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Search Results: 1 - 10 of 31875 matches for " Thomas Quinn "
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AIDS in Africa: a retrospective
Quinn Thomas C.
Bulletin of the World Health Organization , 2001,
Abstract:
A Statistical Examination of the Short Term Stability of the Upsilon Andromedae System
Rory Barnes,Thomas Quinn
Physics , 2000, DOI: 10.1086/319782
Abstract: Because of the high eccentricities (~0.3) of two of the possible planets about the star Upsilon Andromeda, the stability of the system requires careful study. We present results of 1000 numerical simulations which explore the orbital parameter space as constrained by the observations. The orbital parameters of each planet are chosen from a Gaussian error distribution, and the resulting configuration is integrated for 1,000,000 years. We find that 84% of these integrations are stable. Configurations in which the eccentricity of the third planet is <0.3 are always stable, but when the eccentricity is >0.45 the system is always unstable, typically producing a close encounter between the second and third planets. A similar exercise with the gas giants in our own Solar System sampled with the same error distribution was performed. Approximately 81% of these simulations were stable for 10^6 years.
Finite domination and Novikov rings. Iterative approach
Thomas Huettemann,David Quinn
Mathematics , 2011, DOI: 10.1017/S0017089512000419
Abstract: Suppose C is a bounded chain complex of finitely generated free modules over the Laurent polynomial ring L = R[x,1/x]. Then C is R-finitely dominated, ie, homotopy equivalent over R to a bounded chain complex of finitely generated projective R-modules, if and only if the two chain complexes C((x)) and C((1/x)) are acyclic, as has been proved by Ranicki. Here C((x)) is the tensor product over L of C with the Novikov ring R((x)) = R[[x]][1/x] (also known as the ring of formal Laurent series in x); similarly, C((1/x)) is the tensor product over L of C with the Novikov ring R((1/x)) = R[[1/x]][x]. In this paper, we prove a generalisation of this criterion which allows us to detect finite domination of bounded below chain complexes of projective modules over Laurent rings in several indeterminates.
Finite domination and Novikov rings. Laurent polynomial rings in two variables
Thomas Huettemann,David Quinn
Mathematics , 2012, DOI: 10.1142/S0219498815500553
Abstract: Let C be a bounded cochain complex of finitely generated free modules over the Laurent polynomial ring L = R[x,1/x,y,1/y]. The complex C is called R-finitely dominated if it is homotopy equivalent over R to a bounded complex of finitely generated projective R-modules. Our main result characterises R-finitely dominated complexes in terms of Novikov cohomology: C is R-finitely dominated if and only if eight complexes derived from C are acyclic; these complexes are obtained by tensoring C over L with R[[x,y]][1/xy] and R[x,1/x][[y]][1/y], and their variants obtained by swapping x and y, and replacing either indeterminate by its inverse.
Finite domination and Novikov rings. Laurent polynomial rings in several variables
Thomas Huettemann,David Quinn
Mathematics , 2014,
Abstract: We present a homological characterisation of those chain complexes of modules over a Laurent polynomial ring in several indeterminates which are finitely dominated over the ground ring (that is, are a retract up to homotopy of a bounded complex of finitely generated free modules). The main tools, which we develop in the paper, are a non-standard totalisation construction for multi-complexes based on truncated products, and a high-dimensional mapping torus construction employing a theory of cubical diagrams that commute up to specified coherent homotopies.
Reassessing the Source of Long-Period Comets
Nathan A. Kaib,Thomas Quinn
Physics , 2009, DOI: 10.1126/science.1172676
Abstract: We present numerical simulations to model the production of observable long-period comets (LPCs) from the Oort Cloud, a vast reservoir of icy bodies surrounding the Sun. We show that inner Oort Cloud objects can penetrate Jupiter's orbit via a largely unexplored dynamical pathway, and they are an important, if not the dominant, source of known LPCs. We use this LPC production to place observationally motivated constraints on the population and mass of the inner Oort Cloud, which are consistent with giant planet formation theory. These constraints indicate that only one comet shower producing late Eocene bombardment levels has likely occurred since the Cambrian Explosion, making these phenomena an improbable cause of additional extinction events.
