1,086 research outputs found
Mean Curvature Flow on Ricci Solitons
We study monotonic quantities in the context of combined geometric flows. In
particular, focusing on Ricci solitons as the ambient space, we consider
solutions of the heat type equation integrated over embedded submanifolds
evolving by mean curvature flow and we study their monotonicity properties.
This is part of an ongoing project with Magni and Mantegazzawhich will treat
the case of non-solitonic backgrounds \cite{a_14}.Comment: 19 page
Irreversibility of World-sheet Renormalization Group Flow
We demonstrate the irreversibility of a wide class of world-sheet
renormalization group (RG) flows to first order in in string theory.
Our techniques draw on the mathematics of Ricci flows, adapted to
asymptotically flat target manifolds. In the case of somewhere-negative scalar
curvature (of the target space), we give a proof by constructing an entropy
that increases monotonically along the flow, based on Perelman's Ricci flow
entropy. One consequence is the absence of periodic solutions, and we are able
to give a second, direct proof of this. If the scalar curvature is everywhere
positive, we instead construct a regularized volume to provide an entropy for
the flow. Our results are, in a sense, the analogue of Zamolodchikov's
-theorem for world-sheet RG flows on noncompact spacetimes (though our
entropy is not the Zamolodchikov -function).Comment: Minor changes, added one citation, version accepted for publicatio
Retrotransposon mapping in spider monkey genomes of the family Atelidae (Platyrrhini, Primates) shows a high level of LINE-1 amplification
To investigate the distribution of LINE-1 repeat sequences, a
LINE-1 probe was Fluorescence In Situ Hybridized (FISH) on the
chromosomes of Ateles geoffroyi and Ateles fusciceps (Atelidae); a
LINE-1 probe was also mapped on Cebuella pygmaea (Cebidae)
and used as an outgroup for phylogenetic comparison. Ateles spider
monkeys have a highly rearranged genome and are an ideal model
for testing whether LINE-1 is involved in genome evolution. The
LINE-1 probe has been mapped in the two Atelidae species for the
first time, revealing a high accumulation of LINE-1 sequences
along chromosomal arms, including telomeres, and a scarcity of
LINE-1 signals at centromere positions. LINE-1 mapping in C. pygmaea (Cebidae) revealed signals at centromere positions and along
chromosome arms, which was consistent with previous published
data from other Cebidae species. In a broader sense, the results were
analyzed in light of published data on whole-chromosomal human
probes mapped in these genomes. This analysis allows us to speculate about the presence of LINE-1 sequences at the junction of
human chromosomal syntenies, as well as a possible link between
these sequences and chromosomal rearrangements
A simple proof of Perelman's collapsing theorem for 3-manifolds
We will simplify earlier proofs of Perelman's collapsing theorem for
3-manifolds given by Shioya-Yamaguchi and Morgan-Tian. Among other things, we
use Perelman's critical point theory (e.g., multiple conic singularity theory
and his fibration theory) for Alexandrov spaces to construct the desired local
Seifert fibration structure on collapsed 3-manifolds. The verification of
Perelman's collapsing theorem is the last step of Perelman's proof of
Thurston's Geometrization Conjecture on the classification of 3-manifolds. Our
proof of Perelman's collapsing theorem is almost self-contained, accessible to
non-experts and advanced graduate students. Perelman's collapsing theorem for
3-manifolds can be viewed as an extension of implicit function theoremComment: v1: 9 Figures. In this version, we improve the exposition of our
arguments in the earlier arXiv version. v2: added one more grap
Investigating Off-shell Stability of Anti-de Sitter Space in String Theory
We propose an investigation of stability of vacua in string theory by
studying their stability with respect to a (suitable) world-sheet
renormalization group (RG) flow. We prove geometric stability of (Euclidean)
anti-de Sitter (AdS) space (i.e., ) with respect to the simplest
RG flow in closed string theory, the Ricci flow. AdS space is not a fixed point
of Ricci flow. We therefore choose an appropriate flow for which it is a fixed
point, prove a linear stability result for AdS space with respect to this flow,
and then show this implies its geometric stability with respect to Ricci flow.
The techniques used can be generalized to RG flows involving other fields. We
also discuss tools from the mathematics of geometric flows that can be used to
study stability of string vacua.Comment: 29 pages, references added in this version to appear in Classical and
Quantum Gravit
Biosecurity assessment of Argentinian pig farms
The pig industry is growing very fast in Argentina with an increasing need for replacement animals, feedstuff and transportation of animals. One of the main competitive advantages of the Argentinian pig industry is its being free of most major pig diseases. Within this context, applying measures aimed to reduce the risk of introduction and spread of pathogens is critical. The aim of the present study was to assess the biosecurity of Argentinian pig farms. Two types of farms were assessed: firstly, all official suppliers of high-genetic-value (n = 110) and secondly, a sample from commercial farms (n = 192). Data on the external and internal biosecurity practices applied on the farms was collected with a questionnaire. Data was analysed using a correspondence analysis and a hierarchical clustering analysis, which allowed identification of types of farms with regard to the biosecurity measures applied. Key variables characterizing the clusters were identified through an indicator value analysis. In addition, the external biosecurity of the farms was evaluated by using risk assessment tools with respect to the potential introduction of porcine epidemic diarrhoea virus. Results made evident three clusters: the first one which, amongst other measures, applied several barriers to prevent the entry of people, trucks and other vehicles, and could be considered as a group of high biosecurity, and the two other groups which applied a lower number of external and internal biosecurity measures. The results of the risk assessment showed that the routes with the highest risk of disease introduction were: replacement animals, vehicles transporting feed or animals, and visitors. The assessment of the external biosecurity showed that most Argentinian farms were not prepared for the contingency of a pathogen such as porcine epidemic diarrhoea virus. Special efforts should be made in official suppliers of high-genetic-value farms with poor biosecurity scores since they are at the top of the pig production chain and can be key for the spread of diseases.info:eu-repo/semantics/acceptedVersio
Ricci flow and black holes
Gradient flow in a potential energy (or Euclidean action) landscape provides
a natural set of paths connecting different saddle points. We apply this method
to General Relativity, where gradient flow is Ricci flow, and focus on the
example of 4-dimensional Euclidean gravity with boundary S^1 x S^2,
representing the canonical ensemble for gravity in a box. At high temperature
the action has three saddle points: hot flat space and a large and small black
hole. Adding a time direction, these also give static 5-dimensional
Kaluza-Klein solutions, whose potential energy equals the 4-dimensional action.
