From the industrial city to global cities: non-linear interactions, emergencies, assisted self-organization

Abstract

In this work we aim to show the complex view of the urban phenomenon, considered in its international context, according to a theoretical approach of contemporary science, transversal to many disciplines. In paragraph 1 we are going to introduce the central question about the need of a complex vision of the urban phenomenon, in particular in the contemporary world, where cities are no longer isolated realities, but they are deeply rooted in the territory, and, at the same time, deeply connected to each other. In paragraph 2 we are going to refer briefly to the sociological urban theories that developed from the age of the industrial city, in the late nineteenth century, to the present. Such theories focused attention on those aspects of the cities that refer to the quality of urban life, highlighting how the top-down dirigistic approach is scarcely effective in a complex city and why it is necessary, in order to manage the urban complexity, to adopt a bottom-up approach which encourages the phenomena of spontaneous self-organization that tend to appear. In paragraph 3 we are going to illustrate the current concept of the so called global cities, a concept that sees some of the most important cities in the world, for size, as essentially uprooted from their territory and, instead, strongly connected to each other because of communication networks which step over urban limits and extend over the administrative, governmental and geographical boundaries. In paragraph 4 we are going to discuss some aspects of urban complexity that are of noteworthy relevance, showing a typical example of a self-organized city, the gentrification of the historical town centres, and highlighting the fundamental idea of assisted self-organization as an inescapable approach for an effective management of the urban complexity. In paragraph 5 we are going to briefly mention, without technical details, the logic framework of agent-based modeling for social, economic and urban systems that is quickly spreading in scientific environments and that allows the setting up of an empirical-study laboratory, where it is possible to model the evolution of a complex system over time without using differential or finite-difference equations, which was how it was carried out up until not so long ago.

1. Introduction

The conceptualizations aimed at placing the urban phenomenon and the theories aimed at describing it and interpreting it have been numerous and they have changed also because of the shifts that have taken place in the phenomenon itself. Policies and actions in the urban field that are based on such conceptualizations and theories, or that are based on paradigms from which these conceptualizations and theories move, have been following one another, in general with some delay with respect to the socio-economic phenomena that stimulated them, often without considering neither the approval nor the ideas of the citizens, sometimes aiming at the pursuit of goals of a certain social class or group of interest.

As it is known, a lot of nation-states have lost certain powers and competencies in the economic field. Even after this shift, lived in a different traumatic ways by the nation-states that have suffered it, the idea of a broader and broader diffusion in the world of the said change, and of its spreading to the social, political and even institutional fields seems to be now widely accepted. This decrease of sovereignty of states occurs in a process in which the authority of states is being replaced, often in a partial way, by that of other powers (economic, social, political and institutional) at a supranational and sub-national level (or briefly, local). Clearly visible is the change of the role of states that inside supranational regimes of governance, not always well defined, are no longer absolute protagonists, but actors whose preponderance is being challenged by multinational enterprises, by inter-governmental organizations, by a global civic society, by trans-national social movements and – what is more interesting – by cities either in the shape of urban megalopolis and in a net of global cities that combine people of different ethnic groups and cultures. In this framework, we find that actors adopting functions and powers that previously were proper of states, are numerous and linked among them by relations of diverse nature.

Among these actors, cities are underlined like emerging items in the scenery of global governance. Cities delineate and carry out policies in sectors traditionally attributed to the states’ competence, sometimes creating strains that can lead to institutional conflicts. Several cities have reached the point of attributing themselves competencies which were once centralized, establishing economic alliances with other cities or even entering foreign politics, rediscovering or even devising their own local identities for this purpose.

To the acquisition of new powers and competencies by cities, there corresponds a multiplicity of analyses that, starting from different interpretative models, share the relevance given to the urban (or local) dimension. On the one hand, there are the studies that deal with global cities and with the differentiated role that they can take on, in relation to their position in the informative, productive and financial flows. On the other hand the initiative of local projects that consider territories as strategic elements to affirm a globalization model, basically converging or tentatively diverging from the one in action, has started to make its way. Providing that any judgment on the globalization process is at this point historically determined, the globalization process has not only

gone ahead, but it also seems unstoppable and ready to lead to a kind of globalization, following a dynamics that passes through mostly unpredictable steps. This kind of globalization has, in any case, a positive aspect. It is becoming clearer and clearer to everybody that human beings are meant to have a common destiny, to become a community of destiny (Morin and Kern 1993; Morin, 2011). In this sea, all human beings must swim. In this sea we will become aware of the risks for this community of destiny (risks that we are already experiencing, even without having a full perception of them, as we are not or do not perceive to be part of a community of destiny yet).

This must be taken into consideration by social scientists, urban operators, scholars and the assisted self-organization and strategic planning operators.

In the following paragraphs we are going to present, from the historical point of view, some phases of the evolution of urban concept, from the arrival of the first sociological conceptualizations, to the contemporary visions of global cities: two ways of looking at the urban phenomenon according to the perspective which sees cities as complex self-organizing systems. We are going to underline that the effective management of urban complexity, since cities themselves are complex systems, must comply with the endogenous bottom-up push, assisting their evolution in strict abidance by the rules, without managing it in a ‘dirigistic’ way. We will finally briefly refer to a particularly effective modeling technique, to study complex systems. It has claimed great attention in recent years, as it consents the realization of the complex approach to the urban phenomenon effectively: the agent-based modeling.

Before proceeding according to what has already been stated, it must be mentioned that this work lays in the scope of the initiative undertaken by Corrado Beguinot and by the Aldo della Rocca Foundation (as in Beguinot, ed. 2011) that he presides over, in view of the official statement of the UN on the «Right to the city». Moreover this will make it easier to understand better the conclusion of this text¹.

2. Reference to urban sociological theories

A number of scholars, especially from the late 1800s, dealt with the conceptualization of the urban phenomenon in all its many and different articulations, from the physical ones to the social and economic, proposing both analysis schemes and interpretative theories and lines of control according to points of view and different methods. There are urban theories that have formed within different disciplinary areas: economic, geographic, ecological, anthropological, sociological, and many others that we are not going to discuss here, which the reader may refer to (e.g. Bertuglia and Vaio, eds. 1997, 2003, 2005, 2011a, 2011b; some significant cornerstones, without expecting completeness: Lowry, 1964, 1966; Wilson, 1974, 1981; Batty, 1976; Dendrinos and Mullally, 1985; La Cecla, 1988, 1993; Pumain, Sanders and Saint-Julien, 1989; Sanglier and Allen, 1989; Bertuglia, Leonardi and Wilson, eds. 1990; Dendrinos, 1992; Cori et al., eds. 1993; Mela, 1996; Nijkamp and Reggiani, 1998; Mela and Davico, 2002; Pugliese and Spaziante, eds. 2003; Dematteis and Lanza, 2011). In this work, we intend to underline, even if briefly, the evolution of the idea of a city as a complex system composed of individuals (also called actors or agents) in a close interaction according to non-linear and non-mathematical forms. These individuals are far from the idea of homo oeconomicus, omniscient and optimizing, and in competition with other humans like him, assumed by the current ecological and economic theories (Bertuglia and Vaio, eds. 1997, 2003, 2005, 2011a, 2011b).

