12,507 research outputs found

    One Out of Every Five: Teen Mothers and Subsequent Childbearing

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    The birth of a child to a teenager puts the young family at risk for negative social and health consequences; the birth of additional children can further impede the family's financial, academic, and social success. Though there is a national interest in reducing the teen birth rate, strategies designed to achieve this goal often insufficiently target a readily identifiable group -teens who are already mothers. Of those programs that do target teen mothers, few have been able to demonstrate success. Teen mothers should be targeted for pregnancy prevention not only because they contribute to the teen birth rate with its attendant consequences, but also because second and higher-order births to teenaged mothers often limits life options further than having only one child.Compared to a teen mother with one child, a teenager with two or more children typically faces: lower educational attainment;greater likelihood of poverty; andimpaired health for the infant

    Impact of Electron-Electron Cusp on Configuration Interaction Energies

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    The effect of the electron-electron cusp on the convergence of configuration interaction (CI) wave functions is examined. By analogy with the pseudopotential approach for electron-ion interactions, an effective electron-electron interaction is developed which closely reproduces the scattering of the Coulomb interaction but is smooth and finite at zero electron-electron separation. The exact many-electron wave function for this smooth effective interaction has no cusp at zero electron-electron separation. We perform CI and quantum Monte Carlo calculations for He and Be atoms, both with the Coulomb electron-electron interaction and with the smooth effective electron-electron interaction. We find that convergence of the CI expansion of the wave function for the smooth electron-electron interaction is not significantly improved compared with that for the divergent Coulomb interaction for energy differences on the order of 1 mHartree. This shows that, contrary to popular belief, description of the electron-electron cusp is not a limiting factor, to within chemical accuracy, for CI calculations.Comment: 11 pages, 6 figures, 3 tables, LaTeX209, submitted to The Journal of Chemical Physic

    Editorial Updates

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    Dynamical Monte Carlo Study of Equilibrium Polymers : Static Properties

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    We report results of extensive Dynamical Monte Carlo investigations on self-assembled Equilibrium Polymers (EP) without loops in good solvent. (This is thought to provide a good model of giant surfactant micelles.) Using a novel algorithm we are able to describe efficiently both static and dynamic properties of systems in which the mean chain length \Lav is effectively comparable to that of laboratory experiments (up to 5000 monomers, even at high polymer densities). We sample up to scission energies of E/kBT=15E/k_BT=15 over nearly three orders of magnitude in monomer density ϕ\phi, and present a detailed crossover study ranging from swollen EP chains in the dilute regime up to dense molten systems. Confirming recent theoretical predictions, the mean-chain length is found to scale as \Lav \propto \phi^\alpha \exp(\delta E) where the exponents approach αd=δd=1/(1+γ)0.46\alpha_d=\delta_d=1/(1+\gamma) \approx 0.46 and αs=1/2[1+(γ1)/(νd1)]0.6,δs=1/2\alpha_s = 1/2 [1+(\gamma-1)/(\nu d -1)] \approx 0.6, \delta_s=1/2 in the dilute and semidilute limits respectively. The chain length distribution is qualitatively well described in the dilute limit by the Schulz-Zimm distribution \cN(s)\approx s^{\gamma-1} \exp(-s) where the scaling variable is s=\gamma L/\Lav. The very large size of these simulations allows also an accurate determination of the self-avoiding walk susceptibility exponent γ1.165±0.01\gamma \approx 1.165 \pm 0.01. ....... Finite-size effects are discussed in detail.Comment: 15 pages, 14 figures, LATE

    Mechanical compatibility of sol–gel annealing with titanium for orthopaedic prostheses

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    Sol–gel processing is an attractive method for large-scale surface coating due to its facile and inexpensive preparation, even with the inclusion of precision nanotopographies. These are desirable traits for metal orthopaedic prostheses where ceramic coatings are known to be osteoinductive and the effects may be amplified through nanotexturing. However there are a few concerns associated with the application of sol–gel technology to orthopaedics. Primarily, the annealing stage required to transform the sol–gel into a ceramic may compromise the physical integrity of the underlying metal. Secondly, loose particles on medical implants can be carcinogenic and cause inflammation so the coating needs to be strongly bonded to the implant. These concerns are addressed in this paper. Titanium, the dominant material for orthopaedics at present, is examined before and after sol–gel processing for changes in hardness and flexural modulus. Wear resistance, bending and pull tests are also performed to evaluate the ceramic coating. The findings suggest that sol–gel coatings will be compatible with titanium implants for an optimum temperature of 500 °C

    Into the Future, Darkly

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    What physical exam techniques are useful to detect malingering?

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    No examination technique objectively proves malingering (strength of recommendation [SOR]: C, expert opinion). Waddell's signs are associated with poor treatment outcomes but cannot discriminate organic from nonorganic causes (SOR: B, systematic review of low-quality studies). Hoover's and the Abductor sign indicate nonorganic paralysis (SOR: C, small, lower-quality case-control studies)
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