equilibria”—that have a precise interpretation in terms of
subgraphs of the social network. For large network games, the
number of such equilibria is exponentially large in the number of
players. When incentives to defect are small, equilibria are supported
by local structures whereas when incentives exceed a threshold
they acquire a nonlocal nature,which requires a “critical mass” of
more than a given fraction of the players to collaborate. Therefore,
when incentives are high, an individual deviation typically causes
the collapse of collaboration across the whole system. At the same
time, higher incentives to defect typically support equilibria with a
higher density of collaborators. The resulting picture conforms
with several results in sociology and in the experimental literature
on game theory, such as the prevalence of collaboration in denser
groups and in the structural hubs of sparse networks
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