Institute of Geography

Recent studies have shown that a system composed from several randomly interdependent networks is extremely vulnerable to random failure. However, real interdependent networks are usually not randomly interdependent, rather a pair of dependent nodes are coupled according to some regularity which we coin inter-similarity. For example, we study a system composed from an interdependent world wide port network and a world wide airport network and show that well connected ports tend to couple with well connected airports. We introduce two quantities for measuring the level of inter-similarity between networks (i) Inter degree-degree correlation (IDDC) (ii) Inter-clustering coefficient (ICC). We then show both by simulation models and by analyzing the port-airport system that as the networks become more inter-similar the system becomes significantly more robust to random failure.

European integration is for its essential part reflected in the building of urban networks that are increasing interdependencies between cities. Evidence of this process is brought through the observation of a variety of networks whose nodes are located in cities, especially the development of international functions. Analysis of airlines networks as well as the economic linkages between multinational firms (1990,1996) provide a good illustration reflecting the dominant features of such a process . But other international networks as those created by cultural or scientific activities introduce may have distinctive patterns compared to the previous ones . From these analyses, four major modes can be identified in the spatial construction of urban networks in Europe :

We model the spreading of a crisis by constructing a global economic network and applying the Susceptible-Infected-Recovered (SIR) epidemic model with a variable probability of infection. The probability of infection depends on the strength of economic relations between the pair of countries, and the strength of the target country. It is expected that a crisis which originates in a large country, such as the USA, has the potential to spread globally, like the recent crisis. Surprisingly we show that also countries with much lower GDP, such as Belgium, are able to initiate a global crisis. Using the {\it k}-shell decomposition method to quantify the spreading power (of a node), we obtain a measure of ``centrality'' as a spreader of each country in the economic network. We thus rank the different countries according to the shell they belong to, and find the 12 most central countries. These countries are the most likely to spread a crisis globally. Of these 12 only six are large economies, while the other six are medium/small ones, a result that could not have been otherwise anticipated. Furthermore, we use our model to predict the crisis spreading potential of countries belonging to different shells according to the crisis magnitude.

Scaling world city size distributions are analyzed in terms of q-exponentials, the distributions which naturally emerge within nonextensive statistical mechanics. These distributions allow to estimate hierarchy and scale parameters for 29 historical periods, thus conveniently replacing the Zipfian scaling law by an historically sensitive and nuanced model of how long-term oscillations in urban hierarchies co-evolve with urban industrial, commercial and financial innovations. Our present model emerges from attempts to understand our scaling results by validation tests using multiple political and economic variables. We investigate the model so as to explore the extent and by-play of endogenous dynamics and exogenous shocks as causes of synchronization. One such step consists of identifying a network macro-structural variable, the average urban hub adjacency ratio, to help explain interactive network dynamics of endogenous synchronization. We find, further, in examining our validation-check variables, that innovation clusters in space and time -the leading sectors of urban economies -are differentially embedded in periods of alternating hierarchical and heterarchical urban scaling regimes. Urban infrastructure and demography, as expected, change slowly compared to the pace of Schumpeterian K-cyclic tendencies in bursts of competitive economic innovation, but the synchrony of these changes in time and space, while a surprising discovery, is consistent with our network-economic theory. [DOUG: I do not understand the sentence above.] Sandwiched in an intermediate layer of change in terms of its spatiotemporal scalings we find the shifts in the leading cities and states, as studied by Modelski and Thompson, forming a synchronously-embedded three-layer model of economic, political, and urban structural oscillations. These findings have potential for unifying a new perspective on glacially slow and then quickly triggered structural changes in the city hierarchies and infrastructure with perspectives on long-cycles as studied through the world political and economic perspectives.

