1,760 research outputs found
Determinants of product lines
We study the determinants of product lines in a multi-product Cournot model which allows for cost asymmetries and which eliminates strategic motivations (such as a desire to soften competition) for product-line choice. We identify two distinct forces that lead firms to offer multiple qualities. We then investigate how the interaction of asymmetric competition with these forces shapes equilibrium product lines. Our model predicts a rich variety of possible outcomes, ranging from headto-
head competition (in which all firms offer all products) to complete separation of product lines (so that no product is offer by two or more firms) and including the intermediate case of partial separation. In an international trade context, we predict whether a disadvantaged foreign producer will specialize in higher or lower qualities. If cost factors are the main drivers of quality-based discrimination (so that there are
decreasing returns to quality) then foreign firms sell only lower qualities. However, if demand factors are the main drivers (so that, other things equal, the demand for
higher qualities is less elastic) then foreign firms sell only high qualities
Analysis of some global optimization algorithms for space trajectory design
In this paper, we analyze the performance of some global search algorithms on a number of space trajectory design problems. A rigorous testing procedure is introduced to measure the ability of an algorithm to identify the set of ²-optimal solutions. From the analysis of the test results, a novel algorithm is derived. The development of the novel algorithm starts from the redefinition of some evolutionary heuristics in the form of a discrete dynamical system. The convergence properties of this discrete dynamical system are used to derive a hybrid evolutionary algorithm that displays very good performance on the particular class of problems presented in this paper
Information use and acquisition in price-setting oligopolies
Asymmetric price-setting multi-product suppliers have access to multiple sources of information about demand conditions, where the publicity of each source corresponds to the cross-industry correlation of signals received from it. A signal’s influence on suppliers’ prices is increasing in its publicity as well as in its precision. The emphasis on relatively public information is stronger for smaller suppliers who control narrower product portfolios. When information is endogenously acquired, suppliers listen to only a subset of information sources. This subset is smaller when products are less differentiated and when the industry is
less concentrated. Smaller suppliers focus attention on fewer information sources. The inefficiencies arising from information acquisition and use are identified. The
associated externalities depend upon the extent of product differentiation, the concentration of the industry, and the degree of decreasing returns to scale
Prevalence study of yaws in the Democratic Republic of Congo using the lot quality assurance sampling method.
BACKGROUND: Until the 1970s the prevalence of non-venereal trepanomatosis, including yaws, was greatly reduced after worldwide mass treatment. In 2005, cases were again reported in the Democratic Republic of the Congo. We carried out a survey to estimate the village-level prevalence of yaws in the region of Equator in the north of the country in order to define appropriate strategies to effectively treat the affected population. METHODOLOGY/PRINCIPAL FINDINGS: We designed a community-based survey using the Lot Quality Assurance Sampling method to classify the prevalence of active yaws in 14 groups of villages (lots). The classification into high, moderate, or low yaws prevalence corresponded to World Health Organization prevalence thresholds for identifying appropriate operational treatment strategies. Active yaws cases were defined by suggestive clinical signs and positive rapid plasma reagin and Treponema pallidum hemagglutination serological tests. The overall prevalence in the study area was 4.7% (95% confidence interval: 3.4-6.0). Two of 14 lots had high prevalence (>10%), three moderate prevalence (5-10%) and nine low prevalence (<5%.). CONCLUSIONS/SIGNIFICANCE: Although yaws is no longer a World Health Organization priority disease, the presence of yaws in a region where it was supposed to be eradicated demonstrates the importance of continued surveillance and control efforts. Yaws should remain a public health priority in countries where previously it was known to be endemic. The integration of sensitive surveillance systems together with free access to effective treatment is recommended. As a consequence of our study results, more than 16,000 people received free treatment against yaws
Dynamical decoherence in a cavity with a large number of two-level atoms
We consider a large number of two-level atoms interacting with the mode of a
cavity in the rotating-wave approximation (Tavis-Cummings model). We apply the
Holstein-Primakoff transformation to study the model in the limit of the number
of two-level atoms, all in their ground state, becoming very large. The unitary
evolution that we obtain in this approximation is applied to a macroscopic
superposition state showing that, when the coherent states forming the
superposition are enough distant, then the state collapses on a single coherent
state describing a classical radiation mode. This appear as a true dynamical
effect that could be observed in experiments with cavities.Comment: 9 pages, no figures. This submission substitutes paper
quant-ph/0212148 that was withdrawn. Version accepted for publication in
Journal of Physics B: Atomic, Molecular & Optical Physic
Binary Bose-Einstein Condensate Mixtures in Weakly and Strongly Segregated Phases
We perform a mean-field study of the binary Bose-Einstein condensate mixtures
as a function of the mutual repulsive interaction strength. In the phase
segregated regime, we find that there are two distinct phases: the weakly
segregated phase characterized by a `penetration depth' and the strongly
segregated phase characterized by a healing length. In the weakly segregated
phase the symmetry of the shape of each condensate will not take that of the
trap because of the finite surface tension, but its total density profile still
does. In the strongly segregated phase even the total density profile takes a
different symmetry from that of the trap because of the mutual exclusion of the
condensates. The lower critical condensate-atom number to observe the complete
phase segregation is discussed. A comparison to recent experimental data
suggests that the weakly segregated phase has been observed.Comment: minor change
Generation of macroscopic quantum-superposition states by linear coupling to a bath
We demonstrate through an exactly solvable model that collective coupling to
any thermal bath induces effectively nonlinear couplings in a quantum many-body
(multi-spin) system. The resulting evolution can drive an uncorrelated
large-spin system with high probability into a macroscopic
quantum-superposition state. We discuss possible experimental realizations.Comment: 4 pages, 2 figures, Physical Review Letters (in press
Spin correlation and Discrete symmetry in Spinor Bose-Einstein Condensates
We study spin correlations in Bose-Einstein condensates of spin 1 bosons with
scatterings dominated by a total spin equal 2 channel. We show the low energy
spin dynamics in the system can be mapped into an nonlinear sigma
model(NLM). at the zero magnetic field limit and in the
presence of weak magnetic fields. In an ordered phase, the ground state has a
hidden symmetry and is degenerate under the group . We explore consequences of the hidden symmetry and propose some
measurements to probe it.Comment: 4 pages; published version in Phys. Rev. Lett. vol 87, 080401-1(2001
Decoherence in a single trapped ion due to engineered reservoir
The decoherence in trapped ion induced by coupling the ion to the engineered
reservoir is studied in this paper. The engineered reservoir is simulated by
random variations in the trap frequency, and the trapped ion is treated as a
two-level system driven by a far off-resonant plane wave laser field. The
dependence of the decoherence rate on the amplitude of the superposition state
is given.Comment: 4 pages, 2 figure
Two-species magneto-optical trap with 40K and 87Rb
We trap and cool a gas composed of 40K and 87Rb, using a two-species
magneto-optical trap (MOT). This trap represents the first step towards cooling
the Bose-Fermi mixture to quantum degeneracy. Laser light for the MOT is
derived from laser diodes and amplified with a single high power semiconductor
amplifier chip. The four-color laser system is described, and the
single-species and two-species MOTs are characterized. Atom numbers of 1x10^7
40K and 2x10^9 87Rb are trapped in the two-species MOT. Observation of trap
loss due to collisions between species is presented and future prospects for
the experiment are discussed.Comment: 4 pages, 4 figures; accepted for publication in Physical Review
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