A stronger topology for the Brownian web

We propose a metric space of coalescing pairs of paths on which we are able to prove (more or less) directly convergence of objects such as the persistence probability in the (one dimensional, nearest neighbor, symmetric) voter model or the diffusively rescaled weight distribution in a silo model (as well as the equivalent output distribution in a river basin model), interpreted in terms of (dual) diffusively rescaled coalescing random walks, to corresponding objects defined in terms of the Brownian web.Comment: 22 page

Prevalence and associated risk factors of asymptomatic bacteriuria in ante-natal clients in a large teaching hospital in Ghana

Introduction: Asymptomatic bacteriuria, the presence of bacteria in urine without symptoms of acute urinary tract infection, predisposes pregnant women to the development of urinary tract infections and pyelonephritis, with an attendant pregnancy related complications.Objective: To measure the prevalence of asymptomatic bacteriuria among ante-natal clients at the Korle-Bu Teaching Hospital in Ghana and its’ associated risk factors.Methods: A cross-sectional study involving 274 antenatal clients was conducted over a period of 4 weeks. A face to face questionnaire was completed and midstream urine collected for culture and antimicrobial susceptibility testing.Results: The prevalence of asymptomatic bacteriuria was 5.5%. It was associated with sexual activity during pregnancy (Fisher’s Exact 5.871, p-value 0.0135), but not with sexual frequency. There were no significant associations with educational status, parity, gestational age, marital status and the number of foetuses carried. The commonest organism isolated was Enterococcus spp (26.7%) although the enterobacteriaceae formed the majority of isolated organisms (46.7%). Nitrofurantoin was the antibiotic with the highest sensitivity to all the isolated organisms.Conclusions: The prevalence of asymptomatic bacteriuria among ante-natal clients at this large teaching hospital in Ghana is 5.5%, which is lower than what has been found in other African settings. Enterococcus spp was the commonest causative organism. However, due to the complications associated with asymptomatic bacteriuria, a policy to screen and treat- all pregnant women attending the hospital, is worth considering.Key words: Asymptomatic bacteriuria, ante-natal clients, antibiotic sensitivity, tertiary hospital, Ghan

Searching for network modules

When analyzing complex networks a key target is to uncover their modular structure, which means searching for a family of modules, namely node subsets spanning each a subnetwork more densely connected than the average. This work proposes a novel type of objective function for graph clustering, in the form of a multilinear polynomial whose coefficients are determined by network topology. It may be thought of as a potential function, to be maximized, taking its values on fuzzy clusterings or families of fuzzy subsets of nodes over which every node distributes a unit membership. When suitably parametrized, this potential is shown to attain its maximum when every node concentrates its all unit membership on some module. The output thus is a partition, while the original discrete optimization problem is turned into a continuous version allowing to conceive alternative search strategies. The instance of the problem being a pseudo-Boolean function assigning real-valued cluster scores to node subsets, modularity maximization is employed to exemplify a so-called quadratic form, in that the scores of singletons and pairs also fully determine the scores of larger clusters, while the resulting multilinear polynomial potential function has degree 2. After considering further quadratic instances, different from modularity and obtained by interpreting network topology in alternative manners, a greedy local-search strategy for the continuous framework is analytically compared with an existing greedy agglomerative procedure for the discrete case. Overlapping is finally discussed in terms of multiple runs, i.e. several local searches with different initializations.Comment: 10 page

Increased soil stable nitrogen isotopic ratio following phosphorus enrichment: historical patterns and tests of two hypotheses in a phosphorus-limited wetland

Abstract We used a P enrichment gradient in the Everglades to investigate patterns of the stable N isotopic ratio (d 15 N) in peat profiles as an indicator of historic eutrophication of this wetland. We also tested two hypotheses to explain the effects of P on increased d 15 N of organic matter including: (1) increased N mineralization/N loss, and (2) reduced isotopic discrimination during macrophyte N uptake. Spatial patterns of d 15 N in surface litter and soil (0-10 cm) mimic those of the aboveground macrophytes (Typha domingensis Pers. and Cladium jamaicense Crantz). Peat profiles also show increased d 15 N in the peat accumulated in areas near the historic P discharges since the early 1960s. The increased d 15 N of bulk peat correlated well with both measured increases in soil total P and the historical beginning of nutrient discharges into this wetland. In 15-day bottle incubations of soil, added P had no effect on the