Now, let us come to the urban sociological theories. The German sociologist Georg Simmel is one of the first to become interested, from a sociological viewpoint, in the phenomena linked to the industrial cities and to the big metropolitan areas, in the years between the XVIII and the XIX century (Simmel, 1903). Simmel’s interest in sociology lies in the fact that it studies the forms of social interaction without insisting on an explanation for the cause of the action, since the action is seen as the result of individual spontaneity.

The crucial intuition of Simmel’s thought consists of the universal interaction of all phenomena: thus, Simmel anticipates the complex view of the social systems. The subject of Simmel’s sociology is represented by the forms of the reciprocal influence relations among individuals: the society emerges as an effect of interaction among individuals only when many individuals get into a mutual interaction.

Simmel analyzes the social effects of the modernization of the cities of his time, referring to three main topics: the size of the city, the division of labour, the relation money-rationality. In the shifting from a small group to a big group, new forms of relations develop, where the individual feels more and more alone. The division of labour is at the basis of the fragmentation of social life which encourages individualism and selfishness. Money becomes the expression of metropolitan rationality: thus, it becomes something impersonal that makes any qualitative value disappear.

Simmel believed that the modern city – the metropolis – is the cause of an alienated life. In the metropolitan individual, the domains of the family and neighbourhood, typical of a small community, are substituted by the domain of the thousands of superficial contacts. The metropolitan individual lives in a world where images frantically follow one another, causing a decline of the ability to react to stimuli, and a dramatic weakening of spiritual activity, generating boredom and disillusion. Simmel calls the bored and cynical citizen of a metropolis the ‘blasé person’. The blasé person is forced to look for shelter in the gaps of associated life, where the search for the ‘elsewhere’ is substantiated and where the strict influence of the social context is absent.

The theorization that another important German sociologist, Max Weber, makes of the city in an essay published posthumously in 1921, the result of 30 years reflection, is partly linked to Simmel’s work of 1903, in fact he agrees with Weber on the complex approach to the social urban system. The main question that Weber asks himself is why in western countries we witness the development of citizenships that aim at realizing a home rule. To answer this question, Weber compares different types of urban models, starting from ancient cities and then going on to medieval ones both in the West and the East. He investigates these cities both from their relations with other cities and from the relations with those parts of the urban society seen as components of the social and political order.

In his analysis, Weber observes that nearly everywhere in ancient times, there was a form of capitalism controlled by politics, that he calls political capitalism. At a certain point in their history, and in Western countries this happened mostly during the Middle Ages, some cities passed to a form of market capitalism which was not subject to political power, that Weber calls rational capitalism, and which had never come about in ancient times. For Weber the medieval city is the ideal type of city. The importance of the medieval city, according to Weber, comes from commercial importance and not from a military one: it is a city where the inhabitants are governed by groups from the business world, not from the religious world; it is a city where authority is based on a rational choice and not on charisma.

Weber sees the city as a multidimensional object made up of interrelated parts which must be studied from several perspectives: sociological, economic, political and administrative. Like Simmel he criticizes cities like metropolis, as the paradigm of a dehumanized life where social relations are completely destroyed. He rejects Simmel’s idea that a city can be defined even by its size, and gives a conceptualization of it where public and private lives, as well as entrepreneurial and religious activities are integrated. According to Weber’s economic perspective, the city is the centre of commerce, considering that real cities represent a mix of the consumer’s city and the producer’s city, which can be distinguished by their main component.

Approximately in the same period, in America the Sociological School of Chicago was established and it took on an important role. Right from the end of the 1910s it reached the peak of originality and success in the scientific field.

Scientific contributions belonging to several disciplinary areas enriched these studies. Researchers used the theory of urban ecology, that proposed that cities be analyzed as systems, that nowadays we would call complex, just like they used to do for natural ecologic systems. A new element was established by the fact that cities were studied using the Darwinian evolutionary approach, that considered competition in the urban system as the origin of the selection of some features of the cities. This approach underlined the competition for space and for resources among different social or ethnic groups as well as the processes that led to the making of ecological niches of certain groups of the population. In fact it was observed that the groups of population

that became wealthy, moved to the suburbs, which then became privileged areas, whereas the urban centre which had been abandoned became an economic and social decay. Moreover, with this approach, they tried to solve the social problems of that time connected with crime, from juvenile crime to the homeless, which were seen as mechanical consequences (and this maybe constitutes the main limit of the analysis) of poverty and unemployment.

The scholars of the School of Chicago were the first ones to face the systematic study of the city from a sociological point of view through the direct empirical analysis of urban society. The school studied the inter-ethnical relations and crime in the big cities in the United States, in particular in Chicago, catalyzer of consistent immigration from Europe and from the south of the United States. Chicago was in fact a real social laboratory for researches of that kind, where they investigated causes and effects of social problems, provoked by massive changes that had rapidly turned a commercial city and port into a big industrial city².

The School of Chicago, adopting a systemic vision of society in which the bottom-up push, that is the interacting individuals, determines the dynamics, developed an optimistic vision of immigration, as it considered diversity, if supported by appropriate policies of integration, as a resource to enrich the society, and devised and supported an individual who becomes a hybrid: an individual that closely shares both the culture of the native country and that of the host country, even if he is never fully accepted in either of them.

Among the most important conclusions adopted by the sociological School of Chicago, all of them anticipating future positions not without difficulty worked out by other researchers, there is one that considers race-mixing, that derives from the heterogeneity of the population, is in fact an enrichment to society, not an impoverishment. Heterogeneity keeps the social system alive, it keeps it in continuous evolution and it makes it flexible, more easily suitable to unpredictable changes of the environment in which the said system is contained, than what it would be if it were homogeneous. A hybrid social system is more resilient compared to a social system of individuals similar to each other and less adaptable, elements which make this social system more fragile. A fundamental merit of the scholars of the School of Chicago is that they were the first to support and manage what in current language is defined as the complexity of social systems, now universally identified as a value.

