Showing papers by "Vito Latora published in 2006"

The network analysis of urban streets: A dual approach

Abstract: The application of the network approach to the urban case poses several questions in terms of how to deal with metric distances, what kind of graph representation to use, what kind of measures to investigate, how to deepen the correlation between measures of the structure of the network and measures of the dynamics on the network, what are the possible contributions from the GIS community. In this paper, the author considers six cases of urban street networks characterized by different patterns and historical roots. The authors propose a representation of the street networks based firstly on a primal graph, where intersections are turned into nodes and streets into edges. In a second step, a dual graph, where streets are nodes and intersections are edges, is constructed by means of a generalization model named Intersection Continuity Negotiation, which allows to acknowledge the continuity of streets over a plurality of edges. Finally, the authors address a comparative study of some structural properties of the dual graphs, seeking significant similarities among clusters of cases. A wide set of network analysis techniques are implemented over the dual graph: in particular the authors show that the absence of any clue of assortativity differentiates urban street networks from other non-geographic systems and that most of the considered networks have a broad degree distribution typical of scale-free networks and exhibit small-world properties as well.

The Network Analysis of Urban Streets: A Primal Approach:

Abstract: The network metaphor in the analysis of urban and territorial cases has a long tradition, especially in transportation or land-use planning and economic geography. More recently, urban design has brought its contribution by means of the ‘space syntax’ methodology. All these approaches-though under different terms like ‘accessibility’, ‘proximity’, ‘integration’ ‘connectivity’, ‘cost’, or ‘effort’-focus on the idea that some places (or streets) are more important than others because they are more central. The study of centrality in complex systems, however, originated in other scientific areas, namely in structural sociology, well before its use in urban studies; moreover, as a structural property of the system, centrality has never been extensively investigated metrically in geographic networks as it has been topologically in a wide range of other relational networks such as social, biological, or technological ones. After a previous work on some structural properties of the primal graph representation of...

Centrality measures in spatial networks of urban streets

Abstract: We study centrality in urban street patterns of different world cities represented as networks in geographical space. The results indicate that a spatial analysis based on a set of four centrality indices allows an extended visualization and characterization of the city structure. A hierarchical clustering analysis based on the distributions of centrality has a certain capacity to distinguish different classes of cities. In particular, self-organized cities exhibit scale-free properties similar to those found in nonspatial networks, while planned cities do not.

Structural properties of planar graphs of urban street patterns.

Abstract: Recent theoretical and empirical studies have focused on the structural properties of complex relational networks in social, biological, and technological systems. Here we study the basic properties of twenty 1-square-mile samples of street patterns of different world cities. Samples are turned into spatial valued graphs. In such graphs, the nodes are embedded in the two-dimensional plane and represent street intersections, the edges represent streets, and the edge values are equal to the street lengths. We evaluate the local properties of the graphs by measuring the meshedness coefficient and counting short cycles (of three, four, and five edges), and the global properties by measuring global efficiency and cost. We also consider, as extreme cases, minimal spanning trees (MST) and greedy triangulations (GT) induced by the same spatial distribution of nodes. The measures found in the real and the artificial networks are then compared. Surprisingly, cities of the same class, e.g., grid-iron or medieval, exhibit roughly similar properties. The correlation between a priori known classes and statistical properties is illustrated in a plot of relative efficiency vs cost.

Centrality in networks of urban streets

Abstract: Centrality has revealed crucial for understanding the structural properties of complex relational networks. Centrality is also relevant for various spatial factors affecting human life and behaviors in cities. Here, we present a comprehensive study of centrality distributions over geographic networks of urban streets. Five different measures of centrality, namely degree, closeness, betweenness, straightness and information, are compared over 18 1-square-mile samples of different world cities. Samples are represented by primal geographic graphs, i.e., valued graphs defined by metric rather than topologic distance where intersections are turned into nodes and streets into edges. The spatial behavior of centrality indices over the networks is investigated graphically by means of color-coded maps. The results indicate that a spatial analysis, that we term multiple centrality assessment, grounded not on a single but on a set of different centrality indices, allows an extended comprehension of the city structure, nicely capturing the skeleton of most central routes and subareas that so much impacts on spatial cognition and on collective dynamical behaviors. Statistically, closeness, straightness and betweenness turn out to follow similar functional distribution in all cases, despite the extreme diversity of the considered cities. Conversely, information is found to be exponential in planned cities and to follow a power-law scaling in self-organized cities. Hierarchical clustering analysis, based either on the Gini coefficients of the centrality distributions, or on the correlation between different centrality measures, is able to characterize classes of cities.