This study aims to understand the current state of research in urban resilience and to open a discussion about multi-level perspectives for this concept. Starting with the history of the concept of resilience, we identify three main stages in resilience concept’s evolution: conceptualization, contextualization and operationalization. Confusion occurs between sustainability and resilience, therefore we clearly separate these two concepts by creating conceptual maps. Such maps also underline the specificities of urban and regional resilience discourses. We illustrate that urban resilience research, operating within intra-urban processes, is oriented towards natural disasters, while regional resilience research, operating mostly within inter-urban processes, is oriented towards economic shocks. We show that these two approaches to resilience – urban and regional – are complementary, and we propose to integrate them into a multi-level perspective. By combining these tw...

The paper unveils « new reticular territories » implicitly defined in air transport networks. We describe an approach based on weighted graphs (induced from city to city air passenger traffic) from which we infer strongly connected components, themselves organized into sub-networks. The computation can be iterated within each component (or cluster) until each node or city appears as such in a component. The method leads to a multilevel presentation of the most connected cities in the world, underlining the necessary steps to go through when traveling worldwide, described as a path through the different levels and components. These components sometimes appear as being founded on spatial rules, but we also observed specialized subgroups emerging from economical or institutional logics such as partnership strategies. Our methods open new perspectives to study and represent networks or spatial interaction of cities systems at every level of geographical scale, from local to global.

Recent years have seen tremendous growth of many online social networks such as Facebook, LinkedIn and MySpace. People connect to each other through these networks forming large social communities providing researchers rich datasets to understand, model and predict social interactions and behaviors. New contacts in these networks can be formed either due to an individual's demographic profile such as age group, gender, geographic location or due to network's structural dynamics such as triadic closure and preferential attachment, or a combination of both demographic and structural characteristics. A number of network generation models have been proposed in the last decade to explain the structure, evolution and processes taking place in different types of networks, and notably social networks. Network generation models studied in the literature primarily consider structural properties, and in some cases an individual's demographic profile in the formation of new social contacts. These models do not present a mechanism to combine both structural and demographic characteristics for the formation of new links. In this paper, we propose a new network generation algorithm which incorporates both these characteristics to model growth of a network. We use different publicly available Facebook datasets as benchmarks to demonstrate the correctness of the proposed network generation model.

While maritime transport ensures about 90% of world trade volumes, it has not yet attracted as much attention as other transport systems from a graph perspective. As a result, the relative situation and the evolution of seaports within maritime networks are not well understood. This paper wishes verifying to what extent the hub-and-spoke strategies of ports and ocean carriers have modified the structure of a maritime network, based on the Atlantic case. We apply graph measures and clustering methods on liner movements in 1996 and 2006. The methodology also underlines which ports are increasing their position by carriers' circulation patterns on various scales. This research demonstrates that the polarization of the Atlantic network by few dominant ports occurs in parallel with the increased spatial integration of this area by shipping lines.

Population en milliers d'habitants en 1990 (agglomérations de 200 000 habitants et plus, sauf Luxembourg et Metz) 3 500 80 Villes de la zone d'étude A. La population des villes européennes en 1990 1. Dans la suite du texte, les termes « agglomération » et « ville » sont employés indifféremment. 2. Luxembourg faisait partie de l'étude Les Villes européennes de 1989. 1. Luxembourg et Metz ont moins de 200 000 habitants, mais figurent dans la sélection. Seul le nom de la ville principale est mentionné pour les conurbations suivantes: Cologne

