3,918 research outputs found
Detecting and Describing Dynamic Equilibria in Adaptive Networks
We review modeling attempts for the paradigmatic contact process (or SIS
model) on adaptive networks. Elaborating on one particular proposed mechanism
of topology change (rewiring) and its mean field analysis, we obtain a
coarse-grained view of coevolving network topology in the stationary active
phase of the system. Introducing an alternative framework applicable to a wide
class of adaptive networks, active stationary states are detected, and an
extended description of the resulting steady-state statistics is given for
three different rewiring schemes. We find that slight modifications of the
standard rewiring rule can result in either minuscule or drastic change of
steady-state network topologies.Comment: 14 pages, 10 figures; typo in the third of Eqs. (1) correcte
Complex networks analysis in socioeconomic models
This chapter aims at reviewing complex networks models and methods that were
either developed for or applied to socioeconomic issues, and pertinent to the
theme of New Economic Geography. After an introduction to the foundations of
the field of complex networks, the present summary adds insights on the
statistical mechanical approach, and on the most relevant computational aspects
for the treatment of these systems. As the most frequently used model for
interacting agent-based systems, a brief description of the statistical
mechanics of the classical Ising model on regular lattices, together with
recent extensions of the same model on small-world Watts-Strogatz and
scale-free Albert-Barabasi complex networks is included. Other sections of the
chapter are devoted to applications of complex networks to economics, finance,
spreading of innovations, and regional trade and developments. The chapter also
reviews results involving applications of complex networks to other relevant
socioeconomic issues, including results for opinion and citation networks.
Finally, some avenues for future research are introduced before summarizing the
main conclusions of the chapter.Comment: 39 pages, 185 references, (not final version of) a chapter prepared
for Complexity and Geographical Economics - Topics and Tools, P.
Commendatore, S.S. Kayam and I. Kubin Eds. (Springer, to be published
Dynamics and steady-state properties of adaptive networks
Tese de doutoramento, FÃsica, Universidade de Lisboa, Faculdade de Ciências, 2013Collective phenomena often arise through structured interactions among a system's
constituents. In the subclass of adaptive networks, the interaction structure
coevolves with the dynamics it supports, yielding a feedback loop that is common
in a variety of complex systems. To understand and steer such systems, modeling
their asymptotic regimes is an essential prerequisite. In the particular case of a
dynamic equilibrium, each node in the adaptive network experiences a perpetual
change in connections and state, while a comprehensive set of measures characterizing
the node ensemble are stationary. Furthermore, the dynamic equilibria
of a wide class of adaptive networks appear to be unique, as their characteristic
measures are insensitive to initial conditions in both state and topology.
This work focuses on dynamic equilibria in adaptive networks, and while it does
so in the context of two paradigmatic coevolutionary processes, obtained results
easily generalize to other dynamics. In the rst part, a low-dimensional framework
is elaborated on using the adaptive contact process. A tentative description
of the phase diagram and the steady state is obtained, and a parameter region
identi ed where asymmetric microscopic dynamics yield a symmetry between node
subensembles. This symmetry is accounted for by novel recurrence relations, which
predict it for a wide range of adaptive networks. Furthermore, stationary nodeensemble
distributions are analytically generated by these relations from one free
parameter.
Secondly, another analytic framework is put forward that detects and describes
dynamic equilibria, while assigning to them general properties that must hold
for a variety of adaptive networks. Modeling a single node's evolution in state
and connections as a random walk, the ergodic properties of the network process
are used to extract node-ensemble statistics from the node's long-term behavior.
These statistical measures are composed of a variety of stationary distributions
that are related to one another through simple transformations. Applying this
fully self-su cient framework, the dynamic equilibria of three di erent
avors of
the adaptive contact process are subsequently described and compared.
Lastly, an asymmetric variant of the coevolutionary voter model is motivated and
proposed, and as for the adaptive contact process, a low-dimensional description
is given. In a parameter region where a dynamic equilibrium lets the in nite
system display perpetual dynamics, this description can be further reduced to a
one-dimensional random walk. For nite system sizes, this allows to analytically
characterize longevity of the dynamic equilibrium, with results being compared to
the symmetric variant of the process. A nontrivial parameter combination is identi
ed for which, in the low-dimensional description of the process, the asymmetric
coevolutionary model emulates symmetric voter dynamics without topological coevolution.
