Models and average properties of scale-free directed networks

We extend the merging model for undirected networks by Kim et al. [Eur. Phys. J. B 43, 369 (2004)] to directed networks and investigate the emerging scale-free networks. Two versions of the directed merging model, friendly and hostile merging, give rise to two distinct network types. We uncover that some non-trivial features of these two network types resemble two levels of a certain randomization/non-specificity in the link reshuffling during network evolution. Furthermore the same features show up, respectively, in metabolic networks and transcriptional networks. We introduce measures that single out the distinguishing features between the two prototype networks, as well as point out features which are beyond the prototypes.Comment: 7 pages, 8 figure

Optimization and Scale-freeness for Complex Networks

Complex networks are mapped to a model of boxes and balls where the balls are distinguishable. It is shown that the scale-free size distribution of boxes maximizes the information associated with the boxes provided configurations including boxes containing a finite fraction of the total amount of balls are excluded. It is conjectured that for a connected network with only links between different nodes, the nodes with a finite fraction of links are effectively suppressed. It is hence suggested that for such networks the scale-free node-size distribution maximizes the information encoded on the nodes. The noise associated with the size distributions is also obtained from a maximum entropy principle. Finally explicit predictions from our least bias approach are found to be born out by metabolic networks.Comment: 8 pages, 4 figure

Maximum Entropy, Word-Frequency, Chinese Characters, and Multiple Meanings

The word-frequency distribution of a text written by an author is well accounted for by a maximum entropy distribution, the RGF (random group formation)-prediction. The RGF-distribution is completely determined by the a priori values of the total number of words in the text (M), the number of distinct words (N) and the number of repetitions of the most common word (k_max). It is here shown that this maximum entropy prediction also describes a text written in Chinese characters. In particular it is shown that although the same Chinese text written in words and Chinese characters have quite differently shaped distributions, they are nevertheless both well predicted by their respective three a priori characteristic values. It is pointed out that this is analogous to the change in the shape of the distribution when translating a given text to another language. Another consequence of the RGF-prediction is that taking a part of a long text will change the input parameters (M, N, k_max) and consequently also the shape of the frequency distribution. This is explicitly confirmed for texts written in Chinese characters. Since the RGF-prediction has no system-specific information beyond the three a priori values (M, N, k_max), any specific language characteristic has to be sought in systematic deviations from the RGF-prediction and the measured frequencies. One such systematic deviation is identified and, through a statistical information theoretical argument and an extended RGF-model, it is proposed that this deviation is caused by multiple meanings of Chinese characters. The effect is stronger for Chinese characters than for Chinese words. The relation between Zipf's law, the Simon-model for texts and the present results are discussed.Comment: 15 pages, 10 figures, 2 table

Anomalous dynamic response in the two-dimensional lattice Coulomb gas model: Effects of pinning

It is demonstrated through Monte Carlo simulations that the one component lattice Coulomb gas model in two dimensions under certain conditions display features of an anomalous dynamic response. We suggest that pinning, which can either be due to the underlying discrete lattice or induced by disorder, is an essential ingredient behind this anomalous behavior. The results are discussed in relation to other situations where this response type appears, in particular the two components neutral Coulomb gas below the Kosterlitz-Thouless transition, as well as in relation to other findings from theory, simulations, and experiments on superconductors.Comment: 8 pages including 8 figures, to appear in PR

The likely determines the unlikely

We point out that the functional form describing the frequency of sizes of events in complex systems (e.g. earthquakes, forest fires, bursts of neuronal activity) can be obtained from maximal likelihood inference, which, remarkably, only involve a few available observed measures such as number of events, total event size and extremes. Most importantly, the method is able to predict with high accuracy the frequency of the rare extreme events. To be able to predict the few, often big impact events, from the frequent small events is of course of great general importance. For a data set of wind speed we are able to predict the frequency of gales with good precision. We analyse several examples ranging from the shortest length of a recruit to the number of Chinese characters which occur only once in a text.Comment: 7 pages, 5 figures, 2 table

Analysis of current-voltage characteristics of two-dimensional superconductors: finite-size scaling behavior in the vicinity of the Kosterlitz Thouless transition

It has been suggested [Pierson et al., Phys. Rev. B 60, 1309 (1999); Ammirata et al., Physica C 313, 225 (1999)] that for 2D superconductors there exists a phase transition with the dynamic critical exponent z\approx 5.6. We perform simulations for the 2D RSJ model and compare the results with the experimental data in Repaci et al. obtained for an ultrathin YBCO sample [Phys. Rev. B 54, R9674 (1996)]. We then use a different method of analyzing dynamic scaling than in Pierson et al., and conclude that both the simulations and the experiments are consistent with a conventional Kosterlitz Thouless (KT) transition in the thermodynamic limit for which z=2. For finite systems, however, we find both in simulations and experiments that the change in the current-voltage (IV) characteristics caused by the finite size shows a scaling property with an exponent alpha \approx 1/6, seemingly suggesting a vanishing resistance at a temperature for which z=1/alpha. It is pointed out that the dynamic critical exponent found in Pierson et al. corresponds to the exponent 1/alpha. It is emphasized that this scaling property does not represent any true phase transition since in reality the resistance vanishes only at zero temperature. Nevertheless, the observed scaling behavior associated with alpha \approx 1/6 appears to be a common and intriguing feature for the finite size caused change in the IV characteristics around the KT transition.Comment: 12 pages, 14 figures, accepted in Phys.Rev.

Dynamic critical behaviors of three-dimensional XY models related to superconductors/superfluids

The dynamic critical exponent z is determined from numerical simulations for the three-dimensional XY model subject to two types of dynamics, i.e. relaxational dynamics and resistively shunted junction (RSJ) dynamics, as well as for two different treatments of the boundary, i.e., periodic boundary condition (PBC) and fluctuating twist boundary condition (FTBC). In case of relaxational dynamics, finite size scaling at the critical temperature gives $z\approx 2$ for PBC and 1.5 for FTBC, while for RSJ dynamics $z\approx 1.5$ is obtained in both cases. The results are discussed in the context of superfluid/superconductors and vortex dynamics, and are compared with what have been found for other related models.Comment: 7 pages, 5 figures with europhys.sty, to appear in Europhys. Let