As scholars have long observed the cities that have experienced the process of the growth of industry, marked by rational capitalism, in many cases have experienced the havoc wrought on the urban and architectural fabrics formed in previous centuries. Several scholars, have recently studied from different points of view the decline of some aspects of the quality of life during the industrial growth³. During the 1960s Jane Jacobs, an American journalist, strongly denounced in one of her most successful books the progressive degradation of the quality of life in the big American cities that was already clearly visible while she was writing her book. Jacobs maintained that the essence of the urban form is ‘organized complexity’ (Jacobs, 1961; see also Bertuglia and Vaio, 2009, 2011a, 2011b). The evolution of the city is considered a problem of organized complexity: cities are described through many variables, which continuously change value and are inextricably related to each other. Furthermore she claimed that the urban planning got bogged down in a fundamental incomprehension of the nature of said problem which was considered nothing but a mechanical issue, where the sciences of life had already developed the central idea of organized complexity which was what urban planning needed. Organized complexity, which we will be discussing in paragraph 4, in the next few years will be the core from which the new idea of assisted self-organization of social processes will develop, the idea which expresses that social processes must be left to evolve, subject to their internal bottom-up push, governed by precise rules, without being forced from the outside.

In this outline of urban theories of the last decades of the XX century, great attention must be focused on the concept of the city proposed by the French philosopher and sociologist Henri Lefebvre. In the 1960s and in the 1970s, Lefebvre drew up a theory of the city, referring to the problems connected to the use of urban space, and to the sense of space itself, outstanding for the originality of the systemic vision that anticipated many conceptions of complexity and system self-organization (on this topic, make reference to the works of 1968, 1972, 1974).

Lefebvre denounced, in those years, what he called «the misery of habitat» and the misery of the inhabitant of the city, subject to an everyday life mostly organized ‘from the top’ by a kind of power that hardly considered the systemic view and the push, that nowadays we would call self-organizing, bottom-up. Lefebvre maintained the thesis according to which the practices of urban planning and of architectural design, even if they were generally perceived as forms of positive rationality, they were in actual fact implied in the strategies of a domain imposed ‘from the top’. He claimed, in other words, that there was a relationship between analytical knowledge ascribable to the urban planning and to the architectural project and to the building of an urban space marked by practices of separation that determined exclusion. The human being, for Lefebvre, has anthropological needs, that were not taken into consideration by theories on the city and its organization, for example the need of imagination. Lefebvre thus expressed the need to affirm a new right, called «right to the city» intended as the right to a good quality of urban life, the right to be part of a collective self-organized process of civilization that mirrors itself in the urban space (Lefebvre, 1968, 1972).

In his famous book «La production de l’espace» (1974) Lefebvre proposes the idea of space as a product of the society: each society produces its own respective space: and it is in this space that we have the reproduction of social relations, the relations that make up the society as a complex system. According to Lefebvre space is a social product that contains elements very different from each other: natural and cultural, channels of exchange of materials and information. Space thus consists of a group of relations where the subject settles, by activating practices of production and consumption that adapt to the transformations which the space system falls into.

According to Lefebvre, space, a social product, consists of three elements: (i) the ‘perceived space’ (le perçu), made up of the spatial practices of everyday life and of the ways in which spaces are used; (ii) the ‘conceived space’ (le conçu), a system of signs intellectually elaborated by urban scientists and town planners, made of representations of space that include the produced designs and the envisaged places, as well as a material representation of those designs in the developed environment, that is the urban shape itself; (iii) the ‘lived space’ (le vécu), made up of the symbolic values of the inhabitants, which is connected to the artistic creativity and lives by images and symbols that become a tool for imagination. The three elements, le perçu, le conçu and le vécu, interacting and integrating themselves, express a continuity of the evolution of the public sphere, and offer a description of the different ways in which individuals interact in public space: an alternative description compared to the traditional and simplistic vision of public space. The three stages of this spatiality theorized by Lefebvre, constitute a space, so to say, anti-hegemonic for a critical dialogue.

He also maintained that as each society produces its own space, the city of the antique world cannot be considered as just an agglomerate of people and things collocated in one space: the intellectual climate of the city was closely related to the social production of its own space. Max Weber saw the medieval city as a space where merchants could work, a city for the bourgeois, where they could stipulate agreements and where political power exerted the law in all its forms, like the right to property. Lefebvre claims that the city of the XX century, with its public transport, health, housing systems for low income groups and state schools, has become the city of the organized workforce, that is the space that organizes workers and their political activity. Cities have become spaces that raise both a social force representing the urban footprint of the globalized capital, which modifies urban space in its materiality and transforms it into political space, and a political force that is a mix of the ‘socially disadvantaged’ which produces the politics through the materiality of the urban space.

Furthermore, in the frame of the concept of space defined as a social production, a fundamental element of Lefebvre’s thought is the idea that the nature of the urban space builds on the relation between two important concepts: the oeuvre, meant as usage value, and the product, meant as exchange value. The oeuvre is something unique created through a process that does not run out in itself, but that becomes enriched with creative contribution. The product is the result of repetitive moves, instead; it is reproducible, it is the outcome of a production process founded on work only. The difference between the two concepts is basically in the nature and in the forms of the production process and of the implied social relations. The oeuvre refers to an activity that involves the whole society horizontally. The product refers, instead, to processes structured hierarchically. The product is the result of intention, which goes from an abstract thought to a real object, which is its material translation, in a one-way path that implies a well defined relation between a predominant group, that elaborates and formulates the idea, and the dominated group, that accomplishes. According to Lefebvre, the creative capacity, on the contrary, is always referred to a community, to its social practice made of ideas, desires, different strategies; it is capable of expressing in a social, unique project, which extends over the urban space which citizens belong to, as elements of a system that involves them individually and that evolves through bottom-up pushes, self-organizing and producing the social creation: the oeuvre. The oeuvre is the result of a shared symbolic system, of a process of social sedimentation, of a kind of a diffused social creativity stream. Lefebvre considers the urban society, that experiences its best phase when it creates the oeuvre, as a system of individuals interacting with each other: the ‘beautiful’ city, the city-oeuvre, it is the result of the self-organization of the bottom-up social system of citizens.

According to Lefebvre, it is the quality of the relation between power and community that determines the result of the generative process of the urban space. When power does nothing but command, forcing the alienated community only to the fulfillment, thus breaking the relation between the totality of population and the process, the city does not become an ‘oeuvre’, but a product responding to market and to profit. The city-oeuvre is a city where the space exploits a special surplus, the one given by the style that characterizes it, that makes it beautiful and that makes it pleasant to live in. A typical example of city-oeuvre, for Lefebvre, is Venice, but the same can be said of other medieval and Renaissance Italian – and European too – cities-oeuvre (see e.g. Secchi, 2000, 2005; Le Galès, 2002; Bocchi and Ceruti, 2009; Bocchi and Peters, 2009). And it is from this surplus given by the style of the shared city-oeuvre, that the common affection to urban space generates; it is the general and shared contribute to its beauty that is at the basis of a common sense of civic belonging⁴. For centuries the city has been an equilibrium between oeuvre and product. This did not mean less conflict between who has the power and who is subject to it, but the recognition of a commonly shared battle field: the political fights between the common people and aristocracy took place in the cities, as they actually were at stake, too. In this sense, Lefebvre sees the city as a self-organization of individuals’ actions, both of common people and aristocracy, who are part of it. The breaking point of this urban history happened, according to Lefebvre, with industrialization. Industrialization generated an overwhelming process of urbanization, in a city that became product and not oeuvre: a «de-urbanizing and de-urbanized urbanization» (as Lefebvre calls it, 1968) that destroyed the traditional city, and broke the equilibrium between oeuvre and product, subduing the first one to the second one.

