18,399 research outputs found
Adaptation of NASA technology for the optimization of orthopedic knee implants
The NASA technology originally developed for the optimization of composite structures (engine blades) is adapted and applied to the optimization of orthopedic knee implants. A method is developed enabling the tailoring of the implant for optimal interaction with the environment of the tibia. The shape of the implant components are optimized, such that the stresses in the bone are favorably controlled to minimize bone degradation and prevent failures. A pilot tailoring system is developed and the feasibility of the concept is elevated. The optimization system is expected to provide the means for improving knee prosthesis and individual implant tailoring for each patient
Noncommutative geometry and motives: the thermodynamics of endomotives
We combine aspects of the theory of motives in algebraic geometry with
noncommutative geometry and the classification of factors to obtain a
cohomological interpretation of the spectral realization of zeros of
-functions. The analogue in characteristic zero of the action of the
Frobenius on l-adic cohomology is the action of the scaling group on the cyclic
homology of the cokernel (in a suitable category of motives) of a restriction
map of noncommutative spaces. The latter is obtained through the thermodynamics
of the quantum statistical system associated to an endomotive (a noncommutative
generalization of Artin motives). Semigroups of endomorphisms of algebraic
varieties give rise canonically to such endomotives, with an action of the
absolute Galois group. The semigroup of endomorphisms of the multiplicative
group yields the Bost-Connes system, from which one obtains, through the above
procedure, the desired cohomological interpretation of the zeros of the Riemann
zeta function. In the last section we also give a Lefschetz formula for the
archimedean local L-factors of arithmetic varieties.Comment: 52 pages, amslatex, 1 eps figure, v2: final version to appea
Partisan Polarization And Resistance To Elite Messages: Results From Survey Experiments On Social Distancing
COVID-19 compelled government officials in the U.S. and elsewhere to institute social distancing policies, shuttering much of the economy. At a time of low trust and high polarization, Americans may only support such disruptive policies when recommended by same-party politicians. A related concern is that some may resist advice from “elite” sources such as government officials or public health experts. We test these possibilities using novel data from two online surveys with embedded experiments conducted with approximately 2,000 Pennsylvania residents each, in spring 2020 (Study 1 in April and Study 2 in May-June). We uncover partisan differences in views on several coronavirus-related policies, which grew larger between surveys. Yet overall, Study 1 respondents report strong support for social distancing policies and high trust in medical experts. Moreover, an experiment in Study 1 finds no evidence of reduced support for social distancing policies when advocated by elites, broadly defined. A second experiment in Study 2 finds no backlash for a policy described as being backed by public health experts, but a cross-party decline in support for the same policy when backed by government officials. This suggests that, in polarized times, public health experts might be better advocates for collectively beneficial public policies during public health crises than government officials
How Do Disks Survive Mergers?
We develop a physical model for how galactic disks survive and/or are
destroyed in interactions. Based on dynamical arguments, we show gas primarily
loses angular momentum to internal torques in a merger. Gas within some
characteristic radius (a function of the orbital parameters, mass ratio, and
gas fraction of the merging galaxies), will quickly lose angular momentum to
the stars sharing the perturbed disk, fall to the center and be consumed in a
starburst. A similar analysis predicts where violent relaxation of the stellar
disks is efficient. Our model allows us to predict the stellar and gas content
that will survive to re-form a disk in the remnant, versus being violently
relaxed or contributing to a starburst. We test this in hydrodynamic
simulations and find good agreement as a function of mass ratio, orbital
parameters, and gas fraction, in simulations spanning a wide range in these
properties and others, including different prescriptions for gas physics and
feedback. In an immediate sense, the amount of disk that re-forms can be
understood in terms of well-understood gravitational physics, independent of
details of ISM gas physics or feedback. This allows us to explicitly quantify
the requirements for such feedback to (indirectly) enable disk survival, by
changing the pre-merger gas content and distribution. The efficiency of disk
destruction is a strong function of gas content: we show how and why
sufficiently gas-rich major mergers can, under general conditions, yield
systems with small bulges (B/T<0.2). We provide prescriptions for inclusion of
our results in semi-analytic models.Comment: 32 pages, 16 figures, accepted to ApJ (minor revisions to match
accepted version
A Theoretical Interpretation of the Black Hole Fundamental Plane
We examine the origin and evolution of correlations between properties of
supermassive black holes (BHs) and their host galaxies using simulations of
major galaxy mergers, including the effects of gas dissipation, cooling, star
formation, and BH accretion and feedback. We demonstrate that the simulations
predict the existence of a BH 'fundamental plane' (BHFP), of the form M_BH
sigma^(3.0+-0.3)*R_e^(0.43+-0.19) or M_BH
M_bulge^(0.54+-0.17)*sigma^(2.2+-0.5), similar to relations found
observationally. The simulations indicate that the BHFP can be understood
roughly as a tilted intrinsic correlation between BH mass and spheroid binding
energy, or the condition for feedback coupling to power a pressure-driven
outflow. While changes in halo circular velocity, merger orbital parameters,
progenitor disk redshifts and gas fractions, ISM gas pressurization, and other
parameters can drive changes in e.g. sigma at fixed M_bulge, and therefore
changes in the M_BH-sigma or M_BH-M_bulge relations, the BHFP is robust. Given
the empirical trend of decreasing R_e for a given M_bulge at high redshift, the
BHFP predicts that BHs will be more massive at fixed M_bulge, in good agreement
with recent observations. This evolution in the structural properties of merger
remnants, to smaller R_e and larger sigma (and therefore larger M_BH,
conserving the BHFP) at a given M_bulge, is driven by the fact that bulge
progenitors have characteristically larger gas fractions at high redshifts.
Adopting the observed evolution of disk gas fractions with redshift, our
simulations predict the observed trends in both R_e(M_bulge) and M_BH(M_bulge).Comment: 22 pages, 19 figures, replaced with version accepted to ApJ.
Companion paper to arXiv:0707.400
Toward Innovation In Global Food Regime
The market-oriented focus of the global food regime, as it functioned from the late 1940s to the early 1970s, has proved inadequate. Preoccupation with perfecting markets led food policy makers to underemphasize the need for increased production in the Third World. It also led them to exaggerated attention to short-term surplus disposal and too little concern about scarcity. The regime emerged from a context in which unilateral actions and domestic considerations prevailed. This resulted in regime pathologies in which mutually beneficial international food solutions were not reached and multilateral coordination to analyze and solve food problems was discouraged. Such regime inadequacies cumulated over time; while they did not cause the food crisis of 1973-74, they blunted international responses to it. Reform of the global food regime is needed to (1) raise priorities accorded to rural modernization in Third World countries, (2) increase attention to malnutrition and chronic hunger, (3) provide resources for development, and (4) structure and stabilize the market so as to provide security of supply and income. The legitimacy of multilateral forums and processes also must be enhanced
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