18,671 research outputs found
Toe Tip Morphology in Six Species of Salamanders, genus Ambystoma (Caudata: Ambystomatidae) from Arkansas Using Scanning Electron Microscopy
The toe tip friction surface in six species of Ambystoma (A. annulatum, A.maculatum, A.opacum, A. talpoideum, A. texanum, and A. tigrinum) from Arkansas was examined using scanning electron microscopy. We found no sexual dimorphism in cell surface ultrastructure. Variation within and between species was considerable. The most active burrower, A. tigrinum, possessed the most disorganized cell surface, whereas the least active burrowers (A.annulatum, A.maculatum, and A. opacum) had morphologically similar and relatively smooth toe tips. In A. talpoideum and A. texanum, cell surfaces exhibited microprojections. Only these two species possessed mucous pores in close proximity to the friction surface. The microstructure of cell surfaces transcended species groups in Ambystoma and would not represent a reliable taxonomic tool
Environmental chemical exposures and breast cancer
As a hormone-sensitive condition with no single identifiable cause, breast cancer is a major health problem. It is characterized by a wide range of contributing factors and exposures occurring in different combinations and strengths across a lifetime that may be amplified during periods of enhanced developmental susceptibility and impacted by reproductive patterns and behaviours. The vast majority of cases are oestrogen-receptor positive and occur in women with no family history of the disease suggesting that modifiable risk factors are involved. A substantial body of evidence now links oestrogen-positive breast cancer with environmental exposures. Synthetic chemicals capable of oestrogen mimicry are characteristic of industrial development and have been individually and extensively assessed as risk factors for oestrogen-sensitive cancers. Existing breast cancer risk assessment tools do not take such factors into account. In the absence of consensus on causation and in order to better understand the problem of escalating incidence globally, an expanded, integrated approach broadening the inquiry into individual susceptibility breast cancer is proposed. Applying systems thinking to existing data on oestrogen-modulating environmental exposures and other oestrogenic factors characteristic of Westernisation and their interactions in the exposure, encompassing social, behavioural, environmental, hormonal and genetic factors, can assist in understanding cancer risks and the pursuit of prevention strategies. A new conceptual framework based on a broader understanding of the “system” that underlies the development of breast cancer over a period of many years, incorporating the factors known to contribute to breast cancer risk, could provide a new platform from which government and regulators can promulgate enhanced and more effective prevention strategies
Nondestructive evaluation of advanced ceramics
A review is presented of Lewis Research Center efforts to develop nondestructive evaluation techniques for characterizing advanced ceramic materials. Various approaches involved the use of analytical ultrasonics to characterize monolythic ceramic microstructures, acousto-ultrasonics for characterizing ceramic matrix composites, damage monitoring in impact specimens by microfocus X-ray radiography and scanning ultrasonics, and high resolution computed X-ray tomography to identify structural features in fiber reinforced ceramics
QCD Constituent Counting Rules for Neutral Vector Mesons
QCD constituent counting rules define the scaling behavior of exclusive
hadronic scattering and electromagnetic scattering amplitudes at high momentum
transfer in terms of the total number of fundamental constituents in the
initial and final states participating in the hard subprocess. The scaling laws
reflect the twist of the leading Fock state for each hadron and hence the
leading operator that creates the composite state from the vacuum. Thus, the
constituent counting scaling laws can be used to identify the twist of exotic
hadronic candidates such as tetraquarks and pentaquarks. Effective field
theories must consistently implement the scaling rules in order to be
consistent with the fundamental theory. Here we examine how one can apply
constituent counting rules for the exclusive production of one or two neutral
vector mesons in annihilation, processes in which the can
couple via intermediate photons. In case of a (narrow) real , the photon
virtuality is fixed to a precise value , in effect treating
the as a single fundamental particle. Each real thus contributes to
the constituent counting rules with . In effect, the leading
operator underlying the has twist 1. Thus, in the specific physical case
of single or double on-shell production via intermediate photons, the
predicted scaling from counting rules coincides with Vector Meson Dominance
(VMD), an effective theory that treats as an elementary field. However,
the VMD prediction fails in the general case where the is not coupled