La ville est autant formée du dedans que du dehors, c'est à dire constituée par ses relations internes et externes. A travers les réseaux et les stratégies qui gouvernent à leur organisation et leur pratique, on peut tenter de déceler différents niveaux de formation et de mutation des structures urbaines. C'est par l'approche des réseaux « petits-mondes », croisée avec les stratégies d'organismes en réseau multiterritoriaux comme les entreprises multinationales que nous tentons de redéfinir la ville. Celle-ci peut, en effet, être réinterprétée à un niveau méso-géographique à partir de ses réseaux sociaux et économiques, de leur cohésion et de leur forme, entre un niveau micro des acteurs, et un niveau macro des stratégies globales. Mots clefs : Ville, système urbain, réseaux, « petits-mondes », entreprises multinationales Les nouveaux concepts de la science géographique apparaissent dans les sociétés qui les produisent et sont retranscrits, nommés, interprétés, voire anticipés par les chercheurs qui en observant le monde à la manière des chroniqueurs du Moyen-Âge, sont à l'écoute de ses transformations et s'en inspirent. Ces chercheurs ne créent rien, mais comprennent le monde dans une démarche empathique et participative. La recherche est une rencontre entre un chercheur, qui porte en lui sa discipline scientifique mais aussi sa personnalité (son histoire et son sens de la vie), et le monde. Serait-ce comme dans la création artistique où Marc Chagall avouera à propos de ses représentations imaginaires « je n'invente rien ; tout est inscrit en moi ; je ne fais que recopier » (in Lévy-Kuentz, 2003) ? C'est probablement dans cette intention que se situe l'oeuvre de Jean-Bernard Racine et des plus proches collaborateurs de sa carrière, dont nombre sont présents dans cet ouvrage. Certes, cet élan, s'il s'arrêtait à une simple quête des sens du chercheur, pourrait sembler une approche « cartésienne optimiste de la connaissance » (Popper, 1985). Mais, collectivement, ce mouvement ne cesse d'aller à la rencontre de nouveaux objets et de nouveaux concepts, remettant en question les précédents, dans un élan de soif du monde et de don de soi jamais assouvi. C'est cette « en-vie » de comprendre les mutations de la ville dans laquelle il vit et s'engage qui a mené Jean-Bernard Racine de mondes en mondes, de concepts en théories, du multidisciplinaire en « pro-actionnaire » du monde associatif. Car la ville, est bien davantage qu'un objet d'étude pour un chercheur engagé : c'est un ensemble de projets, projets sociétaux (« citadins ») dans lesquels le chercheur peut se placer, soit au service des gouvernants, en adhérant pleinement à leur vision d'un « projet de ville », soit se placer en position d'intermédiaire pour agir comme un catalyseur et un anticipateur des habitants, de l'air du temps et des transformations contemporaines issues des processus émergents. En explicitant les cadres théoriques des processus sociaux à l'oeuvre, le chercheur donne à lire la transformation des systèmes de relations, et contribue à renforcer le sens des projets urbains, en accord avec les sociétés qui s'y projettent. Il y a en effet dans une ville autant de projets que d'individus, que de collaborations ou de confrontations d'individus. Leurs objectifs, désirs et perceptions de leur position relative vont déterminer leurs stratégies et leurs actions. Si la ville est la « maximisation de l'interaction », c'est bien pour accéder aux désirs de chacun, pour que chaque individu élabore des stratégies, se rende plus fort par rapport aux autres mais aussi grâce aux autres. L'interaction crée de l'interdépendance qui sera d'autant plus élevée que les liens sont étroits ou nombreux. Plus une ville grandit en population, et plus les potentiels d'interactions augmentent en nombre mais pas forcément en intensité. En effet, de multiples réseaux sociaux au sens très général se développent, offrant un choix élevé de possibilités de connexions pour chacun. C'est le paradoxe du sentiment de liberté de la grande ville qui semble d'un côté plus contraignante par sa foule, mais qui offre un vaste choix de réseaux interpersonnels, où l'on peut (si on le peut, et c'est bien là la question) butiner ici ou là dans des réseaux très différents. Ces réseaux naissent et meurent, certains perdurent sur plusieurs générations en se reproduisant et en évoluant : leur adaptation renforce leur stabilité et leur donne un caractère « résilient ». C'est par exemple les réseaux

This article presents a study on the position of the European cities within general urban system of Europe in 2000 with the purpose of assessing its level and capacity of supranational integration, and more precisely to explore the specific position of Mediterranean cities in this context. The competitiveness of different territories today is integrated in the logic of urban networks, in which cities have a fundamental paper. In this paper, the position of 180 urban areas of more than 200,000 inhabitants of the European Union of the 15; along with Norway and Switzerland is analyzed using fifteen different indicators of the population, culture, tourism and finances. From the resulting classification some conclusions are obtained: 1) the importance of the urban and national structures in the concentration of international functions; 2) the cooperation and association between cities in different space scales and the importance that this "interurban competition" for the development of these networks has, and 3) the absence of homogeneity of the European Mediterranean cities from the perspective of its international position.

