Homepage of Claudio Castellano


Dr C Castellano
 


Dr. Claudio Castellano

Istituto dei Sistemi Complessi, (ISC-CNR) and Dipartimento di Fisica, "Sapienza" Universita' di Roma

Mailing address:
Claudio Castellano
Istituto dei Sistemi Complessi (ISC-CNR)
Via dei Taurini 19, 00185 Roma, Italy

Tel: +39 06 4993 7511
Fax: +39 06 4993 7440

 
 
 

I am a research scientist at the Istituto dei Sistemi Complessi (Institute of Complex Systems, ISC-CNR), part of CNR, the National Research Council of Italy.

Some recent papers

  • The stochastic addition of either vertices or connections in a network leads to the observation of the percolation transition, a structural change with the appearance of a connected component encompassing a finite fraction of the system. Percolation has always been regarded as a substrate-dependent but model-independent process, in the sense that the critical exponents of the transition are determined by the geometry of the system, but they are identical for the bond and site percolation models. Here, we report a violation of such assumption. We provide analytical and numerical evidence of a difference in the values of the critical exponents between the bond and site percolation models in networks with null percolation thresholds, such as scale-free graphs with diverging second moment of the degree distribution. We discuss possible implications of our results in real networks, and provide additional insights on the anomalous nature of the percolation transition with null threshold. reported.spreading in coevolving, coupled, and time-varying networks is reported.       
        F. Radicchi and C. Castellano
        Breaking of the site-bond percolation universality in networks
        Nat. Commun. 6, 10196 (2015).

  • In recent years the research community has accumulated overwhelming evidence for the emergence of complex and heterogeneous connectivity patterns in a wide range of biological and sociotechnical systems. The complex properties of real-world networks have a profound impact on the behavior of equilibrium and nonequilibrium phenomena occurring in various systems, and the study of epidemic spreading is central to our understanding of the unfolding of dynamical processes in complex networks. The theoretical analysis of epidemic spreading in heterogeneous networks requires the development of novel analytical frameworks, and it has produced results of conceptual and practical relevance. A coherent and comprehensive review of the vast research activity concerning epidemic processes is presented, detailing the successful theoretical approaches as well as making their limits and assumptions clear. Physicists, mathematicians, epidemiologists, computer, and social scientists share a common interest in studying epidemic spreading and rely on similar models for the description of the diffusion of pathogens, knowledge, and innovation. For this reason, while focusing on the main results and the paradigmatic models in infectious disease modeling, the major results concerning generalized social contagion processes are also presented. Finally, the research activity at the forefront in the study of epidemic spreading in coevolving, coupled, and time-varying networks is reported.

       R. Pastor-Satorras, C. Castellano, P. Van Mieghem and A. Vespignani
       Epidemic processes in complex networks
       Rev. Mod. Phys. 87, 925 (2015). [arxiv:1408.2701]




  • Empirical evidence shows that the rate of irregular usage of English verbs exhibits discontinuity as a function of their frequency: the most frequent verbs tend to be totally irregular. We aim to qualitatively understand the origin of this feature by studying simple agent-based models of language dynamics, where each agent adopts an inflectional state for a verb and may change it upon interaction with other agents. At the same time, agents are replaced at some rate by new agents adopting the regular form. In models with only two inflectional states (regular and irregular), we observe that either all verbs regularize irrespective of their frequency, or a continuous transition occurs between a low-frequency state, where the lemma becomes fully regular, and a high-frequency one, where both forms coexist. Introducing a third (mixed) state, wherein agents may use either form, we find that a third, qualitatively different behavior may emerge, namely, a discontinuous transition in frequency. We introduce and solve analytically a very general class of three-state models that allows us to fully understand these behaviors in a unified framework. Realistic sets of interaction rules, including the well-known naming game (NG) model, result in a discontinuous transition, in agreement with recent empirical findings. We also point out that the distinction between speaker and hearer in the interaction has no effect on the collective behavior. The results for the general three-state model, although discussed in terms of language dynamics, are widely applicable.
        F. Colaiori, C. Castellano, C. F. Cuskley, V. Loreto, M. Pugliese and F. Tria
        General three-state model with biased population replacement: Analytical solution and application to language dynamics
        Phys. Rev. E. 91, 012808 (2015). [arxiv:1411.4852]


  • Human languages are rule governed, but almost invariably these rules have exceptions in the form of irregularities. Since rules in language are efficient and productive, the persistence of irregularity is an anomaly. How does irregularity linger in the face of internal (endogenous) and external (exogenous) pressures to conform to a rule? Here we address this problem by taking a detailed look at simple past tense verbs in the Corpus of Historical American English. The data show that the language is open, with many new verbs entering. At the same time, existing verbs might tend to regularize or irregularize as a consequence of internal dynamics, but overall, the amount of irregularity sustained by the language stays roughly constant over time. Despite continuous vocabulary growth, and presumably, an attendant increase in expressive power, there is no corresponding growth in irregularity. We analyze the set of irregulars, showing they may adhere to a set of minority rules, allowing for increased stability of irregularity over time. These findings contribute to the debate on how language systems become rule governed, and how and why they sustain exceptions to rules, providing insight into the interplay between the emergence and maintenance of rules and exceptions in language.
        C. F. Cuskley, M. Pugliese, C. Castellano, F. Colaiori, V. Loreto and F. Tria
        Internal and external dynamics in language: Evidence from verb regularity in a historical corpus of English
        PLoS ONE 9(8): e102882 (2014). [arxiv:1408.2699]


  • We develop an analytical approach to the susceptible-infected-susceptible (SIS) epidemic model that allows us to unravel the true origin of the absence of an epidemic threshold in heterogeneous networks. We find that a delicate balance between the number of high degree nodes in the network and the topological distance between them dictates the existence or absence of such a threshold. In particular, small-world random networks with a degree distribution decaying slower than an exponential have a vanishing epidemic threshold in the thermodynamic limit.
        M. Boguñá,  C. Castellano and R. Pastor-Satorras
        The nature of the epidemic threshold for the susceptible-infected-susceptible dynamics in networks
        Phys. Rev. Lett. 111, 068701 [arxiv:1305.4819]


Subpages (2): Publications Research