3. City networks and global cities

The current interest for urban realities as a global phenomenon comes about and develops in the context of the analysis on globalization. If it is true that the big urban centre has always had to a certain degree, the role of political and economic actor, sometimes even autonomously from nation-states, it is also true that in the last two decades, with the end of the world bipolar order and the birth of a new form of worldwide governance, a new key to understand the contemporary urban phenomenon seen on a world-wide scale is defined: the one contained in the concept of global city. Scholars give different interpretations to this new concept, especially with regard to historic evolution, but all of them recognize that, on the basis of the current transnational network of non-material communication of information and capital, the globalization both of the economies and of the urban phenomenon itself is produced: the city virtually expanding beyond its geographical and historic boundaries, breaking with the link to the territory where it was born, and getting closer to other cities, even the geographically distant ones.

The English sociologist Anthony King (1990a, 1990b, ed. 1996) is one of the first scholars to use the expression ‘global city’⁵, attributing to urban centers a pivotal role related to the new distribution of information and of money flows in the global economy. According to King, global cities are the basis of the big banks and of multinational communications: from this basis, the network of electronic and informative communications develops, and through them the worldwide economy fundamental decisions are transmitted, and capitals are transferred. King states that global cities are not a direct consequence of the transnationalization of the economy, but a result of colonialism. Those that today are global cities, were, in the past, imperial cities, like Paris, London, and even with less importance, Madrid and Lisbon. They were hubs of colonial empires, crossroads for commerce, money and information. The global cities include also the cities that King calls colonial, struggles for a different type of globalization. Even though such organizations are not necessarily urban, for their nature and orientation, the distribution of their operations is present in a big number of cities (Sassen, 1994).

Catalan sociologist Manuel Castells’ analysis of the phenomenon of global cities is closely linked to Sassen’s elaboration; he develops his own reflection on global cities from the network society. According to Castells (1989, 1996-1998; see also Borja and Castells, 1997), the city is global because of its role of knot in the global network of information flows. The city system has been much influenced by the fast globalization and by the shift of advanced economies from the production of goods to the production, management and communication of information; a transformation that Castells (1996-1998) recognizes to be as relevant as the shift from the agricultural economy to the industrial one, in the XVIII and XIX century. In the developed countries, a wide majority of jobs is in services, in particular those connected to information, according to the growing figures over the last decades. This process has increased the relevance of the cities set on a higher hierarchical level, the world cities, even aggregating them in a whole global city, an urban knot, at different levels and with different functions, spreading over the planet and functioning as neuralgic center of the new economy: an interactive system where companies and cities have to adapt continuously. The changeable relations with this network determine the destiny of the network cities and, of course, of their inhabitants (Castells, ed. 2004).

Thus, Castells extends Sassens’ view: according to Castells there is a global urban system, expressing a single reticular global city. In this framework, the city loses its dimension as a place, with its history, and its socio-cultural and geographical features. Yet, Castells does not believe, that these features are all dissolved in the urban global system; he admits in fact the existence of a tension between the globalism of the spaces of flows and the physical size of the city. The space of flows is taking on greater relevance; this establishes an electronic connection among physically separated places and creates a network of relations between activities and individuals, compared to the physical space that organizes experiences in the limits of a geographical location. Cities are at the same time structured and deconstructed by these two logics. The metropolis does not disappear in virtual networks, but modifies itself through the interaction between electronic communication and physical relations (Castells, ed. 2004). This tension makes cities like political arenas at a global level, as a consequence of the action of new social actors who claim new rights.

According to Castells, the internationalization of big cities, reflects, in institutional terms, in the active involvement of urban governments and of the main actors of the local development to international life. This happens, for example, participating in associations of cities, joining networks, developing city marketing and attending at international events. The transnationalization of economic and cognitive flows, along with the cut of nation-state power, is re-designing a new global political geography, where what dominates the scenario is the urban system whose knots are organized according to the economy needs.

For the first time in history, we have cities, global cities, which are the product of a global economy and live independently from their territory. Some of these global cities, do not even have their own territory or their own market, like Singapore and Hong Kong, before being integrated into the Chinese Popular Republic; other global cities are located in poor, developing countries, like Kuala Lumpur and Mexico City.

4. Complexity in the urban context, the assisted self-organization

For some decades, the view on the complexity has permeated natural sciences and social sciences; among the latter, urban and regional sciences too (a few examples: Pumain, Sanders and Saint-Julien, 1989; Donato and Lucchi Basili, 1996; Bertuglia and Vaio, eds. 1997; Nijkamp and Reggiani, 1998; Bertuglia and Staricco, 2000; Bertuglia and Vaio, 2003, 2005, 2009, 2011a, 2011b, 2011c; Batty, 2005, 2008; Reggiani and Nijkamp, eds. 2006; Lane et al., eds. 2009). In this approach, it’s shown that unbalanced systems, as they are subject to continuous exchange flows with the environment and characterized by non-linear interactions between the elements composing them, are able to self-organize and they generate unpredictable and unexpected phenomena endogenously, called emerging phenomena. The positive feedback, in general, does not bring the system back to the state of the previously abandoned equilibrium, and plays a determining role in the birth of new structures and new equilibriums, not computable a priori in a reductionist view. The positive feedback, in general, does not bring back the system to a state of previously abandoned equilibrium, and plays a determining role in the emerging of new structures and new states of equilibrium, not foreseeable in a reductionist view. There are a number of self-organization phenomena: from the vortex in a fluid, in particular conditions, to the flight of starlings when, without any central coordination imposed by a single leader, and just observing the closest individuals behaviour, they perform coordinated evolutions of the flock, which is a typical example of an emerging phenomenon in a complex self-organizing system.

There have been developments in the theory of self-organization in various disciplinary areas, especially in those concerned with the study of society, and the role that cognitive processes play in the economic and social field, that is the effect produced by all the single events that lead to formation and knowledge. The acquisition of knowledge in a person, actually takes place through manifold cultural and intellectual paths linked to one other, and determined, among the other things, by the particular social communities which the individual belongs to and by the experiences he lives through. So, growing attention has been focused on the study of the conditions under which the birth of new structures as a result of local non linear interactions among adaptive individuals, that is individuals who are both able to decide according to fixed rules and to understand and modify (adapt) their own behaviour after the interaction with other individuals and with the environment.

