85,146 research outputs found
A space station onboard scheduling assistant
One of the goals for the Space Station is to achieve greater autonomy, and have less reliance on ground commanding than previous space missions. This means that the crew will have to take an active role in scheduling and rescheduling their activities onboard, perhaps working from preliminary schedules generated on the ground. Scheduling is a time intensive task, whether performed manually or automatically, so the best approach to solving onboard scheduling problems may involve crew members working with an interactive software scheduling package. A project is described which investigates a system that uses knowledge based techniques for the rescheduling of experiments within the Materials Technology Laboratory of the Space Station. Particular attention is paid to: (1) methods for rapid response rescheduling to accommodate unplanned changes in resource availability, (2) the nature of the interface to the crew, (3) the representation of the many types of data within the knowledge base, and (4) the possibility of applying rule-based and constraint-based reasoning methods to onboard activity scheduling
Methods of assessing structural integrity for space shuttle vehicles
A detailed description and evaluation of nondestructive evaluation (NDE) methods are given which have application to space shuttle vehicles. Appropriate NDE design data is presented in twelve specifications in an appendix. Recommendations for NDE development work for the space shuttle program are presented
Should the United States hold foreign currency reserves?
An argument that for countries with well-developed money markets and flexible exchange rates, there is little to be gained from holding a vast foreign exchange portfolio and intervening in the world's currency markets.Foreign exchange - Law and legislation
BEC in Nonextensive Statistical Mechanics
We discuss the Bose-Einstein condensation (BEC) for an ideal gas of bosons in
the framework of Tsallis's nonextensive statistical mechanics. We study the
corrections to the standard BEC formulas due to a weak nonextensivity of the
system. In particular, we consider three cases in the D-dimensional space: the
homogeneous gas, the gas in a harmonic trap and the relativistic homogenous
gas. The results show that small deviations from the extensive Bose statistics
produce remarkably large changes in the BEC transition temperature.Comment: LaTex, 7 pages, no figures, to be published in Mod. Phys. Lett. B;
corrected a typo in Eq. (2
Oil, the economy, and monetary policy
An examination of the theoretical impact of an oil shortage on the U.S. economy and the problems that such shortages present for monetary policy.Power resources - Prices ; Monetary policy
Spontaneous superconductivity and optical properties of high-Tc cuprates
We suggest that the high temperature superconductivity in cuprate compounds
may emerge due to interaction between copper-oxygen layers mediated by in-plane
plasmons. The strength of the interaction is determined by the c-axis geometry
and by the ab-plane optical properties. Without making reference to any
particular in-plane mechanism of superconductivity, we show that the interlayer
interaction favors spontaneous appearance of the superconductivity in the
layers. At a qualitative level the model describes correctly the dependence of
the transition temperature on the interlayer distance, and on the number of
adjacent layers in multilayered homologous compounds. Moreover, the model has a
potential to explain (i) a mismatch between the optimal doping levels for
critical temperature and superconducting density and (ii) a universal scaling
relation between the dc-conductivity, the superfluid density, and the
superconducting transition temperature.Comment: 4.4 pages, 2 figures; v2 matches the published version (clarifying
remarks and references are added
Quantum Monte Carlo study of confined fermions in one-dimensional optical lattices
Using quantum Monte Carlo (QMC) simulations we study the ground-state
properties of the one-dimensional fermionic Hubbard model in traps with an
underlying lattice. Since due to the confining potential the density is space
dependent, Mott-insulating domains always coexist with metallic regions, such
that global quantities are not appropriate to describe the system. We define a
local compressibility that characterizes the Mott-insulating regions and
analyze other local quantities. It is shown that the momentum distribution
function, a quantity that is commonly considered in experiments, fails in
giving a clear signal of the Mott-insulator transition. Furthermore, we analyze
a mean-field approach to these systems and compare it with the numerically
exact QMC results. Finally, we determine a generic form for the phase diagram
that allows us to predict the phases to be observed in the experiments.Comment: RevTex file, 13 pages, 19 figures, published versio
Oscillatory decay of a two-component Bose-Einstein condensate
We study the decay of a two-component Bose-Einstein condensate with negative
effective interaction energy. With a decreasing atom number due to losses, the
atom-atom interaction becomes less important and the system undergoes a
transition from a bistable Josephson regime to the monostable Rabi regime,
displaying oscillations in phase and number. We study the equations of motion
and derive an analytical expression for the oscillation amplitude. A quantum
trajectory simulation reveals that the classical description fails for low
emission rates, as expected from analytical considerations. Observation of the
proposed effect will provide evidence for negative effective interaction.Comment: 4 pages, 3 figue
Monotonicity and logarithmic convexity relating to the volume of the unit ball
Let stand for the volume of the unit ball in for
. In the present paper, we prove that the sequence
is logarithmically convex and that the sequence
is strictly
decreasing for . In addition, some monotonic and concave properties of
several functions relating to are extended and generalized.Comment: 12 page
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