728 research outputs found
On a Tree and a Path with no Geometric Simultaneous Embedding
Two graphs and admit a geometric simultaneous
embedding if there exists a set of points P and a bijection M: P -> V that
induce planar straight-line embeddings both for and for . While it
is known that two caterpillars always admit a geometric simultaneous embedding
and that two trees not always admit one, the question about a tree and a path
is still open and is often regarded as the most prominent open problem in this
area. We answer this question in the negative by providing a counterexample.
Additionally, since the counterexample uses disjoint edge sets for the two
graphs, we also negatively answer another open question, that is, whether it is
possible to simultaneously embed two edge-disjoint trees. As a final result, we
study the same problem when some constraints on the tree are imposed. Namely,
we show that a tree of depth 2 and a path always admit a geometric simultaneous
embedding. In fact, such a strong constraint is not so far from closing the gap
with the instances not admitting any solution, as the tree used in our
counterexample has depth 4.Comment: 42 pages, 33 figure
Marine Mammal Strandings
Since the 1980s, there has been growing concern about the health of marine mammal populations in coastal waters and in particular with respect to a decline in harbour porpoise (Phocoena phocoena) numbers. A variety of possible causes have been proposed including infectious diseases, changes in food supply, pollution and entanglement in fishing gear. Recent studies linking contaminant data with disease levels in cetaceans suggest that higher contaminant levels are generally found in animals with a higher number of diseases i.e. that chronic exposure to PCBs or trace metals negatively influences the health status of some cetacean species by predisposing individuals to mortality associated with infectious disease (Jepson et al., 1999; Siebert et al., 1999). The reverse may also be true, that high levels of disease may disable the animal to the extent that coping with contaminants is not possible and toxins accumulate. The potentially serious role of infectious disease was demonstrated by the phocine distemper epidemic of 1987, which killed approximately 18,000 common seals (Phoca vitulina) in the North Sea and adjacent waters (Kennedy, 1990) and by the subsequent morbillivirus epidemic in striped dolphins (Stenella coeruleoalba) in the Mediterranean sea (Domingo et al., 1990). Rejinders (1986) and Brouwer et al. (1989) demonstrated that feeding captive common seals with fish caught in highly polluted waters had deleterious effects on their health and there was speculation that pollution may have been a factor in the severity of these epidemics (Aguilar and Raga, 1990, Aguilar and Borrell, 1994, deSwart et al., 1994). Relatively little work on the health status and contaminant loadings in cetacean and pinniped populations in the Irish Sea has been undertaken to date (e.g., Morris et al., 1989, Law et al., 1995, Berrow et al., 1998a). Given the need for such data from relatively "high-medium" polluted waters (e.g., the Irish Sea) such a data collection programme is highly desirable. For a large number of cetacean and seal species, the only way to assess their health status and contaminant loadings, bar live capture and/or killing them, is through a Strandings Programme. Such programmes involve the recording and recovering of beach cast animals. Strandings programmes allow for some definition of the distribution of different species, but are primarily used to examine health status and to determine population parameters necessary for management decisions. Such programmes are imperative, as they allow top mammalian predators to be monitored and increase our knowledge of a number of biological parameters (for example, age, reproductive status, diet), parasites and contaminant loadings. Cause of death can reflect disease status and in the absence of observer programmes, can indicate fishing associated mortalities (by-catch).Funder: European Unio
Decision making and risk management in adventure sports coaching
Adventure sport coaches practice in environments that are dynamic and high in risk, both perceived and actual. The inherent risks associated with these activities, individuals’ responses and the optimal exploitation of both combine to make the processes of risk management more complex and hazardous than the traditional sports where risk management is focused almost exclusively on minimization. Pivotal to this process is the adventure sports coaches’ ability to make effective judgments regarding levels of risk, potential benefits and possible consequences. The exact nature of this decision making process should form the basis of coaching practice and coach education in this complex and dynamic field. This positional paper examines decision making by the adventure sports coach in these complex, challenging environments and seeks to stimulate debate whilst offering a basis for future research into this topic
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Electron Beam Alignment Strategy in the LCLS Undulators
The x-ray FEL process puts very tight tolerances on the straightness of the electron beam trajectory (2 {micro}m rms) through the LCLS undulator system. Tight but less stringent tolerances of 80 {micro}m rms vertical and 140 {micro}m rms horizontally are to be met for the placement of the individual undulator segments with respect to the beam axis. The tolerances for electron beam straightness can only be met through beam-based alignment (BBA) based on electron energy variations. Conventional alignment will set the start conditions for BBA. Precision-fiducialization of components mounted on remotely adjustable girders and the use of beam-finder wires (BFW) will satisfy placement tolerances. Girder movement due to ground motion and temperature changes will be monitored continuously by an alignment monitoring system (ADS) and remotely corrected. This stabilization of components as well as the monitoring and correction of the electron beam trajectory based on BPMs and correctors will increase the time between BBA applications. Undulator segments will be periodically removed from the undulator Hall and measured to monitor radiation damage and other effects that might degrade undulator tuning
Construction of Modern Robust Nodal Discontinuous Galerkin Spectral Element Methods for the Compressible Navier-Stokes Equations
Discontinuous Galerkin (DG) methods have a long history in computational
physics and engineering to approximate solutions of partial differential
equations due to their high-order accuracy and geometric flexibility. However,
DG is not perfect and there remain some issues. Concerning robustness, DG has
undergone an extensive transformation over the past seven years into its modern
form that provides statements on solution boundedness for linear and nonlinear
problems.
