34,764 research outputs found
Where is My Attention?
Attention is the key to all learning. Indeed, it is what will save our lives on a daily basis. In all my Communication courses, it is one of the first things I put forward as a question that students may ask of themselves as a check on their “internal considering.”
Philosopher/practitioner George Gurdjieff (1866-1949) called the constant rumble in our minds of random condemnations and re-imaginings of past events as “internal considering.” He felt it was detrimental to ourselves and our relationships with others; it is, I maintain, a constant “self considering” that takes us away from the present and “being here now.” So recognizing that we are often not paying attention to what is happening in the space we are in now, with the people we are with now, is the first step in correcting this distracting state of affairs
Unbiased Instrumental Variables Estimation Under Known First-Stage Sign
We derive mean-unbiased estimators for the structural parameter in
instrumental variables models with a single endogenous regressor where the sign
of one or more first stage coefficients is known. In the case with a single
instrument, there is a unique non-randomized unbiased estimator based on the
reduced-form and first-stage regression estimates. For cases with multiple
instruments we propose a class of unbiased estimators and show that an
estimator within this class is efficient when the instruments are strong. We
show numerically that unbiasedness does not come at a cost of increased
dispersion in models with a single instrument: in this case the unbiased
estimator is less dispersed than the 2SLS estimator. Our finite-sample results
apply to normal models with known variance for the reduced-form errors, and
imply analogous results under weak instrument asymptotics with an unknown error
distribution
Analysis of thermal stress and metal movement during welding
Objectives of study were: investigation of temperature changes caused by welding arc with analysis of temperature distribution; development of system of mathematical statements describing thermal stresses and plastic strains during welding; and development of system of mathematical solutions and computer programs for one-dimensional analysis
Can grain growth explain transition disks?
Aims: Grain growth has been suggested as one possible explanation for the
diminished dust optical depths in the inner regions of protoplanetary
"transition" disks. In this work, we directly test this hypothesis in the
context of current models of grain growth and transport.
Methods: A set of dust evolution models with different disk shapes, masses,
turbulence parameters, and drift efficiencies is combined with radiative
transfer calculations in order to derive theoretical spectral energy
distributions (SEDs) and images.
Results: We find that grain growth and transport effects can indeed produce
dips in the infrared SED, as typically found in observations of transition
disks. Our models achieve the necessary reduction of mass in small dust by
producing larger grains, yet not large enough to be fragmenting efficiently.
However, this population of large grains is still detectable at millimeter
wavelengths. Even if perfect sticking is assumed and radial drift is neglected,
a large population of dust grains is left behind because the time scales on
which they are swept up by the larger grains are too long. This mechanism thus
fails to reproduce the large emission cavities observed in recent
millimeter-wave interferometric images of accreting transition disks.Comment: 11 pages, 5 figures, accepted to A&
Nonlinear spectroscopic studies using sum- and difference-frequency generation
A detailed study is presented of resonant sum- and difference-frequency generation for the mixed-crystal system azulene in naphthalene. The S0 -> S1, S0 -> S2 and S1 -> S2 transitions of azulene provide the 3 needed dipoles for the existence of c(2), and a relatively small static elec. field was used to break the interference between the waves generated in the 2 crystal sublattices. Second-order nonlinear processes can occur sep. in each sublattice. All the theor. predicted resonances, except the DICE effect, were obsd. The generated field intensity satd. at higher fields, and in certain circumstances exhibited hysteresis as a function of the d.c. field strength. The line-narrowing capabilities of these new forms of spectroscopy were explored
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