488 research outputs found
A path integral derivation of -genus
The formula for the Hirzebruch -genus of complex manifolds is a
consequence of the Hirzebruch-Riemann-Roch formula. The classical index
formulae for Todd genus, Euler number, and Signature correspond to the case
when the complex variable 0, -1, and 1 respectively. Here we give a {\it
direct} derivation of this nice formula based on supersymmetric quantum
mechanics.Comment: 5 page
Les étudiant·e·s en médecine mènent une recherche dans La communauté. [Male/female medical students conduct research in the community]
Pendant quatre semaines, les étudiant(e)s en médecine de 3e année de l'Université de Lausanne mènent une enquête dans la communauté sur le sujet de leur choix. L'objectif de ce module est de faire découvrir aux futurs médecins les déterminants non biomédicaux de la santé, de la maladie et de l'exercice de la médecine : les styles de vie, les facteurs psychosociaux et culturels, l'environnement, les décisions politiques, les contraintes économiques, les questions éthiques, etc.
Par groupes de cinq, les étudiant(e)s commencent par définir une question de recherche originale et en explorent la littérature scientifique. Leur travail de recherche les amène à entrer en contact avec le réseau d'acteurs de la communauté concernés, professionnels ou associations de patients dont ils analysent les rôles et influences respectives. Chaque groupe est accompagné par un(e) tuteur(trice), enseignant(e) de la Faculté de biologie et de médecine de l'Université de Lausanne. Les étudiant(e)s présentent la synthèse de leurs travaux pendant un congrès de deux jours à la fin du module. Quatre travaux parmi les plus remarquables ont été choisis pour être publiés dans la Revue Médicale Suisse et Primary Care
Isotropic representation of noncommutative 2D harmonic oscillator
We show that 2D noncommutative harmonic oscillator has an isotropic
representation in terms of commutative coordinates. The noncommutativity in the
new mode, induces energy level splitting, and is equivalent to an external
magnetic field effect. The equivalence of the spectra of the isotropic and
anisotropic representation is traced back to the existence of SU(2) invariance
of the noncommutative model.Comment: 15 pages, RevTex4, no figures; article format, improved version of
the previous paper; new references and aknowledgements adde
Towards a predictive multi-phase model for alpine mass movements and process cascades
Alpine mass movements can generate process cascades involving different materials including rock, ice, snow, and water. Numerical modelling is an essential tool for the quantification of natural hazards. Yet, state-of-the-art operational models are based on parameter back-calculation and thus reach their limits when facing unprecedented or complex events. Here, we advance our predictive capabilities for mass movements and process cascades on the basis of a three-dimensional numerical model, coupling fundamental conservation laws to finite strain elastoplasticity. In this framework, model parameters have a true physical meaning and can be evaluated from material testing, thus conferring to the model a strong predictive nature. Through its hybrid Eulerian–Lagrangian character, our approach naturally reproduces fractures and collisions, erosion/deposition phenomena, and multi-phase interactions, which finally grant accurate simulations of complex dynamics. Four benchmark simulations demonstrate the physical detail of the model and its applicability to real-world full-scale events, including various materials and ranging through five orders of magnitude in volume. In the future, our model can support risk-management strategies through predictions of the impact of potentially catastrophic cascading mass movements at vulnerable sites
A viscoelastic deadly fluid in carnivorous pitcher plants
Background : The carnivorous plants of the genus Nepenthes, widely
distributed in the Asian tropics, rely mostly on nutrients derived from
arthropods trapped in their pitcher-shaped leaves and digested by their
enzymatic fluid. The genus exhibits a great diversity of prey and pitcher forms
and its mechanism of trapping has long intrigued scientists. The slippery inner
surfaces of the pitchers, which can be waxy or highly wettable, have so far
been considered as the key trapping devices. However, the occurrence of species
lacking such epidermal specializations but still effective at trapping insects
suggests the possible implication of other mechanisms. Methodology/Principal
Findings : Using a combination of insect bioassays, high-speed video and
rheological measurements, we show that the digestive fluid of Nepenthes
rafflesiana is highly viscoelastic and that this physical property is crucial
for the retention of insects in its traps. Trapping efficiency is shown to
remain strong even when the fluid is highly diluted by water, as long as the
elastic relaxation time of the fluid is higher than the typical time scale of
insect movements. Conclusions/Significance : This finding challenges the common
classification of Nepenthes pitchers as simple passive traps and is of great
adaptive significance for these tropical plants, which are often submitted to
high rainfalls and variations in fluid concentration. The viscoelastic trap
constitutes a cryptic but potentially widespread adaptation of Nepenthes
species and could be a homologous trait shared through common ancestry with the
sundew (Drosera) flypaper plants. Such large production of a highly
viscoelastic biopolymer fluid in permanent pools is nevertheless unique in the
plant kingdom and suggests novel applications for pest control
Correlated and zonal errors of global astrometric missions: a spherical harmonic solution
We propose a computer-efficient and accurate method of estimation of
spatially correlated errors in astrometric positions, parallaxes and proper
motions obtained by space and ground-based astrometry missions. In our method,
the simulated observational equations are set up and solved for the
coefficients of scalar and vector spherical harmonics representing the output
errors, rather than for individual objects in the output catalog. Both
accidental and systematic correlated errors of astrometric parameters can be
accurately estimated. The method is demonstrated on the example of the JMAPS
mission, but can be used for other projects of space astrometry, such as SIM or
JASMINE.Comment: Accepted by AJ, to be published in 201
Differential geometry construction of anomalies and topological invariants in various dimensions
In the model of extended non-Abelian tensor gauge fields we have found new
metric-independent densities: the exact (2n+3)-forms and their secondary
characteristics, the (2n+2)-forms as well as the exact 6n-forms and the
corresponding secondary (6n-1)-forms. These forms are the analogs of the
Pontryagin densities: the exact 2n-forms and Chern-Simons secondary
characteristics, the (2n-1)-forms. The (2n+3)- and 6n-forms are gauge invariant
densities, while the (2n+2)- and (6n-1)-forms transform non-trivially under
gauge transformations, that we compare with the corresponding transformations
of the Chern-Simons secondary characteristics. This construction allows to
identify new potential gauge anomalies in various dimensions.Comment: 27 pages, references added, matches published versio
T-duality and Generalized Complex Geometry
We find the explicit T-duality transformation in the phase space formulation
of the N=(1,1) sigma model. We also show that the T-duality transformation is a
symplectomorphism and it is an element of O(d,d). Further, we find the explicit
T-duality transformation of a generalized complex structure in this model. We
also show that the extended supersymmetry of the sigma model is preserved under
the T-duality.Comment: 18 pages; added references; published versio
Noncommutative quantum mechanics and the Aharonov-Casher effect
In this work a new method is developed to investigate the Aharonov-Casher
effect in a noncommutative space. It is shown that the holonomy receives
non-trivial kinematical corrections.Comment: 8 pages, Plain Tex, to appear in Eur. Phys. J.
Why Matrix theory works for oddly shaped membranes
We give a simple proof of why there is a Matrix theory approximation for a
membrane shaped like an arbitrary Riemann surface. As corollaries, we show that
noncompact membranes cannot be approximated by matrices and that the Poisson
algebra on any compact phase space is U(infinity). The matrix approximation
does not appear to work properly in theories such as IIB string theory or
bosonic membrane theory where there is no conserved 3-form charge to which the
membranes couple.Comment: 8 pages, 4 figures, revtex; references adde
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