9,604 research outputs found
Improved Algorithms for Radar-based Reconstruction of Asteroid Shapes
We describe our implementation of a global-parameter optimizer and Square
Root Information Filter (SRIF) into the asteroid-modelling software SHAPE. We
compare the performance of our new optimizer with that of the existing
sequential optimizer when operating on various forms of simulated data and
actual asteroid radar data. In all cases, the new implementation performs
substantially better than its predecessor: it converges faster, produces shape
models that are more accurate, and solves for spin axis orientations more
reliably. We discuss potential future changes to improve SHAPE's fitting speed
and accuracy.Comment: 12 pages, 9 figure
Non-equilibrium heat capacity of polytetrafluoroethylene at room temperature
Polytetrafluoroethylene can be considered as a model for calorimetric studies
of complex systems with thermodynamics transitions at ambient temperature. This
polymer exhibits two phase transitions of different nature at 292 K and 303 K.
We show that sensitive ac-calorimetry measurements allow us to study the
thermodynamic behaviour of polytetrafluoroethylene when it is brought out of
thermodynamic equilibrium. Thanks to the thermal modelisation of our
calorimetric device, the frequency dependent complex heat capacity of this
polymer is extracted. The temperature and frequency variations of the real and
imaginary parts of the complex heat capacity are obtained when
polytetrafluoroethylene undergoes its first-order structural phase transition
at 292 K
Prospects of dynamical determination of General Relativity parameter beta and solar quadrupole moment J2 with asteroid radar astronomy
We evaluated the prospects of quantifying the parameterized post-Newtonian
parameter beta and solar quadrupole moment J2 with observations of near-Earth
asteroids with large orbital precession rates (9 to 27 arcsec century).
We considered existing optical and radar astrometry, as well as radar
astrometry that can realistically be obtained with the Arecibo planetary radar
in the next five years. Our sensitivity calculations relied on a traditional
covariance analysis and Monte Carlo simulations. We found that independent
estimates of beta and J2 can be obtained with precisions of
and , respectively. Because we assumed rather conservative
observational uncertainties, as is the usual practice when reporting radar
astrometry, it is likely that the actual precision will be closer to
and , respectively. A purely dynamical determination
of solar oblateness with asteroid radar astronomy may therefore rival the
helioseismology determination.Comment: The astrophysical journal (ApJ), in pres
The hydrogen atom in electric and magnetic fields : Pauli's 1926 article
The results obtained by Pauli, in his 1926 article on the hydrogen atom, made
essential use of the dynamical so(4) symmetry of the bound states. Pauli used
this symmetry to compute the perturbed energy levels of an hydrogen atom in a
uniform electric field (Stark effect) and in uniform electric and magnetic
fields. Although the experimental check of the single Stark effect on the
hydrogen atom has been studied experimentally, Pauli's results in mixed fields
have been studied only for Rydberg states of rubidium atoms in crossedfields
and lithium atoms in parallel fields.Comment: 11 pages, latex file, 2 figure
The Role of Agriculture in Reducing Poverty in Tanzania: A Household Perspective from Rural Kilimanjaro and Ruvuma
This paper explores how farm productivity affects poverty, and how various factor market constraints affect farm productivity. The empirical analysis draws on representative surveys of farm households in Kilimanjaro and Ruvuma, two cash crop growing regions in Tanzania. We find that poorer households do not only possess fewer assets, but are also much less productive. We find that agricultural productivity directly affects household consumption and hence overall poverty and welfare. Stochastic production frontier analysis indicates that many farmers are farming well below best practice in the region. Analysis of allocative efficiency suggests that family labour is substantially over utilized, a sign of considerable excess labour supply. Use of intermediate inputs on the other hand is well below what is commensurate with the estimated value of their marginal productivities. An important reason for low input use is lack of credit to purchase inputs, but difficult access to the inputs themselves, being connected to the economy, and food security and self insurance considerations are also important impediments. Easy access to credit is positively associated with being a member of a savings association or being in a contractual arrangement with a cooperative or firm. The findings support a continuing emphasis on increasing agricultural productivity in designing poverty reduction policies. Better agronomic practices and increased input use will be crucial in this strategy. Financial constraints might be relieved through fostering institutional arrangements facilitating contract enforcement and institutions that facilitate saving by the households themselves. They may also be relieved by the provision of more adequate consumption safety nets.Agricultural development, Factor markets, Rural poverty, Farm productivity, Food Security and Poverty, O13, O120, Q120,
The Importance of Broadband Provision to Knowledge Intensive Firm Location
Despite the volume of literature afforded knowledge work and innovations in information and communications technologies (ICTs), few studies have examined the importance of ICTs to firms in knowledge industries. This study will develop spatial econometric models to examine the relative importance of the level of broadband provision to knowledge intensive firms in select U.S. ĂÂ metropolitan statistical areas (MSAs). Results demonstrate the need for both a spatial econometric and a metropolitan area specific evaluation of this relationship. They also suggest potential spillover effects to knowledge intensive firm location, which may explain why some regional economies are relatively more successful at stimulating firm growth in this increasingly important sector of the U.S economy.