The backbone of a city

Abstract: Recent studies have revealed the importance of centrality measures to analyze various spatial factors affecting human life in cities. Here we show how it is possible to extract the backbone of a city by deriving spanning trees based on edge betweenness and edge information. By using as sample cases the cities of Bologna and San Francisco, we show how the obtained trees are radically different from those based on edge lengths, and allow an extended comprehension of the “skeleton” of most important routes that so much affects pedestrian/vehicular flows, retail commerce vitality, land-use separation, urban crime and collective dynamical behaviours.

Compromise and synchronization in opinion dynamics

Abstract: We discuss two models of opinion dynamics. We first present a brief review of the Hegselmann and Krause (HK) compromise model in two dimensions, showing that it is possible to simulate the dynamics in the limit of an infinite number of agents by solving numerically a rate equation for a continuum distribution of opinions. Then, we discuss the Opinion Changing Rate (OCR) model, which allows to study under which conditions a group of agents with a different natural tendency (rate) to change opinion can find the agreement. In the context of the this model, consensus is viewed as a synchronization process.

Opinion formation models based on game theory

Abstract: A way to simulate the basic interactions between two individuals with different opinions, in the context of strategic game theory, is proposed. Various games are considered, which produce different kinds of opinion formation dynamics. First, by assuming that all individuals (players) are equals, we obtain the bounded confidence model of continuous opinion dynamics proposed by Deffuant et al. In such a model a tolerance threshold is defined, such that individuals with difference in opinion larger than the threshold can not interact. Then, we consider that the individuals have different inclinations to change opinion and different abilities in convincing the others. In this way, we obtain the so-called ``Stubborn individuals and Orators'' (SO) model, a generalization of the Deffuant et al. model, in which the threshold tolerance is different for every couple of individuals. We explore, by numerical simulations, the dynamics of the SO model, and we propose further generalizations that can be implemented.

Opinion dynamics and synchronization in a network of scientific collaborations

Abstract: In this paper we discuss opinion dynamics in the opinion changing rate ( OCR ) model, recently proposed in Pluchino et al. [Int. J. Mod. Phys. C 16(4) (2005) 515–531]. The OCR model allows to study whether and how a group of social agents, with a different intrinsic tendency ( rate ) to change opinion, finds agreement. In particular, we implement the OCR model on a small graph describing the topology of a real social system. The nodes of the graph are scientists participating in the Tepoztlan conference, celebrating Alberto Robledo's 60th birthday, and the links are based on coauthorship in scientific papers. We study how opinions evolve in time according to the frequency rates of the nodes, to the coupling term, and also to the presence of group structures.

Network of sexual contacts and sexually transmitted HIV infection in Burkina Faso

Abstract: Two thirds of the people who have been infected by human immunodeficiency virus (HIV) in the world live in Sub-Saharan African countries. The results of a study measuring the degree distribution of the network of sexual contacts in Burkina Faso are described. Such a network is responsible for the spread of sexually transmitted diseases, and in particular of HIV. It has been found that the number of different sexual partners reported by males is a power law distribution with an exponent gamma = 2.9 (0.1). This is consistent with the degree distribution of scale-free networks. On the other hand, the females can be divided into two groups: the prostitutes with an average of 400 different partners per year, and females with a stable partner, having a rapidly decreasing degree distribution. Such a result may have important implications on the control of sexually transmitted diseases and in particular of HIV. Since scale-free networks have no epidemic threshold, a campaign based on prevention and anti-viral treatment of few highly connected nodes can be more successful than any policy based on enlarged but random distribution of the available anti-viral treatments.

Quantifying the relevance of different mediators in the human immune cell network

Abstract: Motivation: Immune cells coordinate their efforts for the correct and efficient functioning of the immune system (IS). Each cell type plays a distinct role and communicates with other cell types through mediators such as cytokines, chemokines and hormones, among others, that are crucial for the functioning of the IS and its fine tuning. Nevertheless, a quantitative analysis of the topological properties of an immunological network involving this complex interchange of mediators among immune cells is still lacking. Results: Here we present a method for quantifying the relevance of different mediators in the immune network, which exploits a definition of centrality based on the concept of efficient communication. The analysis, applied to the human IS, indicates that its mediators differ significantly in their network relevance. We found that cytokines involved in innate immunity and inflammation and some hormones rank highest in the network, revealing that the most prominent mediators of the IS are molecules involved in these ancestral types of defence mechanisms which are highly integrated with the adaptive immune response, and at the interplay among the nervous, the endocrine and the immune systems. Contact: claudio.franceschi@unibo.it

Olami-Feder-Christensen Model on different Networks

Abstract: We investigate numerically the Self Organized Criticality (SOC) properties of the dissipative Olami-Feder-Christensen model on small-world and scale-free networks. We find that the small-world OFC model exhibits self-organized criticality. Indeed, in this case we observe power law behavior of earthquakes size distribution with finite size scaling for the cut-off region. In the scale-free OFC model, instead, the strength of disorder hinders synchronization and does not allow to reach a critical state.

