2,615 research outputs found
Hermitian Dirac Hamiltonian in time dependent gravitational field
It is shown by a straightforward argument that the Hamiltonian generating the
time evolution of the Dirac wave function in relativistic quantum mechanics is
not hermitian with respect to the covariantly defined inner product whenever
the background metric is time dependent. An alternative, hermitian, Hamiltonian
is found and is shown to be directly related to the canonical field Hamiltonian
used in quantum field theory.Comment: 9 pages, final version, to appear in Class. Quant. Gra
Proximal ulna comminuted fractures: Fixation using a double-plating technique
SummaryIntroductionComminuted fractures of the proximal ulna are severe injuries often associated with bone and ligament injuries of the elbow joint (Monteggia lesion, radial head fractures, dislocation of the elbow). The treatment of these fractures is very demanding and the functional results often fairly mediocre due to associated injuries. Based on a single-center retrospective study, we report the results of the treatment of these fractures fixed using a double-plate technique. The aim was to evaluate the feasibility and reliability of this fixation mode and to compare it with other fractures series using a single plate fixation (in terms of bone union, elbow joint function, and complications stemming from the plates).Patients and methodsEighteen patients sustained a comminuted proximal ulna fracture between 2002 and 2006. The fractures were associated in five cases with a Monteggia type lesion, in two cases with elbow dislocation, and in four cases with a Mason 3 radial head fracture. Four patients had an open fracture. These comminuted ulna fractures included nine Mayo Clinic IIIB fractures. Bone fixation was performed with two third-cylinder tubular plates, one plate on each side of the proximal ulna. This allows more versatile solutions for screw insertion. Functional assessment (according to Broberg and Morrey) and radiological evaluation (bone healing) were provided at 6Â months and at the longest follow-up by an independent surgeon.ResultsSixteen of 18Â patients achieved bone union. No septic complications occurred and no hardware removal was required on patient request. In 67% of the cases, the Morrey score indicated excellent or good results with a mean score of 82.DiscussionThere are no reports in the literature on the technical point of fixation concerning complex fractures of the ulna. Two plates mean the possibility of twice the number of screw insertions for epiphyseal reconstruction . This fixation remains easy to perform and provides stable anatomic reconstruction of the ulna.Level of evidenceLevel IV. Retrospective study
Abrupt sea surface pH change at the end of the Younger Dryas in the central sub-equatorial Pacific inferred from boron isotope abundance in corals (<i>Porites</i>)
The "ÎŽ<sup>11</sup>B-pH" technique was applied to modern and ancient corals <i>Porites</i> from the sub-equatorial Pacific areas (Tahiti and Marquesas) spanning a time interval from 0 to 20.720 calendar years to determine the amplitude of pH changes between the Last Glacial Period and the Holocene. Boron isotopes were measured by Multi-Collector â Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) with an external reproducibility of 0.25‰, allowing a precision of about ±0.03 pH-units for pH values between 8 and 8.3. The boron concentration [B] and isotopic composition of modern samples indicate that the temperature strongly controls the partition coefficient K<sub><i>D</i></sub> for different aragonite species. Modern coral ÎŽ<sup>11</sup>B values and the reconstructed sea surface pH values for different Pacific areas match the measured pH expressed on the seawater scale and confirm the calculation parameters that were previously determined by laboratory calibration exercises. Most ancient sea surface pH reconstructions near Marquesas are higher than modern values. These values range between 8.19 and 8.27 for the Holocene and reached 8.30 at the end of the last glacial period (20.7 kyr BP). At the end of the Younger Dryas (11.50±0.1 kyr BP), the central sub-equatorial Pacific experienced a dramatic drop of up to 0.2 pH-units from the average pH of 8.2 before and after this short event. Using the marine carbonate algorithms, we recalculated the aqueous <i>p</i>CO<sub>2</sub> to be 440±25 ppmV at around 11.5 kyr BP for corals at Marquesas and ~500 ppmV near Tahiti where it was assumed that <i>p</i>CO<sub>2</sub> in the atmosphere was 250 ppmV. Throughout the Holocene, the difference in <i>p</i>CO<sub>2</sub> between the ocean and the atmosphere at Marquesas (Î<i>p</i>CO<sub>2</sub>) indicates that the surface waters behave as a moderate CO<sub>2</sub> sink or source (â53 to 20 ppmV) during El Niño-like conditions. By contrast, during the last glacial/interglacial transition, this area was a marked source of CO<sub>2</sub> (21 to 92 ppmV) for the atmosphere, highlighting predominant La Niña-like conditions. Such conditions were particularly pronounced at the end of the Younger Dryas with a large amount of CO<sub>2</sub> released with Î<i>p</i>CO<sub>2</sub> of +185±25 ppmV. This last finding provides further evidence of the marked changes in the surface water pH and temperature in the equatorial Pacific at the Younger Dryas-Holocene transition and the strong impact of oceanic dynamic on the atmospheric CO<sub>2</sub> content
Chapter 17 - Economics of adaptation
This chapter assesses the literature on the economics of climate change adaptation, building on the Fourth Assessment Report (AR4) and the increasing role that economic considerations are playing in adaptation decisionmaking and policy. AR4 provided a limited assessment of the costs and benefits of adaptation, based on narrow and fragmented sectoral and regional literature (Adger et al, 2007). Substantial advances have been made in the economics of climate change adaptation after AR4
The play's the thing
For very understandable reasons phenomenological approaches predominate in the field of sensory urbanism. This paper does not seek to add to that particular discourse. Rather it takes Rortyâs postmodernized Pragmatism as its starting point and develops a position on the role of multi-modal design representation in the design process as a means of admitting many voices and managing multidisciplinary collaboration.