This chapter takes a constructive approach to introduce a modern incarnation
of the DG spectral element method for the compressible Navier-Stokes equations
in a three-dimensional curvilinear context. The groundwork of the numerical
scheme comes from classic principles of spectral methods including polynomial
approximations and Gauss-type quadratures. We identify aliasing as one
underlying cause of the robustness issues for classical DG spectral methods.
Removing said aliasing errors requires a particular differentiation matrix and
careful discretization of the advective flux terms in the governing equations.Comment: 85 pages, 2 figures, book chapte
Engineering an endocrine Neo-Pancreas by repopulation of a decellularized rat pancreas with islets of Langerhans
Decellularization of pancreata and repopulation of these non-immunogenic
matrices with islets and endothelial cells could provide transplantable,
endocrine Neo- Pancreata. In this study, rat pancreata were perfusion
decellularized and repopulated with intact islets, comparing three perfusion
routes (Artery, Portal Vein, Pancreatic Duct). Decellularization effectively
removed all cellular components but conserved the pancreas specific
extracellular matrix. Digital subtraction angiography of the matrices showed a
conserved integrity of the decellularized vascular system but a contrast
emersion into the parenchyma via the decellularized pancreatic duct. Islets
infused via the pancreatic duct leaked from the ductular system into the peri-
ductular decellularized space despite their magnitude. TUNEL staining and
Glucose stimulated insulin secretion revealed that islets were viable and
functional after the process. We present the first available protocol for
perfusion decellularization of rat pancreata via three different perfusion
routes. Furthermore, we provide first proof-of-concept for the repopulation of
the decellularized rat pancreata with functional islets of Langerhans. The
presented technique can serve as a bioengineering platform to generate
implantable and functional endocrine Neo-Pancreata
Neuronal circuitry for pain processing in the dorsal horn
Neurons in the spinal dorsal horn process sensory information, which is then transmitted to several brain regions, including those responsible for pain perception. The dorsal horn provides numerous potential targets for the development of novel analgesics and is thought to undergo changes that contribute to the exaggerated pain felt after nerve injury and inflammation. Despite its obvious importance, we still know little about the neuronal circuits that process sensory information, mainly because of the heterogeneity of the various neuronal components that make up these circuits. Recent studies have begun to shed light on the neuronal organization and circuitry of this complex region
Measurements of Transverse Energy Flow in Deep-Inelastic Scattering at HERA
Measurements of transverse energy flow are presented for neutral current
deep-inelastic scattering events produced in positron-proton collisions at
HERA. The kinematic range covers squared momentum transfers Q^2 from 3.2 to
2,200 GeV^2, the Bjorken scaling variable x from 8.10^{-5} to 0.11 and the
hadronic mass W from 66 to 233 GeV. The transverse energy flow is measured in
the hadronic centre of mass frame and is studied as a function of Q^2, x, W and
pseudorapidity. A comparison is made with QCD based models. The behaviour of
the mean transverse energy in the central pseudorapidity region and an interval
corresponding to the photon fragmentation region are analysed as a function of
Q^2 and W.Comment: 26 pages, 8 figures, submitted to Eur. Phys.
Searches at HERA for Squarks in R-Parity Violating Supersymmetry
A search for squarks in R-parity violating supersymmetry is performed in e^+p
collisions at HERA at a centre of mass energy of 300 GeV, using H1 data
corresponding to an integrated luminosity of 37 pb^(-1). The direct production
of single squarks of any generation in positron-quark fusion via a Yukawa
coupling lambda' is considered, taking into account R-parity violating and
conserving decays of the squarks. No significant deviation from the Standard
Model expectation is found. The results are interpreted in terms of constraints
within the Minimal Supersymmetric Standard Model (MSSM), the constrained MSSM
and the minimal Supergravity model, and their sensitivity to the model
parameters is studied in detail. For a Yukawa coupling of electromagnetic
strength, squark masses below 260 GeV are excluded at 95% confidence level in a
large part of the parameter space. For a 100 times smaller coupling strength
masses up to 182 GeV are excluded.Comment: 32 pages, 14 figures, 3 table
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