Among the number of fields where the new idea of complexity, in particular emergency and self-organization, mainly recall the attention of scholars, the urban context is getting higher relevance, especially concerning the problems about the city management as a complex system. One of the major problems in big urban areas over the last decades, for example, is the unsatisfactory quality of life. In Italy it has taken on a particular connotation, connected not only to sociological aspects, but also to the discomfort for the loss of beauty of the city, as well as the landscape in general (Settis, 2010), painfully felt in many cities⁶. In Italy this is due not only to the industrial development that characterized a lot of cities, in Europe and North America, particularly until the 1970s, but also, and sometimes above all, after the damage caused by decades of negligence of the urban and rural landscape (Bianchetti, 2011). The complaint is about excessive building compared to the demand, frequently realized without observing the rules, sometimes without control because of the lack of urban plans or because of plans that allowed too much under different types of pressure, and even unauthorized building against existing plans and laws. And this happened despite some significant efforts and sometimes legal acts regulating the market of urban areas⁷.

The problem of expropriation of building areas, was long discussed also while approving the law 167 of 1962, proposed by the senator Camillo Ripamonti, law that Sullo had contributed to pass, as Guido Bodrato recently observed (private communication): «He had realized in a small way what Sullo had thought in a big way». The law 167, in the effort to avoid the rise of prices of the areas destined to popular housing, identified by the town plans, or by projects of production, fixed the price of the areas or the expropriation compensation at the value of the two years before the council decision of the adoption of the plan. The law 167, however, was not refinanced. The new urban law seemed to be realized in 1977 by the minister Pietro Bucalossi. The law Bucalossi ratified the separation between ius aedificandi and property rights. At the beginning of 1980, however, the first of a series of decisions taken by the Constitutional Court cancelled both the innovative contents of the law Bucalossi and the rules of 1971 that facilitated the appeal to expropriation for public utility. The efforts for urban reform failed, and, in the 1980s, the long season of what Vezio De Lucia calls «the urban counter-reformation» (1989, p. 167) started.

The essential news introduced by the view of complexity in urban context, is that complexity, underlining the importance of endogenous emergency of new structures as a result of non-linear local interactions among adaptive individuals that act in a complex system, as the city is, stresses the importance of the phenomenon of bottom-up self-organization. It indicates which way such a phenomenon must be considered by the actors who have the task of operating in, and sometimes on, the urban system, assist and influence its evolution, in order to pursue one or more goals, like a high quality of life.

To this regard, the following points must be highlighted:

• firstly, the so-called actors have to learn how to cooperate with spontaneous processes of organization, commonly indicated as processes of self-organization (representing one of the most significant expressions of the view of complexity) through acts that allow the spontaneous processes the chance to originate and to fully act;

• secondly, the so-called actors, have to learn to assist the spontaneous processes of organization, step by step, compensating for that lack of effectiveness and communication that occurs, in particular, in the urban system that developed and grew very fast;

• finally, the actors mentioned above, always respecting and spurring the capacity of autonomous adaptation of the urban system, can and must, when and if necessary, introduce organizational suggestions, alien to the urban system itself.

As a consequence, it is possible to talk about assisted self-organization; it has to be done. This new concept introduced here, points out that the above mentioned actors can be indicated as actors of assistance to the urban system, they have to try to introduce a principle of order affected by a rational external supervision in the urban system. Such a supervision has to operate in the respect and the compliance of the capacity of autonomous adaptation of the urban system (see: Bertuglia and Vaio, 2011b, 2011c).

The persistence on this last point, highlights how and how much the practice of the assisted self-organization differentiates from the traditional planning practice (where traditional briefly indicates the trend of the historically determined practice, which does not imply any specific judgment of merit different from the one that immediately follows). The former practice contrasts the sometimes too strong dirigisme of the latter with the attention, compliance, respect, education, help, in one word the assistance that it brings along and that make assisted self-organization more appropriate, even in very different situations, for institution and culture. Examples where bottom-up self-organization of citizens generated ‘beautiful’ cities, as we said, are frequent in medieval cities and in particular, Italian cities like Siena⁸, Venice, Florence and Gubbio (besides Weber and Lefebvre already mentioned in paragraph 2, see: Benevolo, 1993; Secchi, 2000, 2005; Le Galès, 2002; De Seta, 2010; Romano, 2010).

While a difference between the practice of assisted self-organization, the peculiarity of which derives directly from the perspective of complexity, and the practice of traditional planning is emphasized, it is recommended to consider that the identified difference, probably, is not marked enough to exclude possible synergies between the two practices, in particular between the practice of assisted self-organization and the version of the practice of traditional planning, defined as strategic planning, which was generated in the late 1900s while attempting to face the problem of de-industrialization; in this attempt, it aimed to be the vehicle to make citizens’ ideas come out, and conveying them towards shared objectives (Pugliese and Spaziante, eds. 2003; Bianchetti, 2011). This latter practice carries the experience made in a challenging attempt. The practice of assisted self-organization carries, as we have strongly stressed above, the attention to let processes of spontaneous organization of actors develop (Lefebvre, 1968, 1972; Benevolo, 1993, edited by Erbani 2011; Le Galès, 2002; Campos Venuti, edited by Oliva 2010; De Seta, 2010; Romano, 2010).

The difficult situation of the Italian urban areas, which grew quickly and were often not controlled enough, subject to the weight of the absolute yield⁹, has long been signaled (among others: Benevolo, 1963, 1993, edited by Erbani 2011; De Lucia, 1989; Calafati, 2009; Campos Venuti, edited by Oliva 2010; Settis, 2010). In the disappointing general picture of Italian town planning, there are, however, some cases of careful control of urban development, carried out on non-dirigistic basis, through town plans designed to discourage the action of private speculation.

These were the town plans of the 1970s, of Bologna, Brescia, Como, Ferrara, Matera, Modena, Venice.