This emerging symmetry is partially con rmed for the full system and
subsequently elaborated on. Slightly varying the original asymmetric model, an
additional asymptotic regime is shown to occur that coexists with all others and
complicates system description.A estrutura das interacções entre os constituintes elementares de um sistema está
frequentemente na origem de comportamentos colectivos não triviais. Em redes
adaptativas, esta estrutura de interacção evolui a par com a dinâaica que nela
assenta, traduzindo uma retroacção que de comum encontrar em vários sistemas
complexos. Resultados analÃticos sobre os estados assimptóticos destes sistemas
são uma peça essencial para a sua compreensão e controlo. O equilÃbrio dinâmico
de um caso particular de estado assimptótico em que cada nodo da rede adaptativa
vai sempre mudando o seu estado e as suas ligações a outros nodos, enquanto que
um conjunto de medidas que caracterizam estatisticamente o ensemble dos nodos
mantêm valores fixos. Alémm disso, uma classe muito geral de redes adaptativas
apresenta equilÃbrios dinâmicos que parecem ser únicos, no sentido em que aqueles
valores estacionários não dependem das condições iniciais, quer em termos do
estados dos nodos quer em termos da topologia da rede.Este trabalho incide no estudo do equilÃbrio dinâmiico de redes adaptativas no contexto
particular de dois modelos paradigmáticos de coevolação, mas os principais
resultados podem ser facilmente generalizados a outros processos. Na primeira
parte, revisita-se e desenvolve-se uma abordagem da variante adaptativa do processo
de contacto baseada num modelo de baixa dimensão. Obtem-se uma descrição
aproximada do diagrama de fases do sistema e do equilÃbrio dinâmico, e
identifica-se nessa fase uma combinação de parâmetros para a qual a dinâmica
microscópica, que de assimétrica nos estados dos nodos, da origem a uma simetria
entre os dois subconjuntos de nodos. Esta simetria é explicada através da
derivação de relações de recorrência para as distribuições de grau, que a preveêm
para uma ampla classe de redes adaptativas. Estas relações permitem também
gerar analiticamente as distribuições de grau estacionárias de cada subconjunto
de nodos a partir de um parâmetro livre.Na segunda parte, desenvolve-se uma outra abordagem analÃtica que permite detectar
e descrever o equilÃbrio dinâmico, a partir de propriedades gerais que se
têm que verificar em muitas redes adaptativas. Na base desta abordagem está a
descrição do processo estocástico associado à evolução do estado e das ligações de
cada nó, e as propriedades ergódicas que permitem obter as estatÃsticas de ensemble
na rede a partir do comportamento a longo termo de um nó. Estas medidas
estatÃsticas podem ser calculadas a partir de várias distribuições estacionárias
que se relacionam umas com as outras através de transformações simples. Como
aplicação desta abordagem completa, os equilÃbrios dinâmicos de três diferentes
variantes do processo de contacto adaptativo são descritos e comparados.
Finalmente, motiva-se e propõe-se uma variante assimétrica do voter model coevolutivo.
A fase activa metastável é tentativamente descrita como uma random
walk ao longo de uma variedade lenta, Ã semelhan ca do que foi feito na literatura
para o modelo simétrico, e os resultados para os dois casos são comparados.É
identicada uma combinação de parâmetros particular para a qual este modelo
assim etrico emula o modelo simétrico em rede fixa, o que é mais um exemplo da
simetria emergente prevista pelas relações de recorrência estabelecidas na primeira
parte. Considera-se ainda uma outra variante assimétrica, mais complexa, do voter
model co-evolutivo, que apresenta um diagrama de fases essencialmente diferente,
e cuja descrição se mostra requerer novas abordagens.Fundação para a Ciência e a Tecnologia (FCT, SFRH/BD/45179/2008
An Interview with Thomas J. Sargent
The rational expectations hypothesis swept through macroeconomics during the 1970’s and permanently altered the landscape. It remains the prevailing paradigm in macroeconomics, and rational expectations is routinely used as the standard solution concept in both theoretical and applied macroeconomic modelling. The rational expectations hypothesis was initially formulated by John F. Muth Jr. in the early 1960s. Together with Robert Lucas Jr., Thomas (Tom) Sargent pioneered the rational expectations revolution in macroeconomics in the 1970s. We interviewed Tom Sargent for Macroeconomic Dynamics .
Genetic Programming for Smart Phone Personalisation
Personalisation in smart phones requires adaptability to dynamic context
based on user mobility, application usage and sensor inputs. Current
personalisation approaches, which rely on static logic that is developed a
priori, do not provide sufficient adaptability to dynamic and unexpected
context. This paper proposes genetic programming (GP), which can evolve program
logic in realtime, as an online learning method to deal with the highly dynamic
context in smart phone personalisation. We introduce the concept of
collaborative smart phone personalisation through the GP Island Model, in order
to exploit shared context among co-located phone users and reduce convergence
time. We implement these concepts on real smartphones to demonstrate the
capability of personalisation through GP and to explore the benefits of the
Island Model. Our empirical evaluations on two example applications confirm
that the Island Model can reduce convergence time by up to two-thirds over
standalone GP personalisation.Comment: 43 pages, 11 figure
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