Geodesic rewriting systems and pregroups

In this paper we study rewriting systems for groups and monoids, focusing on situations where finite convergent systems may be difficult to find or do not exist. We consider systems which have no length increasing rules and are confluent and then systems in which the length reducing rules lead to geodesics. Combining these properties we arrive at our main object of study which we call geodesically perfect rewriting systems. We show that these are well-behaved and convenient to use, and give several examples of classes of groups for which they can be constructed from natural presentations. We describe a Knuth-Bendix completion process to construct such systems, show how they may be found with the help of Stallings' pregroups and conversely may be used to construct such pregroups.Comment: 44 pages, to appear in "Combinatorial and Geometric Group Theory, Dortmund and Carleton Conferences". Series: Trends in Mathematics. Bogopolski, O.; Bumagin, I.; Kharlampovich, O.; Ventura, E. (Eds.) 2009, Approx. 350 p., Hardcover. ISBN: 978-3-7643-9910-8 Birkhause

Effect of Maternal HIV-1 Status and Antiretroviral Drugs on Haematological Profiles of South African Infants in Early Life

Maternal HIV-1 status and antiretroviral drug exposure may influence the haematological profiles of infants. We recruited infants from 118 uninfected control women and from 483 HIV-1 infected women who received no antiretroviral drugs (n=28), or received single-dose Nevirapine (sdNVP) (n=424) or triple-drug combination therapy (n=31) to reduce HIV-1 transmission. Blood was drawn from infants within 24 hours of delivery or 6-12 weeks post-delivery and full blood counts performed using a fully automated AcT-5-diff haematology analyser and reference controls. Exposed uninfected (EU; no NVP) differed from control infants only in having lower basophil counts and percentages. In all infant groups, leukocyte profiles showed characteristic quantitative changes with age in the first 6 weeks of life. HIV-1 infected infants displayed by 6 weeks elevations in white blood cells, lymphocyte, monocyte and basophil counts, and monocyte and basophil percentages, when compared to EU infants. At birth EU NVP-treated infants exhibited elevated monocyte percentages and counts and basophil counts that did not persist at 6 weeks. Interestingly, EU newborns of mothers with high CD4 counts (> 500 cells/μl) that had taken sdNVP had significantly elevated white blood cell, monocyte and basophil counts when compared to newborn infants of mothers with similar CD4 counts that had not taken sdNVP; this was not evident in infants of mothers with CD4 counts <200 cells/μl. These previously undescribed features may affect immune response capability in early life and clinical consequences of such changes need to be further investigated

Time to Switch to Second-line Antiretroviral Therapy in Children With Human Immunodeficiency Virus in Europe and Thailand.

Background: Data on durability of first-line antiretroviral therapy (ART) in children with human immunodeficiency virus (HIV) are limited. We assessed time to switch to second-line therapy in 16 European countries and Thailand. Methods: Children aged <18 years initiating combination ART (≥2 nucleoside reverse transcriptase inhibitors [NRTIs] plus nonnucleoside reverse transcriptase inhibitor [NNRTI] or boosted protease inhibitor [PI]) were included. Switch to second-line was defined as (i) change across drug class (PI to NNRTI or vice versa) or within PI class plus change of ≥1 NRTI; (ii) change from single to dual PI; or (iii) addition of a new drug class. Cumulative incidence of switch was calculated with death and loss to follow-up as competing risks. Results: Of 3668 children included, median age at ART initiation was 6.1 (interquartile range (IQR), 1.7-10.5) years. Initial regimens were 32% PI based, 34% nevirapine (NVP) based, and 33% efavirenz based. Median duration of follow-up was 5.4 (IQR, 2.9-8.3) years. Cumulative incidence of switch at 5 years was 21% (95% confidence interval, 20%-23%), with significant regional variations. Median time to switch was 30 (IQR, 16-58) months; two-thirds of switches were related to treatment failure. In multivariable analysis, older age, severe immunosuppression and higher viral load (VL) at ART start, and NVP-based initial regimens were associated with increased risk of switch. Conclusions: One in 5 children switched to a second-line regimen by 5 years of ART, with two-thirds failure related. Advanced HIV, older age, and NVP-based regimens were associated with increased risk of switch

Time-domain analysis of thermo-acoustic instabilities in a ducted flame

International audienceThe present study focuses on a time-domain approach for investigating the response of laminar premixed flames to acoustic flow perturbations in a straight duct of a uniform cross-section. This alternative perspective has recently been found to facilitate the analysis and interpretation of thermo-acoustic instabilities by providing a more intuitive description of the interaction of the acoustics with the heat-release rate. A framework is first introduced to generate accurate time-series that provide a complete description of the thermo-acoustic system. Advantages include the flexibility to account for different models (delayed, local, and non-local terms), as well as the separation of numerical and modeling errors. Advanced dynamical system techniques are then employed to analyze the low-amplitude (linear) regime. Detailed representation of the acoustic signal inside the duct allows for accurate characterization of the transition from the linear to the non-linear regime, and give access to the coupled dynamics of complex limit-cycles. In particular, the effect of the heat release on the linear and non-linear response of the duct is analyzed in detail. This information forms the basis for an effective use of system reduction techniques. The presented capabilities are found to yield physical insight with relative ease, and are expected to produce interesting results by revisiting existing experimental and numerical strategies for the study of thermo-acoustic instabilities

Microbiology and vegetation of micro-oases and polar desert, Haughton impact crater, Devon Island, Nunavut, Canada

The input of nutrients into arctic polar deserts, aided by some physical processes, can result in localized areas of high biological productivity - "micro-oases." We examined the vegetation cover, and microbial and nematode abundance in the polar desert and in 38 micro-oases at the Haughton impact crater, Devon Island, Arctic Canada. Our sites were split between the alluvial terraces along the banks of the Haughton River and the breccia deposits resulting from the asteroid or comet impact 22 Myr ago that flank the alluvial terraces. The alluvial terraces have a vegetation cover that ranges from 2 to 11% depending on substrate and water availability with a species richness of 5 in most locations. The vegetation cover on the breccia is much lower, between 0.02 and 3% depending on water availability. The micro-oases on both substrates support between 2 and 98% cover, but they are smaller and more sparsely distributed than similar features found in the Truelove Lowland, Devon Island, and on Bathurst Island. Microbial and nematode numbers were an order of magnitude greater inside the micro-oases compared to outside. Micro-oases are often dominated by a particular species, resulting in well-defined groups of micro-oases that were separated by TWINSPAN analysis. The micro-oases at Haughton Crater provide insights into the process of colonization of a substrate resulting from an asteroid or comet impact and the unique biological characteristics of such substrates