Recent CR1 non-LTR retrotransposon activity in coscoroba reveals an insertion site preference
Judy St John, Thomas W Quinn
BMC Genomics , 2008, DOI: 10.1186/1471-2164-9-567
Abstract: Elucidation of the exact octomer repeat sequence (TTCTGTGA) allowed for the identification of younger insertion events. The number of octomer repeats associated with a CR1 element increases after insertion with CR1s having one octomer being youngest. These young CR1s are flanked by regions of low GC content (38%). Furthermore, a bias for specific bases within the first four positions at the site of insertion was revealed.This study focused on those loci where the insertion event has been most recent, as this would tend to minimize noise introduced by post-integration mutational events. Our data suggest that CR1 is not inserting into regions of higher GC content within the coscoroba genome; but rather, preferentially inserting into regions of lower GC content. Furthermore, there appears to be a base preference (TTCT) for the insertion site. The results of this study increase the current level of understanding regarding the elusive CR1 non-LTR retrotransposon.Eukaryotic genomes contain a large percentage of highly and moderately repetitive DNA [1]. Included in the moderately repetitive DNA, are transposable elements (TEs). TEs are categorized into two main classes. DNA transposons (class II) are able to self-excise and move to a new location in the genome while retrotransposons (class I) use an RNA intermediate resulting in a transposed copy. Retrotransposons can further be divided into two categories, those possessing long terminal repeats of 250–600 base pairs (bp) termed LTR retrotransposons and those without LTRs (non-LTR retrotransposons). Non-LTR retrotransposons are thought to be the oldest of the retrotransposons, originating at least 500–600 million years ago [2]. It has been suggested that the non-LTR retrotransposons gave rise to eukaryotic LTRs, which in turn gave rise to myriad viruses including the vertebrate retroviruses [3].Full length (4–6 kb) non-LTR retrotransposons [2], such as L1 and the taxonomically widely distributed chicken repeat 1 (CR1; Fig.
Photoionization and the Formation of Dwarf Galaxies
Thomas Quinn,Neal Katz,George Efstathiou
Physics , 1995, DOI: 10.1093/mnras/278.4.49L
Abstract: It has been argued that a UV photoionizing background radiation field suppresses the formation of dwarf galaxies, and may even inhibit the formation of larger galaxies. In order to test this, we present gas-dynamical simulations of the formation of small objects in a CDM universe with and without a photoionizing background. The objects are selected from a collisionless simulation at a redshift of 2.4, and rerun at higher resolution including the effects of gas dynamics and using a hierarchical grid of particles. Five objects, each with a circular speed of 46 km/sec are simulated. The presence of the photoionizing background has only a small effect on the amount of gas that collapses in these objects, reducing the amount of cold collapsed gas by at most 30%. Analysis of the smaller objects found in the higher resolution simulation indicates that the photoionizing background only significantly affects the formation of objects with a virialized halo mass less than 10^9 soalr masses and circular speeds less than 23 km/sec. However, the ionization balance is greatly changed by the presence of the background radiation field. Typical lines of sight through the objects have 4 orders of magnitude less neutral hydrogen column density when the photoionizing background is included.
Gasoline: An adaptable implementation of TreeSPH
James Wadsley,Joachim Stadel,Thomas Quinn
Physics , 2003, DOI: 10.1016/j.newast.2003.08.004
Abstract: The key algorithms and features of the Gasoline code for parallel hydrodynamics with self-gravity are described. Gasoline is an extension of the efficient Pkdgrav parallel N-body code using smoothed particle hydrodynamics. Accuracy measurements, performance analysis and tests of the code are presented. Recent successful Gasoline applications are summarized. These cover a diverse set of areas in astrophysics including galaxy clusters, galaxy formation and gas-giant planets. Future directions for gasdynamical simulations in astrophysics and code development strategies for tackling cutting edge problems are discussed.
The evolution of dwarf galaxy satellites with different dark matter density profiles in the ErisMod simulations. I. The early infalls
Mihai Tomozeiu,Lucio Mayer,Thomas Quinn
Physics , 2015,
Abstract: We present the first simulations of tidal stirring of dwarf galaxies in the Local Group carried out in a cosmological context. We use the ErisDARK simulation of a MW-sized galaxy to identify some of the most massive subhalos ($M_{vir} > 10^8 M_{\odot}$) that fall into the main host before $z=2$. Subhalos are replaced before infall with high-resolution models of dwarf galaxies comprising a faint stellar disk embedded in a dark matter halo. The set of models contains cuspy halos as well as halos with "cored" profiles (with asymptotic inner slope $\gamma = 0.6$). The simulations are then run to $z=0$ with as many as 54 million particles and resolution as small as $\sim 4$ pc using the N-Body code ChaNGa. The stellar components of all satellites are significantly affected by tidal stirring, losing stellar mass and undergoing a morphological transformation towards a pressure supported spheroidal system. However, while some remnants with cuspy halos maintain significant rotational flattening and disk-like features, all the shallow halo models achieve $v/\sigma < 0.5$ and round shapes typical of dSph satellites of the MW and M31. Mass loss is also enhanced in the latter, and remnants can reach luminosities and velocity dispersions as low as those of Ultra Faint Dwarfs (UFDs). We argue that cuspy progenitors must be the exception rather than the rule among satellites of the MW since all the MW and M31 satellites in the luminosity range of our remnants are dSphs, a result matched only in the simulation with "cored" models.
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