485 research outputs found
Synergists and antagonists of prostigmin
Thesis (Ph.D)--Boston University, 1945. This item was digitized by the Internet Archive
A better step-off algorithm for the knapsack problem
AbstractThe knapsack problem, maximize Σmi = 1 cixi when Σmi = 1 aixi⩽b for integers xi⩾0, can be solved by the classical step-off algorithm. The algorithm develops a series of feasible solutions with ever-increasing objective values. We make a change in the problem so that the step-off algorithm produces a series of solutions of not necessarily increasing objective values. A point is reached when no better solutions can be found and the calculation is stopped
Grease-Resistant O Rings for Joints in Solid Rocket Motors
There is a continuing effort to develop improved O rings for sealing joints in solid-fuel rocket motors. Following an approach based on the lessons learned in the explosion of the space shuttle Challenger, investigators have been seeking O-ring materials that exhibit adequate resilience for effective sealing over a broad temperature range: What are desired are O rings that expand far and fast enough to maintain seals, even when metal sealing surfaces at a joint move slightly away from each other shortly after ignition and the motor was exposed to cold weather before ignition. Other qualities desired of the improved O rings include adequate resistance to ablation by hot rocket gases and resistance to swelling when exposed to hydrocarbon-based greases used to protect some motor components against corrosion. Five rubber formulations two based on a fluorosilicone polymer and three based on copolymers of epichlorohydrin with ethylene oxide were tested as candidate O-ring materials. Of these, one of the epichlorohydrin/ethylene oxide formulations was found to offer the closest to the desired combination of properties and was selected for further evaluation
Methodological Issues in the Study of the Effects of Hemoglobin Variability
We consider estimating the effect of hemoglobin variability on mortality in hemodialysis patients. Causal effects can be defined as comparisons of outcomes under different hypothetical interventions. Defining measures of the effect of hemoglobin variability and clinical outcomes is complicated by the fact that hypothetical interventions on variability used to define its effect inevitably involve manipulation of related variables. We propose a model-based definition of the effect of the hemoglobin variability as a parameter for variability in a causal model for the effect of an overall intervention on hemoglobin levels over time. We consider this problem using history-adjusted marginal structural models, and apply this approach to data from a large observational database. We consider issues arising when the variable of interest is endogenous, and consider in principle alternate estimands
Recommended from our members
Non-propositional analyses of belief.
PhilosophyDoctor of Philosophy (PhD
GALEX Observations of CS and OH Emission in Comet 9P/Tempel 1 During Deep Impact
GALEX observations of comet 9P/Tempel 1 using the near ultraviolet (NUV)
objective grism were made before, during and after the Deep Impact event that
occurred on 2005 July 4 at 05:52:03 UT when a 370 kg NASA spacecraft was
maneuvered into the path of the comet. The NUV channel provides usable spectral
information in a bandpass covering 2000 - 3400 A with a point source spectral
resolving power of approximately 100. The primary spectral features in this
range include solar continuum scattered from cometary dust and emissions from
OH and CS molecular bands centered near 3085 and 2575 A, respectively. In
particular, we report the only cometary CS emission detected during this event.
The observations allow the evolution of these spectral features to be tracked
over the period of the encounter. In general, the NUV emissions observed from
Tempel 1 are much fainter than those that have been observed by GALEX from
other comets. However, it is possible to derive production rates for the parent
molecules of the species detected by GALEX in Tempel 1 and to determine the
number of these molecules liberated by the impact. The derived quiescent
production rates are Q(H2O) = 6.4e27 molecules/s and Q(CS2) = 6.7e24
molecules/s, while the impact produced an additional 1.6e32 H2O molecules and
1.3e29 CS2 molecules, a similar ratio as in quiescent outgassing.Comment: 15 pages, 4 figures, accepted for publication in the Astrophysical
Journa
The Fourth Positive System of Carbon Monoxide in the Hubble Space Telescope Spectra of Comets
The rich structure of the Fourth Positive System (A-X) of carbon monoxide
accounts for many of the spectral features seen in long slit HST-STIS
observations of comets 153P/Ikeya-Zhang, C/2001 Q4 (NEAT), and C/2000 WM1
(LINEAR), as well as in the HST-GHRS spectrum of comet C/1996 B2 Hyakutake. A
detailed CO fluorescence model is developed to derive the CO abundances in
these comets by simultaneously fitting all of the observed A-X bands. The model
includes the latest values for the oscillator strengths and state parameters,
and accounts for optical depth effects due to line overlap and self-absorption.
The model fits yield radial profiles of CO column density that are consistent
with a predominantly native source for all the comets observed by STIS. The
derived CO abundances relative to water in these comets span a wide range, from
0.44% for C/2000 WM1 (LINEAR), 7.2% for 153P/Ikeya-Zhang, 8.8% for C/2001 Q4
(NEAT) to 20.9% for C/1996 B2 (Hyakutake). The subtraction of the CO spectral
features using this model leads to the first identification of a molecular
hydrogen line pumped by solar HI Lyman-beta longward of 1200A in the spectrum
of comet 153P/Ikeya-Zhang. (Abridged)Comment: 12 pages, 11 figures, ApJ accepte
Hubble Space Telescope Observations of Comet 9P/Tempel 1 during the Deep Impact Encounter
We report on the Hubble Space Telescope program to observe periodic comet
9P/Tempel 1 in conjunction with NASA's Deep Impact mission. Our objectives were
to study the generation and evolution of the coma resulting from the impact and
to obtain wide-band images of the visual outburst generated by the impact. Two
observing campaigns utilizing a total of 17 HST orbits were carried out: the
first occurred on 2005 June 13-14 and fortuitously recorded the appearance of a
new, short-lived fan in the sunward direction on June 14. The principal
campaign began two days before impact and was followed by contiguous orbits
through impact plus several hours and then snapshots one, seven, and twelve
days later. All of the observations were made using the Advanced Camera for
Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a
spatial resolution of 36 km (16 km/pixel) at the comet at the time of impact.
Baseline images of the comet, made prior to impact, photometrically resolved
the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement
with the 6.0 +/- 0.2 km diameter derived from the spacecraft imagers. Following
the impact, the HRC images illustrate the temporal and spatial evolution of the
ejecta cloud and allow for a determination of its expansion velocity
distribution. One day after impact the ejecta cloud had passed out of the
field-of-view of the HRC.Comment: 15 pages, 14 postscript figures. Accepted for publication in Icarus
special issue on Deep Impac
- …