Coherent scattering in a small quantum dot

Ballistic transport in an open small (100 nm) three-terminal quantum dot has been analyzed. The dot is based on the high-mobility 2D electron gas of the AlGaAs/GaAs heterojunction. It has been shown that the gate oscillations of the resistance of such a dot arise due to the coherent scattering of electrons on its quasidiscrete levels and these oscillations are suppressed by a weak magnetic field.     

Optimal quasi-circular motion of a spacecraft with an energy storage device

The problem of using an energy storage device in spacecraft low-thrust propulsion systems with a constant-power thruster is considered and solved for optimal quasi-circular maneuvers in the near-Earth space. The maximum payload is the optimality criterion. The optimal control as a function of time and the optimal mass parameters of the spacecraft were determined. Domains of the mass parameters where the use of an energy storage device makes sense are shown. T. G. Shevchenko University, Kiev, Ukraine. Translated from Prikladnaya Mekhanika, Vol. 35, No. 10, pp. 93–100, October, 1999.     

Optimization of spacecraft transfers between distant elliptical orbits

Optimization is made of the trajectories, controls, and the parameters of a low-thrust constant-power engine with energy storage of a spacecraft executing the maneuver of synchronous considerable change in the semimajor axis, eccentricity, and angle of an elliptical orbit in a spherical gravitational field. The gain in payload mass due to the energy storage is estimated. The optimal control law and the optimal ratio for the masses of the propulsion system are found     

Abelian groups admitting a Fréchet-Urysohn pseudocompact topological group topology

We show that every Abelian group G with r₀(G)=|G|=|G|^ω admits a pseudocompact Hausdorff topological group topology T such that the space (G,T) is Fréchet-Urysohn. We also show that a bounded torsion Abelian group G of exponent n admits a pseudocompact Hausdorff topological group topology making G a Fréchet-Urysohn space if for every prime divisor p of n and every integer k≥0, the Ulm-Kaplansky invariant f_p,k of G satisfies (f_p,k)^ω=f_p,k provided that f_p,k is infinite and f_p,k>f_p,i for each i>k.Our approach is based on an appropriate dense embedding of a group G into a Σ-product of circle groups or finite cyclic groups.     

Varieties generated by countably compact Abelian groups

We prove under the assumption of Martin's Axiom that every precompact Abelian group of size belongs to the smallest class of groups that contains all Abelian countably compact groups and is closed under direct products, taking closed subgroups and continuous isomorphic images.

     

Nondiscrete P-groups can be reflexive

We present the first examples of nondiscrete reflexive P-groups (topological groups in which countable intersections of open sets are open) as well as of noncompact reflexive ω-bounded groups (precompact groups in which the closure of every countable set is compact). Our main result implies that every product of discrete Abelian groups equipped with the P-modified topology is reflexive. Taking uncountably many nontrivial factors, we thus answer a question posed by P. Nickolas and solve a problem raised by Ardanza-Trevijano, Chasco, Domínguez, and Tkachenko.New examples of non-reflexive P-groups are also given which are based on a further development of Leptin's technique going back to 1955.     

Peculiarities of the B8-Solid Solution Formation in Ni(Sb,Sn) and Co(Sb,Sn) Systems Studied by Mossbauer Effect Method

Quadrupole splitting and effective characteristic temperatures of ¹¹⁹Sn in Ni(Sb,Sn) and Co(Sb,Sn) solid solutions with B8-type structure have been studied by Mossbauer Effect method. The electric field gradient at metalloid atoms was estimated in point charge model for different variants of local environment. On the basis of the comparison between experimental Quadruple Splitting values and estimated Electric Field Gradient ones it was concluded that MeII-Sn pairs occur more often than MeII-Sb when the amount of transition metal in ditrigonal-dibipyramidal positions is less than 0.10.