The small black hole has a Gross-Perry-Yaffe-type negative mode, and is
therefore unstable under Ricci flow. We numerically simulate the two flows
seeded by this mode, finding that they lead to the large black hole and to hot
flat space respectively, in the latter case via a topology-changing
singularity. In the context of string theory these flows are world-sheet
renormalization group trajectories. We also use them to construct a novel free
energy diagram for the canonical ensemble.Comment: 31 pages, 14 color figures. v2: Discussion of the metric on the space
of metrics corrected and expanded, references adde
a cross-sectional study in six European cities
Background: The relationship between socioeconomic position (SEP) and adolescent physical activity is uncertain, as most evidence is limited to specific settings and a restricted number of SEP indicators. This study aimed to assess the magnitude of socioeconomic differences in adolescent vigorous physical activity (VPA) across various European countries using a wide range of SEP indicators, including family-based (education, family affluence, perceived social standing, parents’ employment, housing tenure) and adolescent-based (academic performance and pocket money) ones. Methods: We used data from a survey among 10,510 students aged 14–17 from 50 schools in six European cities: Namur (BE), Tampere (FI), Hannover (DE), Latina (IT), Amersfoort (NL), Coimbra (PT). The questionnaire included socio-demographic characteristics and the amount of time spent in VPA. Results: The mean time spent practicing VPA was 60.4 min per day, with lower values for Namur (BE) and Latina (IT), and higher values for Amersfoort (NL). In the multivariable analysis, both categories of SEP indicators (family-based and adolescent based indicators) were independently associated with VPA. For each SEP indicator, lower levels of VPA were recorded in lower socioeconomic groups. In the total sample, each additional category of low SEP was associated with a decrease in mean VPA of about 4 min per day. Conclusions: This study showed that across European cities adolescent VPA is positively related to both family-based SEP and adolescents’ own SEP. When analysing socioeconomic differences in adolescent VPA, one should consider the use of multiple indicators of SEP.publishersversionpublishe
A Gradient Flow for Worldsheet Nonlinear Sigma Models
We discuss certain recent mathematical advances, mainly due to Perelman, in
the theory of Ricci flows and their relevance for renormalization group (RG)
flows. We consider nonlinear sigma models with closed target manifolds
supporting a Riemannian metric, dilaton, and 2-form B-field. By generalizing
recent mathematical results to incorporate the B-field and by decoupling the
dilaton, we are able to describe the 1-loop beta-functions of the metric and
B-field as the components of the gradient of a potential functional on the
space of coupling constants. We emphasize a special choice of diffeomorphism
gauge generated by the lowest eigenfunction of a certain Schrodinger operator
whose potential and kinetic terms evolve along the flow. With this choice, the
potential functional is the corresponding lowest eigenvalue, and gives the
order alpha' correction to the Weyl anomaly at fixed points of (g(t),B(t)).
Since the lowest eigenvalue is monotonic along the flow and reproduces the Weyl
anomaly at fixed points, it accords with the c-theorem for flows that remain
always in the first-order regime. We compute the Hessian of the lowest
eigenvalue functional and use it to discuss the linear stability of points
where the 1-loop beta-functions vanish, such as flat tori and K3 manifolds.Comment: Accepted version for publication. Citations added to Friedan and to
Fateev, Onofri, and Zamolodchikov. Introduction modified slightly to discuss
these citations. 25 pages, LaTe
a cross-section analysis on 36 European countries
Background: Schools have a crucial role to play in preventing youth smoking. However, the well-known long-term health consequences of youth smoking may be insufficient to convince education stakeholders to devote efforts to implement school-based programmes. However, if youth smoking were to have short-term consequences, this evidence could prompt education stakeholders' action. In this article, we investigate the link between smoking and school absenteeism. Methods: We used data from the 2011 wave of the European School Survey Project on Alcohol and Other Drugs, on adolescents aged 15-16. We applied logistic models to assess the risk of more than 3 missed school days, by cause, as function of smoking intensity, adjusting for age, sex, socioeconomic status, academic performance, parental involvement and other risk behaviours (alcohol and cannabis consumption). Consistency was assessed by replicating the analyses for each sex and age group and further adjusting for depression and self-esteem. Results: Smoking more than five cigarettes per day was significantly linked to school absenteeism, with a 55% excess risk of missing more than 3 school days per month due to illness (OR = 1.55, 95% CI 1.46-1.64), and a more than two times excess risk due to skipping (OR = 2.29; 95% CI 2.16-2.43). These findings were consistent across age and sex groups. Conclusion: We observed an association between smoking intensity and absenteeism among youth in Europe. This implies that, to the extent that this association is causal, school tobacco control policies may reduce the short-term consequences of smoking on adolescents' education and health.publishersversionpublishe
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