1,720,969 research outputs found
Universal instabilities of rolls, squares and hexagons in time-dependent non-linear convection
No full textNonlinear phase diffusion equations for the long-wave instabilities of hexagons
No full textThe phase instabilities of hexagons are studied using the lowest order amplitude equations. The shapes of the unstable modes and the nonlinear phase diffusion equations which hold close to onset are found. The latter show that the instabilities are subcritical. It is found that the long-wave zigzag and two-dimensional Eckhaus instabilities cannot occur in hexagons
League tables and school effectiveness: a mathematical model
No full text'School performance tables', an alphabetical list of secondary schools along with aggregates of their pupils' performances in national tests, have been published in the UK since 1992. Inevitably, the media have responded by publishing ranked 'league tables'. Despite concern over the potentially divisive effect of such tables, the current government has continued to publish this information in the same form. The effect of this information on standards and on the social make-up of the community has been keenly debated. Since there is no control group available that would allow us to investigate this issue directly, we present here a simple mathematical model. Our results indicate that, while random fluctuations from year to year can cause large distortions in the league-table positions, some schools still establish themselves as 'desirable'. To our surprise, we found that 'value-added' tables were no more accurate than tables based on raw exam scores, while a different method of drawing up the tables, in which exam results are averaged over a period of time, appears to give a much more reliable measure of school performance
The benefits of maternal effects in novel and in stable environments
No full textNatural selection favours phenotypes that match prevailing ecological conditions. A rapid process of adaptation is therefore required in changing environments. Maternal effects can facilitate such responses, but it is currently poorly understood under which circumstances maternal effects may accelerate or slow down the rate of phenotypic evolution. Here, we use a quantitative genetic model, including phenotypic plasticity and maternal effects, to suggest that the relationship between fitness and phenotypic variance plays an important role. Intuitive expectations that positive maternal effects are beneficial are supported following an extreme environmental shift, but, if too strong, that shift can also generate oscillatory dynamics that overshoot the optimal phenotype. In a stable environment, negative maternal effects that slow phenotypic evolution actually minimize variance around the optimum phenotype and thus maximize population mean fitness
Erratum: “Exact dynamic properties of molecular motors” [J. Chem. Phys. 137, 084102 (2012)]
Full text linkExact dynamic properties of molecular motors
Full text linkMolecular motors play important roles within a biological cell, performing functions such as intracellular transport and gene transcription. Recent experimental work suggests that there are many plausible biochemical mechanisms that molecules such as myosin-V could use to achieve motion. To account for the abundance of possible discrete-stochastic frameworks that can arise when modeling molecular motor walks, a generalized and straightforward graphical method for calculating their dynamic properties is presented. It allows the calculation of the velocity, dispersion, and randomness ratio for any proposed system through analysis of its structure. This article extends work of King and Altman [“A schematic method of deriving the rate laws of enzyme-catalyzed reactions,” J. Phys. Chem.60, 1375–1378 (1956)]10.1021/j150544a010 on networks of enzymatic reactions by calculating additional dynamic properties for spatially hopping systems. Results for n-state systems are presented: single chain, parallel pathway, divided pathway, and divided pathway with a chain. A novel technique for combining multiple system architectures coupled at a reference state is also demonstrated. Four-state examples illustrate the effectiveness and simplicity of these method
Modelling pattern formation in CO+O2 on Pt{100}
No full textWe extend a detailed kinetic model for CO + O2 on Pt{1 0 0} to describe pattern formation. The model includes: (i) a non-linear power law to describe the phase transition, (ii) trapping and untrapping processes explicitly considered, and (iii) experimentally determined coverage-dependent sticking probabilities and rate constants. This model is extended to include diffusion and gas global coupling. Diffusion is included through a mass-balance equation which couples the migration of CO with the phase transition. Gas global coupling is introduced considering realistic values of the pumping flow, the reactor volume and the size of the crysta
