559 research outputs found
Local Geometric Invariants of Integrable Evolution Equations
The integrable hierarchy of commuting vector fields for the localized
induction equation of 3D hydrodynamics, and its associated recursion operator,
are used to generate families of integrable evolution equations which preserve
local geometric invariants of the evolving curve or swept-out surface.Comment: 15 pages, AMSTeX file (to appear in Journal of Mathematical Physics
The Conformal Vega Disk
Abstract The relationship between square and circle has intrigued humans since antiquity. Computer visualizations of the conformal equivalence between the two shapes double as mathematical illustrations and as op art after Vasarely. Victor Vasarely created a series of Vega compositions, which explored possibilities for filling a disc with distorted squares, usually creating the illusion of a hemispherical bulge in a planar grid. Inspired by Vasarely (and presumably also Escher), Douglas Dunham [1] recently introduced a non-Euclidean version, in which infinitely many 'hyperbolic squares' fill the Poincare disk (the exterior of which is left blank). The mathematical tool for producin
A Network Analysis of Developmental Change in ADHD Symptom Structure from Preschool to Adulthood
Although there is substantial support for the validity of the diagnosis of attention-deficit/hyperactivity disorder (ADHD), there is considerable disagreement about how to best capture developmental changes in the expression of ADHD symptomatology. This article examines the associations among the 18 individual ADHD symptoms using a novel network analysis approach, from preschool to adulthood. The 1,420 participants were grouped into four age brackets: preschool (ages 3–6, n = 109), childhood (ages 6–12, n = 548), adolescence (ages 13–17, n = 357), and young adulthood (ages 18–36, n = 406). All participants completed a multistage, multi-informant diagnostic process, and self and informant symptom ratings were obtained. Network analysis indicated ADHD symptom structure became more differentiated over development. Two symptoms, often easily distracted and difficulty sustaining attention, appeared as central, or core, symptoms across all age groups. Thus, a small number of core symptoms may warrant extra weighting in future diagnostic systems
Phase Segregation Dynamics in Particle Systems with Long Range Interactions I: Macroscopic Limits
We present and discuss the derivation of a nonlinear non-local
integro-differential equation for the macroscopic time evolution of the
conserved order parameter of a binary alloy undergoing phase segregation. Our
model is a d-dimensional lattice gas evolving via Kawasaki exchange dynamics,
i.e. a (Poisson) nearest-neighbor exchange process, reversible with respect to
the Gibbs measure for a Hamiltonian which includes both short range (local) and
long range (nonlocal) interactions. A rigorous derivation is presented in the
case in which there is no local interaction. In a subsequent paper (part II),
we discuss the phase segregation phenomena in the model. In particular we argue
that the phase boundary evolutions, arising as sharp interface limits of the
family of equations derived in this paper, are the same as the ones obtained
from the corresponding limits for the Cahn-Hilliard equation.Comment: amstex with macros (included in the file), tex twice, 20 page
Phase Separation Kinetics in a Model with Order-Parameter Dependent Mobility
We present extensive results from 2-dimensional simulations of phase
separation kinetics in a model with order-parameter dependent mobility. We find
that the time-dependent structure factor exhibits dynamical scaling and the
scaling function is numerically indistinguishable from that for the
Cahn-Hilliard (CH) equation, even in the limit where surface diffusion is the
mechanism for domain growth. This supports the view that the scaling form of
the structure factor is "universal" and leads us to question the conventional
wisdom that an accurate representation of the scaled structure factor for the
CH equation can only be obtained from a theory which correctly models bulk
diffusion.Comment: To appear in PRE, figures available on reques
Phase Separation of Crystal Surfaces: A Lattice Gas Approach
We consider both equilibrium and kinetic aspects of the phase separation
(``thermal faceting") of thermodynamically unstable crystal surfaces into a
hill--valley structure. The model we study is an Ising lattice gas for a simple
cubic crystal with nearest--neighbor attractive interactions and weak
next--nearest--neighbor repulsive interactions. It is likely applicable to
alkali halides with the sodium chloride structure. Emphasis is placed on the
fact that the equilibrium crystal shape can be interpreted as a phase diagram
