A comunicação da Embrapa Trigo em um dia de campo de inverno 2013.

Editores técnicos: Joseani Mesquita Antunes, Ana Lídia Variani Bonato, Márcia Barrocas Moreira Pimentel

New Complexity Results and Algorithms for the Minimum Tollbooth Problem

The inefficiency of the Wardrop equilibrium of nonatomic routing games can be eliminated by placing tolls on the edges of a network so that the socially optimal flow is induced as an equilibrium flow. A solution where the minimum number of edges are tolled may be preferable over others due to its ease of implementation in real networks. In this paper we consider the minimum tollbooth (MINTB) problem, which seeks social optimum inducing tolls with minimum support. We prove for single commodity networks with linear latencies that the problem is NP-hard to approximate within a factor of $1.1377$ through a reduction from the minimum vertex cover problem. Insights from network design motivate us to formulate a new variation of the problem where, in addition to placing tolls, it is allowed to remove unused edges by the social optimum. We prove that this new problem remains NP-hard even for single commodity networks with linear latencies, using a reduction from the partition problem. On the positive side, we give the first exact polynomial solution to the MINTB problem in an important class of graphs---series-parallel graphs. Our algorithm solves MINTB by first tabulating the candidate solutions for subgraphs of the series-parallel network and then combining them optimally

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 período de inverno em Santa Maria. A estufa permitiu um ganho térmico favorável ao crescimento da alface, da ordem de 256 graus-dia acima da temperatura base de crescimento de 10°C, durante o período de 50 dias. A temperatura do solo também foi mais favorável no interior da estufa, 5,3°C em média, em relação ao exterior. As plantas cultivadas no interior da estufa apresentaram curvas de crescimento similares àquelas cultivadas no exterior, porém com valores mais elevados dos parâmetros de crescimento de área foliar, massa verde da parte aérea, massa seca das folhas, do caule e das raízes. A relação parte aérea/sistema radiador também foi mais elevada no interior da estufa. O número de folhas por planta foi similar nos dois ambientes. Dentre as três 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

Influência de uma estufa coberta de polietileno transparente no crescimento de alface.

Este trabalho teve como objetivo avaliar o desempenho de três 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

Dynamics of conflicts in Wikipedia

In this work we study the dynamical features of editorial wars in Wikipedia (WP). Based on our previously established algorithm, we build up samples of controversial and peaceful articles and analyze the temporal characteristics of the activity in these samples. On short time scales, we show that there is a clear correspondence between conflict and burstiness of activity patterns, and that memory effects play an important role in controversies. On long time scales, we identify three distinct developmental patterns for the overall behavior of the articles. We are able to distinguish cases eventually leading to consensus from those cases where a compromise is far from achievable. Finally, we analyze discussion networks and conclude that edit wars are mainly fought by few editors only.Comment: Supporting information adde