A topological analysis of scientific coauthorship networks

Abstract: We study coauthorship networks based on the preprints submitted to the Los Alamos cond-mat database during the period 2000–2005. In our approach two scientists are considered connected if they have coauthored one or more cond-mat preprints together in the same year. We focus on the characterization of the structural properties of the derived graphs and on the time evolution of such properties. The results show that the cond-mat community has grown over the last six years. This is witnessed by an improvement in the connectivity properties of coauthorship graphs over the years, as confirmed by an increasing size of the largest connected component, of the global efficiency and of the clustering coefficient. We have also found that the graphs are characterized by long-tailed degree and betweenness distributions, assortative degree–degree correlations, and a power-law dependence of the clustering coefficient on the node degree.

MULTIFRACTAL ANALYSIS OF MOUNT St. HELENS SEISMICITY AS A TOOL FOR IDENTIFYING ERUPTIVE ACTIVITY

Abstract: We present a multifractal analysis of Mount St. Helens seismic activity during 1980–2002. The seismic time distribution is studied in relation to the eruptive activity, mainly marked by the 1980 major explosive eruptions and by the 1980–1986 dome building eruptions. The spectrum of the generalized fractal dimensions, i.e. Dq versus q, extracted from the data, allows us to identify two main earthquake time distribution patterns. The first one exhibits a multifractal clustering correlated to the intense seismic swarms of the dome building activity. The second one is characterized by an almost constant value of Dq ≈ 1, as for a random uniform distribution. The time evolution of Dq (for q = 0.2), calculated on a fixed number of events window and at different depths, shows that the brittle mechanical response of the shallow layers to rapid magma intrusions, during the eruptive periods, is revealed by sharp changes, acting at a short time scale (order of days), and by the lowest values of Dq (≈ 0.3). Conversely, for deeper earthquakes, characterized by intense seismic swarms, Dq do not show obvious changes during the whole analyzed period, suggesting that the earthquakes, related to the deep magma supply system, are characterized by a minor degree of clustering, which is independent of the eruptive activity.

Multiple Centrality Assessment. Centralità e ordine complesso nell'analisi spaziale e nel progetto urbano

Abstract: Territorio 1. Introduzione «Non c’è spesso così tanta perfezione nei lavori composti di molte parti e realizzati dalla mano di diversi maestri quanto in quelli sulle quali ha lavorato un solo individuo. [...] Così quelle vecchie città che, avendo cominciato come villaggi, sono diventate col tempo grandi centri urbani, sono in generale così mal composte, se confrontate con quei luoghi regolari tracciati da un ingegnere su un piano seguendo la sua fantasia, che, anche se considerando i loro edifici separatamente uno spesso trova in essi altrettanta arte, se non di più, che nelle altre, nondimeno guardando come sono disposti, uno grande qui, uno piccolo là, e quanto essi rendono le strade storte e disuguali, uno direbbe che è il caso, piuttosto che la volontà di un uomo sicuro che usa la ragione, che li ha messi in tal modo» (Descartes, 1994, p. 27).

Effective spin-glass Hamiltonian for the anomalous dynamics of the HMF model

Abstract: We discuss an effective spin-glass Hamiltonian which can be used to study the glassy-like dynamics observed in the metastable states of the Hamiltonian mean field (HMF) model. By means of the Replica formalism, we were able to find a self-consistent equation for the glassy order parameter which reproduces, in a restricted energy region below the phase transition, the microcanonical simulations for the polarization order parameter recently introduced in the HMF model.

A new analysis of Self-Organized Criticality in the OFC model and in real earthquakes

Abstract: We perform a new analysis on the dissipative Olami-Feder-Christensen model on a small world topology considering avalanche size differences. We show that when criticality appears the Probability Density Functions (PDFs) for the avalanche size differences at different times have fat tails with a q-Gaussian shape. This behaviour does not depend on the time interval adopted and is found also when considering energy differences between real earthquakes. Such a result can be analytically understood if the sizes (released energies) of the avalanches (earthquakes) have no correlations. Our findings support the hypothesis that a self-organized criticality mechanism with long-range interactions is at the origin of seismic events and indicate that it is not possible to predict the magnitude of the next earthquake knowing those of the previous ones.