through an elementary photon field, and then the leading-twist interpolating
operator has twist . Analogous effects appear in scattering
processes.Comment: 15 page
QCD Compositeness as Revealed in Exclusive Vector Boson Reactions through Double-Photon Annihilation: and
We study the exclusive double-photon annihilation processes, and where the is a neutral vector meson produced in the
forward kinematical region: and . We
show how the differential cross sections , as predicted by
QCD, have additional falloff in the momentum transfer squared due to the
QCD compositeness of the hadrons, consistent with the leading-twist
fixed- scaling laws. However, even though they are exclusive
channels and not associated with the conventional electron-positron
annihilation process these total cross
sections and integrated over the dominant forward- and backward-
angular domains, scale as , and thus contribute to the leading-twist
scaling behavior of the ratio . We generalize these results to
exclusive double-electroweak vector-boson annihilation processes accompanied by
the forward production of hadrons, such as and . These results can also be applied to the exclusive production
of exotic hadrons such as tetraquarks, where the cross-section scaling behavior
can reveal their multiquark nature.Comment: 10 page
A map on the space of rational functions
We describe dynamical properties of a map defined on the space
of rational functions. The fixed points of are classified and
the long time behavior of a subclass is described in terms of Eulerian
polynomials
PDEs with Compressed Solutions
Sparsity plays a central role in recent developments in signal processing,
linear algebra, statistics, optimization, and other fields. In these
developments, sparsity is promoted through the addition of an norm (or
related quantity) as a constraint or penalty in a variational principle. We
apply this approach to partial differential equations that come from a
variational quantity, either by minimization (to obtain an elliptic PDE) or by
gradient flow (to obtain a parabolic PDE). Also, we show that some PDEs can be
rewritten in an form, such as the divisible sandpile problem and
signum-Gordon. Addition of an term in the variational principle leads to
a modified PDE where a subgradient term appears. It is known that modified PDEs
of this form will often have solutions with compact support, which corresponds
to the discrete solution being sparse. We show that this is advantageous
numerically through the use of efficient algorithms for solving based
problems.Comment: 21 pages, 15 figure
Collected notes from the Benchmarks and Metrics Workshop
In recent years there has been a proliferation of proposals in the artificial intelligence (AI) literature for integrated agent architectures. Each architecture offers an approach to the general problem of constructing an integrated agent. Unfortunately, the ways in which one architecture might be considered better than another are not always clear. There has been a growing realization that many of the positive and negative aspects of an architecture become apparent only when experimental evaluation is performed and that to progress as a discipline, we must develop rigorous experimental methods. In addition to the intrinsic intellectual interest of experimentation, rigorous performance evaluation of systems is also a crucial practical concern to our research sponsors. DARPA, NASA, and AFOSR (among others) are actively searching for better ways of experimentally evaluating alternative approaches to building intelligent agents. One tool for experimental evaluation involves testing systems on benchmark tasks in order to assess their relative performance. As part of a joint DARPA and NASA funded project, NASA-Ames and Teleos Research are carrying out a research effort to establish a set of benchmark tasks and evaluation metrics by which the performance of agent architectures may be determined. As part of this project, we held a workshop on Benchmarks and Metrics at the NASA Ames Research Center on June 25, 1990. The objective of the workshop was to foster early discussion on this important topic. We did not achieve a consensus, nor did we expect to. Collected here is some of the information that was exchanged at the workshop. Given here is an outline of the workshop, a list of the participants, notes taken on the white-board during open discussions, position papers/notes from some participants, and copies of slides used in the presentations
Periodicity of mass extinctions without an extraterrestrial cause
We study a lattice model of a multi-species prey-predator system. Numerical
results show that for a small mutation rate the model develops irregular
long-period oscillatory behavior with sizeable changes in a number of species.
The periodicity of extinctions on Earth was suggested by Raup and Sepkoski but
so far is lacking a satisfactory explanation. Our model indicates that this is
a natural consequence of the ecosystem dynamics, not the result of any
extraterrestrial cause.Comment: 4 pages, accepted in Phys.Rev.
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