The Disciplined Use of Simplifying Assumptions

Submitted to the ACM SIGSOFT Second Software Engineering Symposium: Workshop on Rapid Prototyping. Columbia, Maryland, April 19-21, 1982.Simplifying assumptions — everyone uses them but no one's programming tool explicitly supports them. In programming, as in other kinds of engineering design, simplifying assumptions are an important method for dealing with complexity. Given a complex programming problem, expert programmers typically choose simplifying assumptions which, though false, allow them to arrive rapidly at a program which addresses the important features of the problem without being distracted by all of its details. The simplifying assumptions are then incrementally retracted with corresponding modifications to the initial program. This methodology is particularly applicable to rapid prototyping because the main questions of interest can often be answered using only the initial program. Simplifying assumptions can easily be misused. In order to use them effectively two key issues must be addressed. First, simplifying assumptions should be chosen which simplify the design problems significantly without changing the essential character of the program which needs to be implemented. Second, the designer must keep track of all the assumptions he is making so that he can later retract them in an orderly manner. By explicitly dealing with these issues, a programming assistant system could directly support the use of simplifying assumptions as a disciplined part of the software development process.MIT Artificial Intelligence Laborator

Study of vibrational excitation mechanisms of carbon dioxide at high temperatures

Vibrational relaxation models of carbon dioxide at high temperature

Study of vibrational excitation mechanisms of CO2 at high temperatures

Calculating vibrational excitation of CO2 for anharmonic coupling and normal mode at high temperatur

The coupling between pulsation and mass loss in massive stars

To what extent can pulsational instabilities resolve the mass-loss problem of massive stars? How important is pulsation in structuring and modulating the winds of these stars? What role does pulsation play in redistributing angular momentum in massive stars? Although I cannot offer answers to these questions, I hope at the very least to explain how they come to be asked.Comment: 12 pages, 1 figure, to appear in proceedings of "Unsolved Problems in Stellar Physics" conference (Cambridge, UK, July 2007

Carbon monoxide consumption and production by wetland peats

Wetland peats were analyzed for their potential to consume and produce carbon monoxide (CO) under aerobic and anaerobic conditions. Kinetic and functional characteristics of anaerobic CO consumption were compared with those of methanogenesis. Inhibitors of methanogenesis and sulfate reduction decreased the rate of CO consumption by 30 and 20%, respectively, suggesting that methanogens and sulfate reducers played secondary roles in CO uptake. Low concentrations of nitrate (0.2 mM) stimulated CO uptake, while high concentrations (20 mM) were partially inhibitory. Sulfate (20 mM), ferric iron (60 μmol cm-3), and acetate (10 mM) had no effect on CO consumption. Formate and glucose (10 mM) temporarily stimulated net CO and H2 production. Aerobic incubations of previously anaerobic peat stimulated transient CO production. Kinetic analysis of anaerobic CO consumption by two sediment types (organic peat and mineral silt) showed that maximum potential uptake velocities (V(maxp)) in each sediment were similar, 1-2 nmol CO cm-3 sediment h-1, with apparent half saturation constants (K(app)) ranging from 5 to 37 nM CO. Anaerobic CO consumption may limit CO accumulation in wetland peats and sediments, thereby affecting CO emissions. Understanding the role and characteristics of wetland CO consumption may help explain current and future patterns in wetland CO dynamics. Copyright (C) 1999 Federation of European Microbiological Societies

Effect of Estrous Synchronization with Natural Service or Fixed-Timed Artificial Insemination with Conventional or Gender-Skewed Semen in Beef Females

The objective of this study was to evaluate estrous synchronization and/or artificial insemination (AI) with conventional (CON) or gender-skewed (SEXED) semen on calving distribution

Deep HST Imaging in NGC 6397: Stellar Dynamics

Multi-epoch observations with ACS on HST provide a unique and comprehensive probe of stellar dynamics within NGC 6397. We are able to confront analytic models of the globular cluster with the observed stellar proper motions. The measured proper motions probe well along the main sequence from 0.8 to below 0.1 M$_\odot$ as well as white dwarfs younger than one gigayear. The observed field lies just beyond the half-light radius where standard models of globular cluster dynamics (e.g. based on a lowered Maxwellian phase-space distribution) make very robust predictions for the stellar proper motions as a function of mass. The observed proper motions show no evidence for anisotropy in the velocity distribution; furthermore, the observations agree in detail with a straightforward model of the stellar distribution function. We do not find any evidence that the young white dwarfs have received a natal kick in contradiction with earlier results. Using the observed proper motions of the main-sequence stars, we obtain a kinematic estimate of the distance to NGC 6397 of $2.2^{+0.5}_{-0.7}$ kpc and a mass of the cluster of $1.1 \pm 0.1 \times 10^5 \mathrm{M}_\odot$ at the photometric distance of 2.53 kpc. One of the main-sequence stars appears to travel on a trajectory that will escape the cluster, yielding an estimate of the evaporation timescale, over which the number of stars in the cluster decreases by a factor of e, of about 3 Gyr. The proper motions of the youngest white dwarfs appear to resemble those of the most massive main-sequence stars, providing the first direct constraint on the relaxation time of the stars in a globular cluster of greater than or about 0.7 Gyr.Comment: 25 pages, 20 figures, accepted for publication in Astrophysical Journa

IUE observations of the clusters of the Magellanic Clouds

IUE observations of 17 clusters in the Large Magellanic Cloud are presented. The derived ultraviolet colors become progressively redder as the cluster's classification in the scheme of Searle, Wilkinson, and Bagnuolo increases. With the aid of theoretical models of the integrated light of coeval clusters of varying ages, it is shown that the SWB classification is one of increasing age accompanied by decreasing metallicity toward later SWB classes. The behavior of the UV absorption line features in the integrated light of the LMC clusters supports the contention