Synchronization Properties of Network Motifs

Abstract: We address the problem of understanding the variable abundance of 3-node and 4-node subgraphs (motifs) in complex networks from a dynamical point of view. As a criterion in the determination of the functional significance of a n-node subgraph, we propose an analytic method to measure the stability of the synchronous state (SSS) the subgraph displays. We show that, for undirected graphs, the SSS is correlated with the relative abundance, while in directed graphs the correlation exists only for some specific motifs.

The Postgenomic Era: Transcriptomic as the New Paradigm of Life

Abstract: Now that complete genome sequences are available for a variety of organism, the elucidation of single gene function involved in the metabolism, includes necessarily a better understanding of the cellular responses on all levels of gene products, mRNA, proteins and metabolites. Such knowledge’s growth is essential since the observable properties of living organisms, so called phenotypes, are the results of the genotype in juxtaposition with the environmental factors. If much has been done and is in continuous evolution to make clear the mRNA and transcript profiling, considerably less effort has been required to characterize the products of gene expression, the metabolites. Now it is possible to represent cellular processes as metabolic networks and to describe them at different levels of resolution, focusing the attention on the connection map of the metabolic network. Here we describing just the structure of a metabolic network, instead of considering the details of the metabolic flux. For each approach, a number of examples have been given and the potential application have been discussed. Mathematical studies of simple metabolic networks show clearly that, already from an oversimplified approach based only on the analysis of the structure of the network, it is possible to reach important conclusions on the network’s design principles. Of course one of the most important constraint on metabolic networks comes from the influences of the external environments. For this reason we focus some examples of the effects of the environments on metabolic networks, making of transcriptomic a new paradigm of the life.

Analyse du réseau des voiries urbaines: une approche directe

Abstract: « Entre lesquelles l’une des premières fut que je m’avisai de considérer que souvent il n’y a pas tant de perfection dans les ouvrages composés de plusieurs pièces, et faits de la main de divers maîtres, qu’en ceux auxquels un seul a travaillé. Ainsi voit-on que les bâtiments qu’un seul architecte a entrepris et achevés ont coutume d’être plus beaux et mieux ordonnés que ceux que plusieurs ont tâché de raccommoder, en faisant servir de vieilles murailles qui avoient été bâties à d’autres fins. Ainsi ces anciennes cités qui, n’ayant été au commencement que des bourgades, sont devenues par succession de temps de grandes villes, sont ordinairement si mal compassées, au prix de ces places régulières qu’un ingénieur trace a sa fantaisie dans une plaine, qu’encore que, considérant leurs édifices chacun à part, on y trouve souvent autant ou plus d’art qu’en ceux des autres, toutefois, à voir comme ils sont arrangés, ici un grand, là un petit, et comme ils rendent les rues courbées et inégales, on diroit que c’est plutôt la fortune que la volonté de quelques hommes usants de raison, qui les a ainsi disposés.» (Descartes, Discours de la méthode). À l’aube de la modernité, Descartes définit la géométrie euclidienne comme le seul système ordonné capable, aux yeux d’un homme raisonnable, de s’appliquer à des motifs environnementaux, comme les jardins, les paysages, les routes et les villes. Presque trois cents ans après, Charles-Edouard Jeanneret-Gris dit Le Corbusier réagit violemment contre le retour, prêché par Sitte, à l’esthétique sociale des entrelacs médiévaux, par cette célèbre invective : « La rue courbe est le chemin des ânes, la rue droite le chemin des Hommes » (1925). Seuls des ânes auraient pu concevoir les cités historiques d’Europe, avec leur fouillis de ruelles étroites et cet horrible, chaotique enchevêtrement d’intersections et de places. Actuellement encore, la géométrie euclidienne exerce une sorte de tyrannie sur les architectes et les aménageurs urbains, une obligation lorsqu’il s’agit de tracer des routes, des quartiers ou des villes. Tout comme lors du renouvellement urbain de naguère, les vieux quartiers restent sousestimés quant à leurs valeurs les plus fondamentales : certes ils sont pittoresques, agréables et intéressants en raison de leurs architectures originales, mais leur structure est dévalorisée : ils sont désordonnés.