and that the details of its structure tell us into which surface orientations
an unstable surface will decompose. We find that, depending on the temperature
and growth conditions, a number of interesting behaviors are expected. For a
crystal in equilibrium with its vapor, these include a low temperature regime
with logarithmically--slow separation into three symmetrically--equivalent
facets, and a higher temperature regime where separation proceeds as a power
law in time into an entire one--parameter family of surface orientations. For a
crystal slightly out of equilibrium with its vapor (slow crystal growth or
etching), power--law growth should be the rule at late enough times. However,
in the low temperature regime, the rate of separation rapidly decreases as the
chemical potential difference between crystal and vapor phases goes to zero.Comment: 16 pages (RevTex 3.0); 12 postscript figures available on request
([email protected]). Submitted to Physical Review E. SFU-JDSDJB-94-0
Profiling Critical Cancer Gene Mutations in Clinical Tumor Samples
Background: Detection of critical cancer gene mutations in clinical tumor specimens may predict patient outcomes and inform treatment options; however, high-throughput mutation profiling remains underdeveloped as a diagnostic approach. We report the implementation of a genotyping and validation algorithm that enables robust tumor mutation profiling in the clinical setting. Methodology: We developed and implemented an optimized mutation profiling platform (“OncoMap”) to interrogate ∼400 mutations in 33 known oncogenes and tumor suppressors, many of which are known to predict response or resistance to targeted therapies. The performance of OncoMap was analyzed using DNA derived from both frozen and FFPE clinical material in a diverse set of cancer types. A subsequent in-depth analysis was conducted on histologically and clinically annotated pediatric gliomas. The sensitivity and specificity of OncoMap were 93.8% and 100% in fresh frozen tissue; and 89.3% and 99.4% in FFPE-derived DNA. We detected known mutations at the expected frequencies in common cancers, as well as novel mutations in adult and pediatric cancers that are likely to predict heightened response or resistance to existing or developmental cancer therapies. OncoMap profiles also support a new molecular stratification of pediatric low-grade gliomas based on BRAF mutations that may have immediate clinical impact. Conclusions: Our results demonstrate the clinical feasibility of high-throughput mutation profiling to query a large panel of “actionable” cancer gene mutations. In the future, this type of approach may be incorporated into both cancer epidemiologic studies and clinical decision making to specify the use of many targeted anticancer agents
Spitzer mapping of molecular hydrogen pure rotational lines in NGC 1333: A detailed study of feedback in star formation
We present mid-infrared spectral maps of the NGC 1333 star forming region,
obtained with the the Infrared Spectrometer on board the Spitzer Space
Telescope. Eight pure H2 rotational lines, from S (0) to S (7), are detected
and mapped. The H2 emission appears to be associated with the warm gas shocked
by the multiple outflows present in the region. A comparison between the
observed intensities and the predictions of detailed shock models indicates
that the emission arises in both slow (12 - 24 km/s) and fast (36 - 53 km/s)
C-type shocks with an initial ortho-to-para ratio of ~ 1. The present H2
ortho-to-para ratio exhibits a large degree of spatial variations. In the
post-shocked gas, it is usually about 2, i.e. close to the equilibrium value (~
3). However, around at least two outflows, we observe a region with a much
lower (~ 0.5) ortho-to-para ratio. This region probably corresponds to gas
which has been heated-up recently by the passage of a shock front, but whose
ortho-to-para has not reached equilibrium yet. This, together with the low
initial ortho-to-para ratio needed to reproduce the observed emission, provide
strong evidence that H2 is mostly in para form in cold molecular clouds. The H2
lines are found to contribute to 25 - 50% of the total outflow luminosity, and
thus can be used to ascertain the importance of star formation feedback on the
natal cloud. From these lines, we determine the outflow mass loss rate and,
indirectly, the stellar infall rate, the outflow momentum and the kinetic
energy injected into the cloud over the embedded phase. The latter is found to
exceed the binding energy of individual cores, suggesting that outflows could
be the main mechanism for core disruption.Comment: Accepted for publication in the Astrophysical Journa
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