175 research outputs found
Long path and cycle decompositions of even hypercubes
We consider edge decompositions of the -dimensional hypercube into
isomorphic copies of a given graph . While a number of results are known
about decomposing into graphs from various classes, the simplest cases of
paths and cycles of a given length are far from being understood. A conjecture
of Erde asserts that if is even, and divides the number
of edges of , then the path of length decomposes . Tapadia et
al.\ proved that any path of length , where , satisfying these
conditions decomposes . Here, we make progress toward resolving Erde's
conjecture by showing that cycles of certain lengths up to
decompose . As a consequence, we show that can be decomposed into
copies of any path of length at most dividing the number of edges of
, thereby settling Erde's conjecture up to a linear factor
Polychromatic Colorings on the Integers
We show that for any set , there exists a
3-coloring of in which every translate of receives all three
colors. This implies that has a codensity of at most , proving a
conjecture of Newman [D. J. Newman, Complements of finite sets of integers,
Michigan Math. J. 14 (1967) 481--486]. We also consider related questions in
, .Comment: 16 pages, improved presentatio
The importance of episodic accretion for low-mass star formation
A star acquires much of its mass by accreting material from a disc. Accretion
is probably not continuous but episodic. We have developed a method to include
the effects of episodic accretion in simulations of star formation. Episodic
accretion results in bursts of radiative feedback, during which a protostar is
very luminous, and its surrounding disc is heated and stabilised. These bursts
typically last only a few hundred years. In contrast, the lulls between bursts
may last a few thousand years; during these lulls the luminosity of the
protostar is very low, and its disc cools and fragments. Thus, episodic
accretion enables the formation of low-mass stars, brown dwarfs and
planetary-mass objects by disc fragmentation. If episodic accretion is a common
phenomenon among young protostars, then the frequency and duration of accretion
bursts may be critical in determining the low-mass end of the stellar initial
mass function.Comment: To appear in the Astrophysical Journal. Press release available at:
http://www.astro.cf.ac.uk/pub/Dimitrios.Stamatellos/News/News.html Full
resolution paper available at http://stacks.iop.org/0004-637X/730/3
Modelling circumstellar discs with 3D radiation hydrodynamics
We present results from combining a grid-based radiative transfer code with a
Smoothed Particle Hydrodynamics code to produce a flexible system for modelling
radiation hydrodynamics. We use a benchmark model of a circumstellar disc to
determine a robust method for constructing a gridded density distribution from
SPH particles. The benchmark disc is then used to determine the accuracy of the
radiative transfer results. We find that the SED and the temperature
distribution within the disc are sensitive to the representation of the disc
inner edge, which depends critically on both the grid and SPH resolution. The
code is then used to model a circumstellar disc around a T-Tauri star. As the
disc adjusts towards equilibrium vertical motions in the disc are induced
resulting in scale height enhancements which intercept radiation from the
central star. Vertical transport of radiation enables these perturbations to
influence the mid-plane temperature of the disc. The vertical motions decay
over time and the disc ultimately reaches a state of simultaneous hydrostatic
and radiative equilibrium.Comment: MNRAS accepted; 15 pages; 17 figures, 4 in colou
A retrospective analysis of geriatric trauma patients: venous lactate is a better predictor of mortality than traditional vital signs
BACKGROUND: Traditional vital signs (TVS), including systolic blood pressure (SBP), heart rate (HR) and their composite, the shock index, may be poor prognostic indicators in geriatric trauma patients. The purpose of this study is to determine whether lactate predicts mortality better than TVS. METHODS: We studied a large cohort of trauma patients age ≥ 65 years admitted to a level 1 trauma center from 2009-01-01 - 2011-12-31. We defined abnormal TVS as hypotension (SBP < 90 mm Hg) and/or tachycardia (HR > 120 beats/min), an elevated shock index as HR/SBP ≥ 1, an elevated venous lactate as ≥ 2.5 mM, and occult hypoperfusion as elevated lactate with normal TVS. The association between these variables and in-hospital mortality was compared using Chi-square tests and multivariate logistic regression. RESULTS: There were 1987 geriatric trauma patients included, with an overall mortality of 4.23% and an incidence of occult hypoperfusion of 20.03%. After adjustment for GCS, ISS, and advanced age, venous lactate significantly predicted mortality (OR: 2.62, p < 0.001), whereas abnormal TVS (OR: 1.71, p = 0.21) and SI ≥ 1 (OR: 1.18, p = 0.78) did not. Mortality was significantly greater in patients with occult hypoperfusion compared to patients with no sign of circulatory hemodynamic instability (10.67% versus 3.67%, p < 0.001), which continued after adjustment (OR: 2.12, p = 0.01). CONCLUSIONS: Our findings demonstrate that occult hypoperfusion was exceedingly common in geriatric trauma patients, and was associated with a two-fold increased odds of mortality. Venous lactate should be measured for all geriatric trauma patients to improve the identification of hemodynamic instability and optimize resuscitative efforts
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