Fitness-dependent topological properties of the World Trade Web

Among the proposed network models, the hidden variable (or good get richer) one is particularly interesting, even if an explicit empirical test of its hypotheses has not yet been performed on a real network. Here we provide the first empirical test of this mechanism on the world trade web, the network defined by the trade relationships between world countries. We find that the power-law distributed gross domestic product can be successfully identified with the hidden variable (or fitness) determining the topology of the world trade web: all previously studied properties up to third-order correlation structure (degree distribution, degree correlations and hierarchy) are found to be in excellent agreement with the predictions of the model. The choice of the connection probability is such that all realizations of the network with the same degree sequence are equiprobable.Comment: 4 Pages, 4 Figures. Final version accepted for publication on Physical Review Letter

Patterns of link reciprocity in directed networks

We address the problem of link reciprocity, the non-random presence of two mutual links between pairs of vertices. We propose a new measure of reciprocity that allows the ordering of networks according to their actual degree of correlation between mutual links. We find that real networks are always either correlated or anticorrelated, and that networks of the same type (economic, social, cellular, financial, ecological, etc.) display similar values of the reciprocity. The observed patterns are not reproduced by current models. This leads us to introduce a more general framework where mutual links occur with a conditional connection probability. In some of the studied networks we discuss the form of the conditional connection probability and the size dependence of the reciprocity.Comment: Final version accepted for publication on Physical Review Letter

The entropy of randomized network ensembles

Randomized network ensembles are the null models of real networks and are extensivelly used to compare a real system to a null hypothesis. In this paper we study network ensembles with the same degree distribution, the same degree-correlations or the same community structure of any given real network. We characterize these randomized network ensembles by their entropy, i.e. the normalized logarithm of the total number of networks which are part of these ensembles. We estimate the entropy of randomized ensembles starting from a large set of real directed and undirected networks. We propose entropy as an indicator to assess the role of each structural feature in a given real network.We observe that the ensembles with fixed scale-free degree distribution have smaller entropy than the ensembles with homogeneous degree distribution indicating a higher level of order in scale-free networks.Comment: (6 pages,1 figure,2 tables

New results on focusing of gamma-rays with Laue lenses

We report on new results on the development activity of broad band Laue lenses for hard X-/gamma-ray astronomy (70/100-600 keV). After the development of a first prototype, whose performance was presented at the SPIE conference on Astronomical Telescopes held last year in Marseille (Frontera et al. 2008), we have improved the lens assembling technology. We present the development status of the new lens prototype that is on the way to be assembled.Comment: 8 pages, 11 figures, to be Published in SPIE Proceedings, vol.7437-19, 200

Development status of a Laue lens project for gamma-ray astronomy

We report the status of the HAXTEL project, devoted to perform a design study and the development of a Laue lens prototype. After a summary of the major results of the design study, the approach adopted to develop a Demonstration Model of a Laue lens is discussed, the set up described, and some results presented.Comment: 11 pages, 11 figures, 2007 SPIE Conference on Optics for EUV, X-Ray, and Gamma-Ray Astronomy II

Exploring the Hard X-/soft gamma-ray Continuum Spectra with Laue Lenses

The history of X-ray astronomy has shown that any advancement in our knowledge of the X-ray sky is strictly related to an increase in instrument sensitivity. At energies above 60 keV, there are interesting prospects for greatly improving the limiting sensitivity of the current generation of direct viewing telescopes (with or without coded masks), offered by the use of Laue lenses. We will discuss below the development status of a Hard X-Ray focusing Telescope (HAXTEL) based on Laue lenses with a broad bandpass (from 60 to 600 keV) for the study of the X-ray continuum of celestial sources. We show two examplesof multi-lens configurations with expected sensitivity orders of magnitude better ($\sim 1 \times 10^{-8}$ photons cm$^{-2}$ s$^{-1}$ keV$^{-1}$ at 200 keV) than that achieved so far. With this unprecedented sensitivity, very exciting astrophysical prospects are opened.Comment: 4 pages, 10 figures, to be published in the Proc. of the 39th ESLAB Symosium, 19-21 April 200

Conservative ideological shift among adolescents in response to system threat.

We examined conservative ideological shift among adolescents by assessing the effect of different types of threat on the self-reported political orientation of 183 New York City high school students and investigated the mediating role of system justification. Participants read one of three newspaper passages: (1) a system-related passage that described flaws in the American social, economic, and political system; (2) a self-related passage that described the deleterious health effects of cell phone use; or (3) a control passage that described house plant cultivation. Participants then completed measures of system justification and political orientation. As hypothesized, a threat to the system (but not the self) increased self-reported conservatism indirectly through its effect on system justification. This suggests that when the overarching social system is threatened, adolescents may be drawn to conservative ideology and that this is attributable, at least in part, to a heightened desire to defend and bolster the societal status quo.Social decision makin