Near-BPS Skyrmions: Non-shell configurations and Coulomb effects
The relatively small binding energy in nuclei suggests that they may be well
represented by near-BPS Skyrmions since their mass is roughly proportional to
the baryon number For that purpose, we propose a generalization of the
Skyrme model with terms up to order six in derivatives of the pion fields and
treat the nonlinear and Skyrme terms as small perturbations. For our
special choice of mass term (or potential) , we obtain well-behaved
analytical BPS-type solutions with non-shell configurations for the baryon
density, as opposed to the more complex shell-like configurations found in most
extensions of the Skyrme model . Along with static and (iso)rotational
energies, we add to the mass of the nuclei the often neglected Coulomb energy
and isospin breaking term. Fitting the four model parameters, we find a
remarkable agreement for the binding energy per nucleon with respect to
experimental data. These results support the idea that nuclei could be near-BPS
Skyrmions.Comment: Correction of minors errors, references adde
The Dunkl oscillator in the plane I : superintegrability, separated wavefunctions and overlap coefficients
The isotropic Dunkl oscillator model in the plane is investigated. The model
is defined by a Hamiltonian constructed from the combination of two independent
parabosonic oscillators. The system is superintegrable and its symmetry
generators are obtained by the Schwinger construction using parabosonic
creation/annihilation operators. The algebra generated by the constants of
motion, which we term the Schwinger-Dunkl algebra, is an extension of the Lie
algebra u(2) with involutions. The system admits separation of variables in
both Cartesian and polar coordinates. The separated wavefunctions are
respectively expressed in terms of generalized Hermite polynomials and products
of Jacobi and Laguerre polynomials. Moreover, the so-called Jacobi-Dunkl
polynomials appear as eigenfunctions of the symmetry operator responsible for
the separation of variables in polar coordinates. The expansion coefficients
between the Cartesian and polar bases (overlap coefficients) are given as
linear combinations of dual -1 Hahn polynomials. The connection with the
Clebsch-Gordan problem of the sl_{-1}(2) algebra is explained.Comment: 25 pages; Added references; Added appendix on anti-Hermicity of the
Dunkl derivativ
The Dunkl oscillator in the plane II : representations of the symmetry algebra
The superintegrability, wavefunctions and overlap coefficients of the Dunkl
oscillator model in the plane were considered in the first part. Here
finite-dimensional representations of the symmetry algebra of the system,
called the Schwinger-Dunkl algebra sd(2), are investigated. The algebra sd(2)
has six generators, including two involutions and a central element, and can be
seen as a deformation of the Lie algebra u(2). Two of the symmetry generators,
J_3 and J_2, are respectively associated to the separation of variables in
Cartesian and polar coordinates. Using the parabosonic creation/annihilation
operators, two bases for the representations of sd(2), the Cartesian and
circular bases, are constructed. In the Cartesian basis, the operator J_3 is
diagonal and the operator J_2 acts in a tridiagonal fashion. In the circular
basis, the operator J_2 is block upper-triangular with all blocks 2x2 and the
operator J_3 acts in a tridiagonal fashion. The expansion coefficients between
the two bases are given by the Krawtchouk polynomials. In the general case, the
eigenvectors of J_2 in the circular basis are generated by the Heun polynomials
and their components are expressed in terms of the para-Krawtchouk polynomials.
In the fully isotropic case, the eigenvectors of J_2 are generated by little -1
Jacobi or ordinary Jacobi polynomials. The basis in which the operator J_2 is
diagonal is then considered. In this basis, the defining relations of the
Schwinger-Dunkl algebra imply that J_3 acts in a block tridiagonal fashion with
all blocks 2x2. The matrix elements of J_3 in this basis are given explicitly.Comment: 33 page
- âŠ