During the 1980s, the trend to neglect rules in the urban field, started to spread, as it was wrongly considered that by disregarding rules room might be given to invention and creativity, with awful results. Actually, rules and the strict respect of them challenge and stimulate invention and creativity; strict rules also improve the processes of self-organization, e.g. those of assisted self-organization (called planned organization). In fact, the town plan of Brescia, drawn up by Leonardo Benevolo in 1970, until that time professor at the University of Rome, was an emblematic example of a town plan founded on the view of the city seen as a complex system, as we would say today, based on the distinction and the management of bottom-up forces, paving the way to the conditions for the self-fulfillment of its residents’ needs. This implied the break of the alliance between house builders and area marketers, and encouraged the alliance between house builders and users (Benevolo, edited by Erbani 2011). The central idea of the town plan of Brescia, was not choosing a model, juxtaposing a model to other models. Instead, it encouraged citizens (the urban agents) to design a civic and integrated environment, suitable to the needs of individuals, families, of other urban agents and of the society, starting from a leading interest: the quality of life. The pursued idea, was that the town planner should accompany the growth of the city, watching it with a global and unitary glance, granted by the public help, letting spontaneous bottom-up forces act; they have to be properly identified, without imposing top-down instructions, but limiting the intervention to the definition of shared rules (Benevolo, edited by Erbani 2011). So, letting the urban system evolve spontaneously, in a form of assisted self-organization. Considering this, in the town plan of Brescia, the choice was, unlike the trend going on in Italy, of keeping the limit of new building low, giving the public Administration the responsibility of buying the degraded buildings of the old town centre. These were restructured with public financing, and then resold at controlled prices to private people that went to live there. It was also planned that the new buildings would be completed in peripheral areas, purchased by the Municipality at a market price, urbanized with public funds and resold, after urbanization, at a controlled price for a number of new buildings, based upon a realistic perspective of the population growth.

Another example of bottom-up self-organization process of the urban systems, has taken place in some areas of the deteriorated historical centers of big and medium Italian cities, where in the last three or four decades, there has been a progressive recovery of such areas, from the degradation they had fallen into.

It is the so called gentrification¹⁰ of the historical centers, a social and economic spontaneous phenomenon, originated from the single decisions of single agents, whose choices influence those of other agents, in a typical form of social bottom-up self-organization.

Gentrification is widespread in many European and North-American cities. Its causes are manifold and can be put down to, for example, demographic reasons, cultural reasons connected to the changes of tastes and styles, political-economic causes referring to the excessive price or unavailability of the houses in the suburbs, reasons referred to the social networks of the community that form around leading figures such as artists and showmen (Palen and London, 1984).

In Italy there are many cases of gentrification, recent or in progress, in several cities: for example Milan, Genoa and Turin (Diappi, ed. 2009).

In Milan gentrification is not a recent phenomenon. The protests of evicted tenants from Corso Garibaldi to leave room for well-off families date back to the 1960s, for example. At the beginning, gentrification interested the historic neighbourhood around the Navigli area, in the framework of a general tertiarization process started in the post-war period. In the 1970s and in the 1980s, a part of the demand for prestigious houses turned more and more to the city centre, harming as a consequence the poor people. In the 1980s the quarter of Brera, transformed a students and artists meeting place into an exclusive quarter, reserved to a social and economic elite. In the 1990s, the phenomenon then invested the less central quarters of the expansion ring of the XIX century, like Ticinese, Lazzaretto and Corso Como (Diappi, Bolchi and Gaeta, 2009).

In the years after 2000, gentrification of the central areas of Milan, continued to expand, sustained by the growth of the property value and by the development of the advanced tertiary. The case of the quarter Isola set in the city’s northern area, not far from the surrounding wall of Bastioni, is significant (Diappi, Bolchi and Gaeta, 2009).

Gentrification in the historical center of Genoa (Gastaldi, 2003, 2009), one of the largest in Europe, started in the early 1990s, when university students coming from other cities were the pioneers of a social change, as many new residents chose to live in this area, attracted by its cultural values and its centrality. Until then, the relocation to the city centre of Genoa had been only a secondary choice for poor categories or for immigrants. The turnaround consolidated when upper middle class young couples and individuals of a good cultural level moved from the prestigious neighbourhoods of the city to the historical center, settling in on the top floors or penthouses of the big central buildings that in the past centuries had been built by – and had belonged to – the wealthy economic aristocracy or the noble class. At the same time, other new residents with a lower income (students, young couples, single with a lower income, craftsman-artists, innovative freelancers, young atypical workers) settled in less prestigious houses, which ensured the advantages of the central position, of the shorter commuting distance and of the possibility to share social and professional relations.

Gentrification in Turin, unlike the two cases mentioned above, can be considered at a really advanced stage (Semi, 2004; Curto et al., 2009). It generated in the framework of a real estate market characterized by a high territorial dynamism, shown in the number of transactions, compared to the housing stock, higher than in other big Italian cities, and with lower average prices, with a ratio price vs. architectural, environmental and building quality, favourable to buyers.

The 1995’s town plan gave back the city of Turin a dynamism that became tangible in territorial as well as cultural transformations (Curto et al., 2009). The plan concerned the recovery and the transformation of more than two million square meters of disused industrial areas. The destination to public-housing of a major part of these areas produced changes both on the plan of the housing model and on the one of the territorial mobility. These territorial changes also invested (but not only) the central area denominated «Roman square»; this area experienced a process that involved citizens and the way they use the historical center, with clear effects at an economic, social and territorial level. The area around the market of Porta Palazzo and that includes the heart of the Roman Turin, was perceived for decades as destroyed on the social, economic, environmental and building plan. It experienced a radical process of gentrification, which is close to its end, with perspectives to be analyzed with regard to the future evolution.

5. The agent-based modeling

Starting from the 1970s, developments on the theory of self-organization have been introduced, concerning the systems of chemistry and of physics (Haken, 1977, 1984, 2004; Nicolis and Prigogine, 1977, 1989) and referring to the biological organization and to the role that cognitive processes have in theoretical economy and natural sciences (Anderson, Arrow and Pines, eds. 1988; Arthur, Durlauf and Lane, eds. 1997; Rizzello and Egidi, eds. 2004; Rosser jr. and Cramer jr., eds. 2004; Lane et al., eds. 2009; Lane and Terna, eds. 2010).

The peculiarity of the systems studied by social sciences, compared to the physical systems, is that the first involve intelligent agents, generally in a far more reduced number compared to the second ones, that are capable of adapting and innovating, instead. For the social systems, the study according to the interdisciplinary approach of complexity focused on the investigation of their capacity of adapting and innovating in contexts of uncertainty and of rules that change as regards social interactions. This is true for urban systems too (Batty and Torrens, 2001; Batty, 2005, 2008; Castle and Crooks, 2006).

For the social scientists engaged in the study of complex systems, the main challenge is to establish explicit links between the attributes of the single actors (for example, learning, invention and adaptation) and the occurrence of unexpected and unpredictable properties of the system (emergent properties) that appear in the global dynamics of the system, made of actors interacting among them, following non-linear forms. The central point of the challenge for modeling is planning the model and tuning it so as it is methodologically parsimonious (that is, avoiding to involve too many hypotheses and articulations) and that is capable of relating, more efficiently than the traditional models, the micro level (referred to the behaviour of single agents) and the macro level (referred to the global system and to the phenomena that are observed in it). As a consequence, it has to be an effective instrument to investigate the theory and to improve the correspondence with the facts observed (Squazzoni, 2012).