Some remarks on a new exotic spacetime for time travel by free fall

This work is essentially a review of a new spacetime model with closed causal curves, recently presented in another paper (Class. Quantum Grav. \textbf{35}(16) (2018), 165003). The spacetime at issue is topologically trivial, free of curvature singularities, and even time and space orientable. Besides summarizing previous results on causal geodesics, tidal accelerations and violations of the energy conditions, here redshift/blueshift effects and the Hawking-Ellis classification of the stress-energy tensor are examined.Comment: 17 pages, 9 figures. Submitted as a contribution to the proceedings of "DOMOSCHOOL - International Alpine School of Mathematics and Physics, Domodossola 2018". Possible text overlaps with my previous work arXiv:1803.08214, of which this is essentially a review. Additional results concerning redshift/blueshift effects and the classification of the stress-energy tensor are presented her

Message-Passing Methods for Complex Contagions

Message-passing methods provide a powerful approach for calculating the expected size of cascades either on random networks (e.g., drawn from a configuration-model ensemble or its generalizations) asymptotically as the number $N$ of nodes becomes infinite or on specific finite-size networks. We review the message-passing approach and show how to derive it for configuration-model networks using the methods of (Dhar et al., 1997) and (Gleeson, 2008). Using this approach, we explain for such networks how to determine an analytical expression for a "cascade condition", which determines whether a global cascade will occur. We extend this approach to the message-passing methods for specific finite-size networks (Shrestha and Moore, 2014; Lokhov et al., 2015), and we derive a generalized cascade condition. Throughout this chapter, we illustrate these ideas using the Watts threshold model.Comment: 14 pages, 3 figure

Functional cartography of complex metabolic networks

High-throughput techniques are leading to an explosive growth in the size of biological databases and creating the opportunity to revolutionize our understanding of life and disease. Interpretation of these data remains, however, a major scientific challenge. Here, we propose a methodology that enables us to extract and display information contained in complex networks. Specifically, we demonstrate that one can (i) find functional modules in complex networks, and (ii) classify nodes into universal roles according to their pattern of intra- and inter-module connections. The method thus yields a ``cartographic representation'' of complex networks. Metabolic networks are among the most challenging biological networks and, arguably, the ones with more potential for immediate applicability. We use our method to analyze the metabolic networks of twelve organisms from three different super-kingdoms. We find that, typically, 80% of the nodes are only connected to other nodes within their respective modules, and that nodes with different roles are affected by different evolutionary constraints and pressures. Remarkably, we find that low-degree metabolites that connect different modules are more conserved than hubs whose links are mostly within a single module.Comment: 17 pages, 4 figures. Go to http://amaral.northwestern.edu for the PDF file of the reprin

Optimizing topological cascade resilience based on the structure of terrorist networks

Complex socioeconomic networks such as information, finance and even terrorist networks need resilience to cascades - to prevent the failure of a single node from causing a far-reaching domino effect. We show that terrorist and guerrilla networks are uniquely cascade-resilient while maintaining high efficiency, but they become more vulnerable beyond a certain threshold. We also introduce an optimization method for constructing networks with high passive cascade resilience. The optimal networks are found to be based on cells, where each cell has a star topology. Counterintuitively, we find that there are conditions where networks should not be modified to stop cascades because doing so would come at a disproportionate loss of efficiency. Implementation of these findings can lead to more cascade-resilient networks in many diverse areas.Comment: 26 pages. v2: In review at Public Library of Science ON

Ising model for distribution networks

An elementary Ising spin model is proposed for demonstrating cascading failures (break-downs, blackouts, collapses, avalanches, ...) that can occur in realistic networks for distribution and delivery by suppliers to consumers. A ferromagnetic Hamiltonian with quenched random fields results from policies that maximize the gap between demand and delivery. Such policies can arise in a competitive market where firms artificially create new demand, or in a solidary environment where too high a demand cannot reasonably be met. Network failure in the context of a policy of solidarity is possible when an initially active state becomes metastable and decays to a stable inactive state. We explore the characteristics of the demand and delivery, as well as the topological properties, which make the distribution network susceptible of failure. An effective temperature is defined, which governs the strength of the activity fluctuations which can induce a collapse. Numerical results, obtained by Monte Carlo simulations of the model on (mainly) scale-free networks, are supplemented with analytic mean-field approximations to the geometrical random field fluctuations and the thermal spin fluctuations. The role of hubs versus poorly connected nodes in initiating the breakdown of network activity is illustrated and related to model parameters