66 research outputs found
Preferential attachment in the growth of social networks: the case of Wikipedia
We present an analysis of the statistical properties and growth of the free
on-line encyclopedia Wikipedia. By describing topics by vertices and hyperlinks
between them as edges, we can represent this encyclopedia as a directed graph.
The topological properties of this graph are in close analogy with that of the
World Wide Web, despite the very different growth mechanism. In particular we
measure a scale--invariant distribution of the in-- and out-- degree and we are
able to reproduce these features by means of a simple statistical model. As a
major consequence, Wikipedia growth can be described by local rules such as the
preferential attachment mechanism, though users can act globally on the
network.Comment: 4 pages, 4 figures, revte
Sampling properties of directed networks
For many real-world networks only a small "sampled" version of the original
network may be investigated; those results are then used to draw conclusions
about the actual system. Variants of breadth-first search (BFS) sampling, which
are based on epidemic processes, are widely used. Although it is well
established that BFS sampling fails, in most cases, to capture the
IN-component(s) of directed networks, a description of the effects of BFS
sampling on other topological properties are all but absent from the
literature. To systematically study the effects of sampling biases on directed
networks, we compare BFS sampling to random sampling on complete large-scale
directed networks. We present new results and a thorough analysis of the
topological properties of seven different complete directed networks (prior to
sampling), including three versions of Wikipedia, three different sources of
sampled World Wide Web data, and an Internet-based social network. We detail
the differences that sampling method and coverage can make to the structural
properties of sampled versions of these seven networks. Most notably, we find
that sampling method and coverage affect both the bow-tie structure, as well as
the number and structure of strongly connected components in sampled networks.
In addition, at low sampling coverage (i.e. less than 40%), the values of
average degree, variance of out-degree, degree auto-correlation, and link
reciprocity are overestimated by 30% or more in BFS-sampled networks, and only
attain values within 10% of the corresponding values in the complete networks
when sampling coverage is in excess of 65%. These results may cause us to
rethink what we know about the structure, function, and evolution of real-world
directed networks.Comment: 21 pages, 11 figure
Comparação do crescimento e desenvolvimento da alface (Lactuca sativa L.) no interior e no exterior de uma estufa de polietileno em Santa Maria, RS.
O trabalho teve por objetivo comparar o crescimento e o desenvolvimento das cultivares de alface Brasil-202, White Boston e Regina, no interior e no exterior de uma estufa de polietileno durante o perĂodo de inverno em Santa Maria. A estufa permitiu um ganho tĂ©rmico favorĂĄvel ao crescimento da alface, da ordem de 256 graus-dia acima da temperatura base de crescimento de 10°C, durante o perĂodo de 50 dias. A temperatura do solo tambĂ©m foi mais favorĂĄvel no interior da estufa, 5,3°C em mĂ©dia, em relação ao exterior. As plantas cultivadas no interior da estufa apresentaram curvas de crescimento similares Ă quelas cultivadas no exterior, porĂ©m com valores mais elevados dos parĂąmetros de crescimento de ĂĄrea foliar, massa verde da parte aĂ©rea, massa seca das folhas, do caule e das raĂzes. A relação parte aĂ©rea/sistema radiador tambĂ©m foi mais elevada no interior da estufa. O nĂșmero de folhas por planta foi similar nos dois ambientes. Dentre as trĂȘs cultivares testadas, a Brasil-202 apresentou maior precocidade e maior acumulação de massa seca. A estufa de polietileno mostrou ser uma alternativa eficiente para a produção de alface nos meses de inverno
InfluĂȘncia de uma estufa coberta de polietileno transparente no crescimento de alface.
Este trabalho teve como objetivo avaliar o desempenho de trĂȘs cultivares de alface cultivadas em ambiente natural e no interior de estufa coberta com polietileno transparente, durante o inverno.bitstream/item/97531/1/CPAF-AP-1999-Influencia-estufa.pd
QAPgrid: A Two Level QAP-Based Approach for Large-Scale Data Analysis and Visualization
Background: The visualization of large volumes of data is a computationally challenging task that often promises rewarding new insights. There is great potential in the application of new algorithms and models from combinatorial optimisation. Datasets often contain âhidden regularitiesâ and a combined identification and visualization method should reveal these structures and present them in a way that helps analysis. While several methodologies exist, including those that use non-linear optimization algorithms, severe limitations exist even when working with only a few hundred objects. Methodology/Principal Findings: We present a new data visualization approach (QAPgrid) that reveals patterns of similarities and differences in large datasets of objects for which a similarity measure can be computed. Objects are assigned to positions on an underlying square grid in a two-dimensional space. We use the Quadratic Assignment Problem (QAP) as a mathematical model to provide an objective function for assignment of objects to positions on the grid. We employ a Memetic Algorithm (a powerful metaheuristic) to tackle the large instances of this NP-hard combinatorial optimization problem, and we show its performance on the visualization of real data sets. Conclusions/Significance: Overall, the results show that QAPgrid algorithm is able to produce a layout that represents the relationships between objects in the data set. Furthermore, it also represents the relationships between clusters that are feed into the algorithm. We apply the QAPgrid on the 84 Indo-European languages instance, producing a near-optimal layout. Next, we produce a layout of 470 world universities with an observed high degree of correlation with the score used by the Academic Ranking of World Universities compiled in the The Shanghai Jiao Tong University Academic Ranking of World Universities without the need of an ad hoc weighting of attributes. Finally, our Gene Ontology-based study on Saccharomyces cerevisiae fully demonstrates the scalability and precision of our method as a novel alternative tool for functional genomics
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