In particular, for urban systems the agreement depends on the way the transformations of the system are expressed at different levels: the problem is how the interactions influence the speed and intensity of the spatial interactions, both on an intra-urban scale and on the inter-urban scale of the systems (networks) of the city.

Social sciences propose to understand not only the behaviour of individuals, but also the way the interactions among many individuals lead to results on a large scale. To understand a social or economic system we must know how individuals act and how this determines that the resulting global behaviour is more than the sum of the behaviours of the single individuals. When interactions among agents (individuals, families, companies, social groups, communities, governmental authorities, institutions, etc.) depend on the experience of the single agent and especially when agents continuously adapt their own behaviour to their experience, the effectiveness of the traditional mathematical analysis in getting the dynamic consequences is extremely limited. In such cases, agent-based modeling may be the only operating methodology of empirical analysis of the system behaviour in a sort of virtual laboratory.

A model of this kind starts from the definitions of the assumption on agents and their interactions and, with the use of computers, generates evolutions of the system over time, carried out under controlled conditions, which can reveal the consequences of initial assumptions. In such a way, the researcher can investigate if and how effects on a large scale emerge from individual micro-processes of interactions among different agents.

A basic feature of agent-based modeling is that the tester that performs the simulation can attribute to them some individual features, both for the behaviour rules and for the preferences and also to keep the results of learning that they take from individual experiences differentiated among the single agents. This is particularly evident for the social sciences, where it is basically wrong, because not corresponding to facts, failing to consider the diversity of agents and making homogeneous groups of agents: that is a hypothetical and unrealistic, average representative agent¹¹. The variety among agents is at the origin of the self-organizational global and emerging phenomenon that, in general, would not appear if all the agents were equal. Considering abstract agents the same as an average agent, is an approach that might work in the description of global phenomena of physics, where agents can differ from each other only for the value of the variables that characterize them¹², but it works less or not at all if the elements of the considered system, the agents, can differ from one another, not only in the value of the variables but also in the behaviour rules. The idea of average agent is traditionally used to transform large and patchy populations in a small block of homogeneous individuals, easier to describe with equations. In many situations, however, it happens that individual differences do not offset one another in the aggregate, so that the representative agent will badly represent the associated consequences of micro-foundations (see e.g. Farjoun and Machover, 1989; Aoki, 2002).

The so called agent-based models represent effective instruments to respond to the challenge of complexity. Through the definition of rules at an individual level, such models can reproduce the traffic of information among social actors (agents), considered different from each other, provided with cognitive skills and capable of making individual choices. The models based on the agent-based approach, then, generate something that deals with all the possible dynamics of the urban system, starting from the decisions and the behaviours of the single individuals that compose it. Such models are easier (in the idea and the realization), more flexible (in the utilization) and more effective (in the results that they produce as regards the modeling of social systems) than the models based on differential or finite-difference equations. Agent-based models can stimulate the formation (emergence) of general structures of various kinds in the system, unpredictable with the traditional calculation, that can be then analyzed with the use of statistics.

Such models are a powerful instrument of knowledge, of education and of training for the decision makers of urban systems, because they allow the creation of artificial urban systems, useful to carry out an empirical study and, more generally, to achieve the development of a sort of laboratory of human and socio-economic geography (see for example: Gilbert, Hawksworth and Sweeney, 2008¹³).

6. Conclusion

Given what was discussed in the previous paragraphs, the authors consider it useful to insist on the possibility that the UN, in the sphere of the extraordinary initiative that it has taken on, promotes experiments and actions moving along the general line of assisted self-organization in the management of the urban phenomenon on a global scale; moving from typical existing urban situations (in particular the most degraded). This has to be carried out by accompanying, assisting, never forcing the actors of these situations, that move bottom-up in the autonomous creation of new urban organizations, determined by the needs for freedom and coexistence, in the sole respect of the minimum standards concerning the functions that are carried out at the house and neighbourhood scales.

New urban organizations will follow, as mentioned above, with new elements that will be stimulated, because of the limits coming from the bottom, by the criteria of sustainability. So, urban organizations conditioned by original cultural situations, tied to the criteria of sustainability, inspired by the needs of freedom and coexistence.

An important experimentation may derive from this, capable of moving large masses of individuals, able to create an unprecedented reflection on the theme. The hope is also to form many managers, at different technical levels, and to produce rich and articulated ideas and suggestions. In conclusion, creating a cultural movement on the city that might be the beginning of huge changes all over the world.

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Thomas W.I., Znaniecki F.W. (1918-1920) The Polish Peasant in Europe and America. Monograph of an Immigrant Group: Volume 1 (1918) Primary-group Organization, University of Chicago Press, Chicago; Volume 2 (1918) Primary-group Organization, University of Chicago Press, Chicago; Volume 3 (1919) Life Record of an Immigrant, Badger, Boston; Volume 4 (1920) Disorganization and Reorganization in Poland, Badger, Boston; Volume 5 (1920) Organization and Disorganization in America, Badger, Boston.

Tosco C. (2011) Petrarca: paesaggi, città, architetture, Quodlibet Studio, Macerata.

Weber M. (1921) Die Stadt. Eine soziologische Untersuchung, Archiv für Social wissenschaft und Sozialpolitik, 47, 621-772.

Wilson A.G. (1974) Urban and Regional Models in Geography and Planning, Wiley, London.

Wilson A.G. (1981) Catastrophe Theory and Bifurcation, Croom Helm, London.

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Notes

1. One of the authors of this text, the older one, does not forget that the Aldo Della Rocca Foundation, on the initiative of Luigi Moretti and Bruno de Finetti, respectively president and vice president of IRMOU – Institute of Mathematical and Operative Research for Town Planning offered him the chance, when he was still a student, of starting the research that led him to write the two works cited in the references as published in 1964 and 1965.

2. Among the most emblematic studies, the research by William I. Thomas, professor of Sociology ay the University of Chicago in Chicago and of the Polish sociologist Florian W. Znaniecki is pointed out, who between 1918 and 1920 published The Polish Paesant in Europe and America. Monograph of an Immigrant Group, a thorough study in 5 volumes on Polish immigration and on the problems about the integration of this population in the American society (see e.g. Bertuglia and Vaio, 2011b).

3. This does not mean that the quality of life has declined in the city: that is both because some aspects of the quality of life in many cities (both American and European) have notably improved in the years after Jane Jacob was writing, and because we do not have yet, and it is not sure we will, at least soon, a theory and a strong and convincing method to measure it.