intitolò uno fra i suoi più celebri articoli A city is not a tree (1965). Con quella metafora intendeva criticare l'impoverimento delle rappresentazioni che derivano dall'incapacità della mente umana a concepire e figurarsi la complessità urbana. In effetti, spesso i teorici della città tendono a semplificare la complessa struttura urbana utilizzando schemi ad albero che separano gli elementi piuttosto che connetterli. Invece, all'interno della città, le reti sociali si intricano, si intersecano e sono dotate di molti rami, incrociati e ridondanti. E appunto questa complessità a costituire la ricchezza della città, formata dalle persone e dalle loro interazioni, dalle mescolanze di risorse e dalle pratiche di potere (Allen, 1999). Le ricchezze costitutive di ogni città si sono sviluppate e possono sussistere e trasformarsi solo grazie alle relazioni intrattenute con altre città. Al contrario del Christopher Alexander entitled one of his best-known articles A City is not a Tree (1965). He employed the metaphor to criticize the poverty of representation that stems from the human mind's inability to comprehend and represent the complexity of the city. Urban theorists often simplify complex urban structures with tree diagrams that tend to separate elements rather than link them. In contrast, the social networks within the city interweave and overlap with no shortage of intersecting and redundant branches. It is in fact this complexity that gives rise to the richness of the city. It is composed of people and their interactions, of a combination of resources and the exercise of power (Allen, 1999). This constituent richness of all cities can only be developed, maintained and transformed through relations with other cities. Unlike the village which capitalizes on essentially local resources (its location), the city owes its existence to the networking of distant locations, based 38 A CITY NETWORK IS NOT A TREE La rete di città non è un albero FORME E TIPOLOGIE DELLA CITTÀ GLOBALE FORMS AND TYPOLOGIES OF THE GLOBAL CITY

While maritime transport ensures about 90% of world trade volumes, it has not yet attracted as much attention as other transport systems from a graph perspective. As a result, the relative situation and the evolution of seaports within maritime networks are not well understood. This paper wishes verifying to what extent the hub-and-spoke strategies of ports and ocean carriers have modified the structure of a maritime network, based on the Atlantic case. We apply graph measures and clustering methods on liner movements in 1996 and 2006. The methodology also underlines which ports are increasing their position by carriers' circulation patterns on various scales. This research demonstrates that the polarization of the Atlantic network by few dominant ports occurs in parallel with the increased spatial integration of this area by shipping lines.

Real data show that interdependent networks usually involve inter-similarity. Intersimilarity means that a pair of interdependent nodes have neighbors in both networks that are also interdependent (Parshani et al [1]). For example, the coupled world wide port network and the global airport network are intersimilar since many pairs of linked nodes (neighboring cities), by direct flights and direct shipping lines exist in both networks. Nodes in both networks in the same city are regarded as interdependent. If two neighboring nodes in one network depend on neighboring nodes in the another we call these links common links. The fraction of common links in the system is a measure of intersimilarity. Previous simulation results suggest that intersimilarity has considerable effect on reducing the cascading failures, however, a theoretical understanding on this effect on the cascading process is currently missing. Here, we map the cascading process with inter-similarity to a percolation of networks composed of components of common links and non common links. This transforms the percolation of inter-similar system to a regular percolation on a series of subnetworks, which can be solved analytically. We apply our analysis to the case where the network of common links is an Erdős-Rényi (ER) network with the average degree K, and the two networks of non-common links are also ER networks. We show for a fully coupled pair of ER networks, that for any K ≥ 0, although the cascade is reduced with increasing K, the phase transition is still discontinuous. Our analysis can be generalized to any kind of interdependent random networks system.