This paper will interrogate some of the concepts underpinning the Sensory Urbanism project to help define the scope of interest in multi-modal representations. It will then explore a range of techniques and approaches developed by artists and designers during the past fifty years or so and comment on how they might inform the question of multi-modal representation. In conclusion I will argue that we should develop a heterogeneous tool kit that adopts, adapts and re-invents existing methods because this will better serve our purposes during the exploratory phase(s) of any design project that deals with complexity
Mathisson-Papapetrou equations in metric and gauge theories of gravity in a Lagrangian formulation
We present a simple method to derive the semiclassical equations of motion
for a spinning particle in a gravitational field. We investigate the cases of
classical, rotating particles (pole-dipole particles), as well as particles
with intrinsic spin. We show that, starting with a simple Lagrangian, one can
derive equations for the spin evolution and momentum propagation in the
framework of metric theories of gravity and in theories based on a
Riemann-Cartan geometry (Poincare gauge theory), without explicitly referring
to matter current densities (spin and energy-momentum). Our results agree with
those derived from the multipole expansion of the current densities by the
conventional Papapetrou method and from the WKB analysis for elementary
particles.Comment: 28 page
Impact de l'introduction des légumineuses dans les systÚmes de culture sur les émissions de protoxyde d'azote
Dans un objectif d'attĂ©nuation du rĂ©chauffement climatique, la mise en place de systĂšmes de culture plus autonomes vis - Ă - vis des engrais minĂ©raux devient incontournable pour rĂ©duire les Ă©missions de gaz Ă effet de serre (GES) tout en diminuant le coĂ»t de la fertilisation. L'insertion de lĂ©gumineuses dans ces systĂšmes semble ĂȘtre un levier prometteur pour y parvenir. Les Ă©conomies d'azote permises par diffĂ©rents modes d'introduction de lĂ©gumineuses et leur impact sur les Ă©missions de GES ont Ă©tĂ© estimĂ©s et Ă©valuĂ©s en quantifiant au prĂ©alable les flux d'azote dans les systĂšmes avec ou sans lĂ©gumineuses Ă partir de donnĂ©es issues de dispositifs expĂ©rimentaux prĂ©existants, disponibles dans la bibliographie ou acquises dans de nouveaux essais. L'introduction de lĂ©gumineuses prĂ©sente rĂ©guliĂšrement un impact positif sur la rĂ©duction de l'emploi des engrais azotĂ©s. Le risque d'augmentation de la lixiviation du nitrate dĂ©pend de leur mode d'introduction et de leur place dans les rotations. Enfin, les Ă©missions mesurĂ©es en cul tures principales de lĂ©gumineuses sont gĂ©nĂ©ralement trĂšs significativement rĂ©duites en comparaison des cultures fertilisĂ©es, cependant la dĂ©gradation des rĂ©sidus des lĂ©gumineuses (en culture ou en couverts) peut gĂ©nĂ©rer des Ă©missions de N 2 O corrĂ©lĂ©es Ă leur impact sur les flux d'azote, variable selon les conditions pĂ©doclimatiques. Ces rĂ©sultats devraient permettre d'Ă©laborer des recommandations pour mieux prendre en compte les lĂ©gumineuses dans la conception de systĂšmes de culture plus autonomes vis - Ă - vis des engrais azotĂ©s de synthĂšse et moins Ă©metteurs de N 2 0
On the equivalence between hierarchical segmentations and ultrametric watersheds
We study hierarchical segmentation in the framework of edge-weighted graphs.
We define ultrametric watersheds as topological watersheds null on the minima.
We prove that there exists a bijection between the set of ultrametric
watersheds and the set of hierarchical segmentations. We end this paper by
showing how to use the proposed framework in practice in the example of
constrained connectivity; in particular it allows to compute such a hierarchy
following a classical watershed-based morphological scheme, which provides an
efficient algorithm to compute the whole hierarchy.Comment: 19 pages, double-colum
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