Reply to 'Comment on "Modelling pattern formation in CO+O2 on Pt{100}" '
No full textWe respond to the comment of V.P. Zhdanov on our earlier Letter [Chem. Phys. Lett. 377 (2003) 69]. We give an explicit account of the derivation of our coarse-grained equations for the spatial variation of CO coverages
Early life predictors of obesity and hypertension comorbidity at midlife: findings from the 1958 National Child Development Study (NCDS)
No full textBackground: early life exposures can increase the risk of both obesity and hypertension in adulthood. In this paper we identify exposures across five pre-hypothesised childhood domains, explore them as predictors of obesity and hypertension comorbidity using the 1958 National Child Development Study (NCDS), and discuss these results in comparison to a similar approach using another birth cohort (the 1970 British Cohort Study (BCS70)).Methods: the analytical sample included 9150 participants. The outcome was obesity (BMI of ≥30) and hypertension (blood pressure>140/90mm Hg) comorbidity at age 44. Domains included: ‘prenatal, antenatal, neonatal and birth’, ‘developmental attributes and behaviour’, ‘child education and academic ability’, ‘socioeconomic factors’ and ‘parental and family environment’. Stepwise backward elimination selected variables for inclusion for each domain, and predicted risk scores of obesity-hypertension for each cohort member within each domain were calculated. We performed multivariable logistic regression analysis including domain-specific risk scores, sex and ethnicity to assess how well the outcome could be predicted taking all domains into account. In additional analysis we included potential adult factors.Results: including all domain-specific risk scores, sex, and ethnicity in the same prediction model the area under the curve was 0.70 (95%CI 0.67-0.72). The strongest domain predictor for obesity-hypertension comorbidity was for the socioeconomic factors domain (OR 1.28 95%CI 1.18-1.38), similar to the BCS70 results. However, the parental and family environment domain was not a significant predictor for obesity-hypertension comorbidity (OR 1.08 95%CI 0.94-1.24) unlike the BCS70 results. After considering adult predictors, robust associations remained to the socioeconomic, education and academic abilities, development and behaviour, and prenatal, antenatal, neonatal and birth domains.Conclusions: in the NCDS some early life course domains were found to be significant predictors of obesity-hypertension comorbidity, supporting previous findings. Shared early-life characteristics could have a role in predicting obesity-hypertension comorbidity, particularly for those who faced socioeconomic disadvantage
Exploring the relationship between early life exposures and the comorbidity of obesity and hypertension: findings from the 1970 the British cohort study (BCS70)
No full textBackground: epidemiological research commonly investigates single exposure-outcome relationships, while children’s experiences across a variety of early lifecourse domains are intersecting. To design realistic interventions, epidemiological research should incorporate information from multiple risk exposure domains to assess effect on health outcomes. In this paper we identify exposures across five pre-hypothesised childhood domains and explored their association to the odds of combined obesity and hypertension in adulthood.Methods: we used data from 17,196 participants in the 1970 British Cohort Study. The outcome was obesity (BMI of ≥30) and hypertension (blood pressure>140/90mm Hg or self-reported doctor’s diagnosis) comorbidity at age 46. Early life domains included: ‘prenatal, antenatal, neonatal and birth’, ‘developmental attributes and behaviour’, ‘child education and academic ability’, ‘socioeconomic factors’ and ‘parental and family environment’. Stepwise backward elimination selected variables for inclusion for each domain. Predicted risk scores of combined obesity and hypertension for each cohort member within each domain were calculated. Logistic regression investigated the association between domain-specific risk scores and odds of obesity-hypertension, controlling for demographic factors and other domains.Results: adjusting for demographic confounders, all domains were associated with odds of obesity-hypertension. Including all domains in the same model, higher predicted risk values across the five domains remained associated with increased odds of obesity-hypertension comorbidity, with the strongest associations to the parental and family environment domain (OR1.11 95%CI 1.05-1.18) and the socioeconomic factors domain (OR1.11 95%CI 1.05-1.17).Conclusions: targeted prevention interventions aimed at population groups with shared early-life characteristics could have an impact on obesity-hypertension prevalence which are known risk factors for further morbidity including cardiovascular disease
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