4. This of course does not mean that the city-oeuvre cannot be the market-city too and that some of its parts cannot respond to the logic of products. Oeuvre and product, in the urban space, are not alternative and do not charge with a judgment of value, since it is the nature of their relation and their balance that is important.

5. The idea of global city, a transnational city composed of cities connected through immaterial networks, was introduced for the first time, some years earlier by the American town planner John Friedmann in his work of 1986 «The World City Hypothesis». The idea of important cities on a global scale, meant as knots of commercial networks that link them, is different; it is older and it was first expressed by the Scottish town planner Patrick Geddes (1915).

6. We can’t exempt from observing how the topic of the beauty of the Italian landscape is not new and was repeatedly faced in different fields in the past centuries, as the notable literary reports of travels (grand tours) by Goethe, Byron, and many other artists, literates, aristocrats, or European well-off bourgeois, that could afford a direct knowledge of Italy. To mention a famous and previous example of traveler, Petrarca, today considered as an essential reference to the birth of the idea of landscape, the poet that first revealed the aesthetic dimension of nature. Petrarca, moving from geography to architecture, to archeology and to literature, mixing the theoretical knowledge with the experiences on the field and even remaining a man of his time, he seems to prefigure, in the XIV century, the ideal of an aesthetic perception of nature and of the role carried out by the human intervention. The poet was fascinated by the housing spaces of cities, not only Italian, but in most part of Europe, and by monumental architecture, both classical and contemporary to his time, even if nothing will compare to the evocative power that the vision of the ruins of the Roman Empire provoked inside him (Tosco, 2011).

7. First of all, the effort, brave but fruitless, constituted by the Bill of reform of the regime of soil proposed in 1963 by Fiorentino Sullo, that provided the chance of expropriation for all the areas destined to housing, and the following auction sale of the sole building right, and not also of the area property (see Bertuglia and Vaio, 2011b). The problem of expropriation of building areas, was long discussed also while approving the law 167 of 1962, proposed by the senator Camillo Ripamonti, law that Sullo had contributed to pass, as Guido Bodrato recently observed (private communication): «He had realized in a small way what Sullo had thought in a big way». The law 167, in the effort to avoid the rise of prices of the areas destined to popular housing, identified by the town plans, or by projects of production, fixed the price of the areas or the expropriation compensation at the value of the two years before the council decision of the adoption of the plan. The law 167, however, was not refinanced. The new urban law seemed to be realized in 1977 by the minister Pietro Bucalossi. The law Bucalossi ratified the separation between ius aedificandi and property rights. At the beginning of 1980, however, the first of a series of decisions taken by the Constitutional Court cancelled both the innovative contents of the law Bucalossi and the rules of 1971 that facilitated the appeal to expropriation for public utility. The efforts for urban reform failed, and, in the 1980s, the long season of what Vezio De Lucia calls «the urban counter-reformation» (1989, p. 167) started.

8. In some pages of «La città del ventesimo secolo» (2005), Bernando Secchi gives a vibrant description of the experience of open space in Siena, described as «the medieval city par excellence» (p. 54) and mirror of a society that actively takes part in the decision-making process, underlining the meaning of open space as a public space that belongs to the common experience (the reference is to Piazza del Campo). «The ones who tried to observe the city square, to observe how during the different seasons shadow and sun move and how its different parts are attended, the ones who tried to seat on the pavements of this square, kept out of the wind, enjoying the soft warmness of bricks, the ones who observed the way the simple pattern of the pavements makes the floating of water during the rainy days easier; the ways in which the same pattern suggests its way of usage without imposing them, can’t but agree that it is especially the big comfort of this space that belongs to common experience and makes it lovable» (our translation). Secchi also considers «what defines the urban space and the territory of Siena is the close relationship established and consolidated in time, between the form and the territory of the city and the role and function performed by each one of the elements which constitute it» (p. 60, our translation). Siena as other medieval cities, Secchi argues, has developed through a continuous individual interpretation of the same structure in the common space, whereas the modern city, on the contrary, has developed trying to provide a collective interpretation of individual needs.

9. It is necessary to notice that the absolute yield is a pathological phenomenon of the market of areas and of housing, and so it is useful to pursuit its elimination. It can be generated by the silent collusion of the owners of building lots or that can be made such, set at the limits of the urban built area: a restrictive policy of the offer of areas that generates from the behaviour that the offer operators, who are almost in the same situation, are led to put on almost without having the need to agree. Thus, the absolute yield generates in the suburbs, but spreads all over the city, adding to the differential yield. As a consequence, it affects the whole city, and the pursuit of its elimination is useful for the entire city, for all the activities set in it and for the whole urban population (see: Bertuglia, 1964, 1965).

10. The word gentrification indicates the progressive socio-cultural changing resulting from an urban area, usually set in the old town, when a number of individuals of a wealthy social class buy and upgrade real estate properties, in those areas generally populated by a community of low income individuals. The consequence of gentrification is that rentals and house prices grow, starting from the ones set close to the upgraded areas, the cost of life increases, the average income of the interested area grows. This causes, along with other socio-economic and demographic phenomena, the expulsion of the previous inhabitants at a low income. The process of gentrification triggers when a certain number of individuals chooses to move in some areas, as the historical center, previously abandoned by the wealthy social classes, and that are in a state of degradation, rediscovering their attractiveness. The growing number of wealthy residents in the area that is undergoing requalification attracts the residents of other wealthy areas, reinforcing the mechanism of gentrification with a positive feedback effect. The process is similar to the process of segregation described by Schelling in 1971 (see also: Schelling, 1978; Bertuglia and Vaio, 2009, 2011a, 2011b, 2011c), but in this case segregation bases not on the colour of the skin but on census. Given the nature of the new settled social class, the quarter upgrades from the urban point of view, the industrial use of soil reduces or disappears and the settlement of commercial activities and social services to wealthy classes is favoured. Maybe, it is not useless to point out that gentrification, more generally self-organization, even though it is a bottom-up process, from single individuals, which in itself might represent a value, is not a process that necessarily has neither a social nor a political sign. If it has it, it is not a given sign, not even given once for all: the assisted self-organization can have a social and political sign, and it is the sign of the authority which provides assistance.

11. A close effort of building an artificial average agent was already made in 1835, with scarce results, by Adolphe Quetelet, the father of social physics of XIX century (see e.g. Bertuglia and Vaio, 2003, 2005, 2011a, 2011b).

12. For example, the model of ideal gas works well in the case of the description of the macroscopic behaviour of a rarefied gas at a high temperature, when considering the trial to reproduce, by a model, the values and the statistical distributions of empirically observed macroscopic quantity, which find, in this way, a suitable reason at the micro level.

13. The model of Gilbert, Hawksworth and Sweeney is described in detail also in Bertuglia and Vaio (2011b).

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