Random walks in directed modular networks

Because diffusion typically involves symmetric interactions, scant attention has been focused on studying asymmetric cases. However, important networked systems underlain by diffusion (e.g. cortical networks and WWW) are inherently directed. In the case of undirected diffusion, it can be shown that the steady-state probability of the random walk dynamics is fully correlated with the degree, which no longer holds for directed networks. We investigate the relationship between such probability and the inward node degree, which we call efficiency, in modular networks. Our findings show that the efficiency of a given community depends mostly on the balance between its ingoing and outgoing connections. In addition, we derive analytical expressions to show that the internal degree of the nodes do not play a crucial role in their efficiency, when considering the Erd\H{o}s-R\'enyi and Barab\'asi-Albert models. The results are illustrated with respect to the macaque cortical network, providing subsidies for improving transportation and communication systems

Réplicas ampliadas de microfósseis em resina odontológica: usos didáticos e científicos em micropaleontologia

O uso de réplicas de microfósseis em escala macrométrica consiste em uma ferramenta didática muito útil para os estudos de micropaleontologia. Analisando um espécime com o auxílio do tato e visão é possível entender os aspectos estruturais e morfológicos dos organismos com maior eficiência. A técnica de confecção de réplicas de microfósseis representa uma alternativa em relação à dependência da infra-estrutura laboratorial necessária para a identificação dos microfósseis. Com esta alternativa é possível estudá-los em qualquer ambiente acadêmico ou eventos que não disponham dos devidos equipamentos para sua visualização, uma vez que se torna mais fácil compreender e estudar os espécimes. As resinas odontológicas utilizadas na confecção oferecem um excelente resultado quanto à pigmentação, textura, brilho, opacidade e resistência, pois estes materiais são o que há de mais fiel na substituição dos tecidos da boca humana, e ao transferir esta técnica para a paleoarte obtem-se excelentes resultados estéticos. Para a confecção dos modelos, primeiramente foram feitas imagens em Microscópio Eletrônico de Varredura (MEV) em diversos ângulos, para melhor visualização dos detalhes. Inicia-se, então, o processo de escultura, usando como base um bloco de cera odontológica do tipo Utilidade (ou Nº 7), bico de Bunsen (ou lamparina de bancada), lamparina de Hannau, escova e espátulas. Concluída a fase de escultura, fabrica-se, através da moldagem tradicional, uma matriz em dois hemisférios, consituída de silicone e gesso para produzir a fase resina, e logo depois da demuflagem, o acabamento com motor de suspensão. Finalmente, executa-se o polimento em polidora química, variando de acordo com a textura de cada espécie. Os exemplares apresentam ótima aceitação no meio científico devido à fidelidade em relação ao espécime original, gerando interesse na confecção de novas esculturas, projetando para o futuro a abrangência desta técnica, desenvolvendo a escultura de diferentes assembléias de microfósseis encontradas em amostras para pesquisa.Sesiones libresFacultad de Ciencias Naturales y Muse

Strong correlations between text quality and complex networks features

Concepts of complex networks have been used to obtain metrics that were correlated to text quality established by scores assigned by human judges. Texts produced by high-school students in Portuguese were represented as scale-free networks (word adjacency model), from which typical network features such as the in/outdegree, clustering coefficient and shortest path were obtained. Another metric was derived from the dynamics of the network growth, based on the variation of the number of connected components. The scores assigned by the human judges according to three text quality criteria (coherence and cohesion, adherence to standard writing conventions and theme adequacy/development) were correlated with the network measurements. Text quality for all three criteria was found to decrease with increasing average values of outdegrees, clustering coefficient and deviation from the dynamics of network growth. Among the criteria employed, cohesion and coherence showed the strongest correlation, which probably indicates that the network measurements are able to capture how the text is developed in terms of the concepts represented by the nodes in the networks. Though based on a particular set of texts and specific language, the results presented here point to potential applications in other instances of text analysis.Comment: 8 pages, 8 figure

How Many Nodes are Effectively Accessed in Complex Networks?

The measurement called accessibility has been proposed as a means to quantify the efficiency of the communication between nodes in complex networks. This article reports important results regarding the properties of the accessibility, including its relationship with the average minimal time to visit all nodes reachable after $h$ steps along a random walk starting from a source, as well as the number of nodes that are visited after a finite period of time. We characterize the relationship between accessibility and the average number of walks required in order to visit all reachable nodes (the exploration time), conjecture that the maximum accessibility implies the minimal exploration time, and confirm the relationship between the accessibility values and the number of nodes visited after a basic time unit. The latter relationship is investigated with respect to three types of dynamics, namely: traditional random walks, self-avoiding random walks, and preferential random walks.Comment: 8 pages and 7 figure

Good practices for a literature survey are not followed by authors while preparing scientific manuscripts

The number of citations received by authors in scientific journals has become a major parameter to assess individual researchers and the journals themselves through the impact factor. A fair assessment therefore requires that the criteria for selecting references in a given manuscript should be unbiased with respect to the authors or the journals cited. In this paper, we advocate that authors should follow two mandatory principles to select papers (later reflected in the list of references) while studying the literature for a given research: i) consider similarity of content with the topics investigated, lest very related work should be reproduced or ignored; ii) perform a systematic search over the network of citations including seminal or very related papers. We use formalisms of complex networks for two datasets of papers from the arXiv repository to show that neither of these two criteria is fulfilled in practice

Modeling Connectivity in Terms of Network Activity

A new complex network model is proposed which is founded on growth with new connections being established proportionally to the current dynamical activity of each node, which can be understood as a generalization of the Barabasi-Albert static model. By using several topological measurements, as well as optimal multivariate methods (canonical analysis and maximum likelihood decision), we show that this new model provides, among several other theoretical types of networks including Watts-Strogatz small-world networks, the greatest compatibility with three real-world cortical networks.Comment: A working manuscript, 5 pages, 3 figures, 1 tabl

Evolving Networks and the Development of Neural Systems

It is now generally assumed that the heterogeneity of most networks in nature probably arises via preferential attachment of some sort. However, the origin of various other topological features, such as degree-degree correlations and related characteristics, is often not clear and attributed to specific functional requirements. We show how it is possible to analyse a very general scenario in which nodes gain or lose edges according to any (e.g., nonlinear) functions of local and/or global degree information. Applying our method to two rather different examples of brain development -- synaptic pruning in humans and the neural network of the worm C. Elegans -- we find that simple biologically motivated assumptions lead to very good agreement with experimental data. In particular, many nontrivial topological features of the worm's brain arise naturally at a critical point.Comment: 16 pages, 4 figures. Accepted for publication in J. Stat. Mec

Beyond the average: Detecting global singular nodes from local features in complex networks

Deviations from the average can provide valuable insights about the organization of natural systems. The present article extends this important principle to the systematic identification and analysis of singular motifs in complex networks. Six measurements quantifying different and complementary features of the connectivity around each node of a network were calculated, and multivariate statistical methods applied to identify singular nodes. The potential of the presented concepts and methodology was illustrated with respect to different types of complex real-world networks, namely the US air transportation network, the protein-protein interactions of the yeast Saccharomyces cerevisiae and the Roget thesaurus networks. The obtained singular motifs possessed unique functional roles in the networks. Three classic theoretical network models were also investigated, with the Barab\'asi-Albert model resulting in singular motifs corresponding to hubs, confirming the potential of the approach. Interestingly, the number of different types of singular node motifs as well as the number of their instances were found to be considerably higher in the real-world networks than in any of the benchmark networks

Biodiversity increases and decreases ecosystem stability

International audienc