On the splitting of groups of central group extensions

It is proved that if G and V are abelian groups, then the group Ext(G, V) of all abelian extensions of the group V with the aid of the group G is extracted by the direct component in the group Opext(G, V) of all central extensions of V with the aid of G.Translated from Matematicheskie Zametki, Vol. 6, No. 2, pp. 197–200, August, 1969.     

Variation of the elasticoviscous characteristics of a heavily filled rubber after periodic shear deformation

The results of an investigation of the recovery of the elasticoviscous characteristics after thixotropic reduction by periodic deformation in the nonlinear zone are reported. The rate of recovery of the mechanical properties depends on the deformation frequency and the previous history of dynamic deformation is shown to affect the dependence of the elasticoviscous characteristics on the shear rate gradient. The effects of stationary and dynamic deformation regimes on the relaxation spectrum of the material are compared.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 528–533, May–June, 1969.     

Random anisotropy in R2CuO4 (R=Nd, Pr, Sm, Gd, Eu) quasi-2D antiferromagnets

The mechanisms of random anisotropy produced by an an effective coupling between rare-earth ion moments and orbital momenta of Cu₂₊ ions through spin fluctuations is studied in R₂CuO₄ crystals. The effective random-anisotropy fields are estimated from an analysis of experimental data for R₂CuO₄ crystals (R=Eu, Pr, Gd). Fiz. Tverd. Tela (St. Petersburg) 41, 1259–1263 (July 1999)     

Wear resistance of polyethylene

The effect of the molecular weight and branching of polyethylene on its wear resistance in direct contact with abrasive particles has been investigated. The abrasive wear resistance of the polyethylene increases with its molecular weight. Copolymers have better wear resistance than homopolymers."Plastolimer" Research and Production Association, Leningrad. Translated from Mekhanika Polimerov, No. 3, pp. 542–544, May–June, 1971.     

Nonlinear theory of channeling of radiation by a ribbon-shaped stream of cyclotron oscillators

The features of the nonlinear stage of radiation from transversely bounded, magnetically steered helical and rectilinear electron streams in a homogeneous medium and in vacuum are investigated theoretically under conditions of the normal and anomalous Doppler effects. The evolution of the transverse distribution of the radiated field is investigated, and the channeling effect of the electron stream is demonstrated. It is shown that in the radiation from a helical stream in a plasmalike medium a practically complete conversion of the energy of gyrational motion of the electrons into energy of electromagnetic oscillations can occur on account of the automatic fine tuning of the radiation angle. Zh. Tekh. Fiz. 69, 9–14 (January 1999)     

Quantitation of atherosclerosis by magnetic resonance imaging and 3-D morphology operators

The objective was to ascertain whether MRI and image processing can be used to quantify atherosclerosis by measuring wall thickness in rabbit aorta. The abdominal aortas of 2 healthy and 5 atherosclerotic rabbits were examined with a gradient-echo inflow angiography sequence (2DI) and a proton density weighted turbo-spin-echo sequence (PDW). Using thresholding by four observers and 3D morphology operators, segmentation of the artery and vein lumina was performed from the 2DI sequence, and of surrounding fat and muscle from the PDW sequence. Remaining voxels adjacent to the aortic lumen were classified as vessel wall. By measuring the vessel wall volume and the lumen volume, the wall percentage was calculated. The values were significantly higher for the diseased animals than for unaffected individuals (p < 0.01). It is concluded that aortic wall thickening in atherosclerotic rabbits can be measured quantitatively by using MRI combined with 3D morphology image processing operators.     

Physical and theoretical aspects of a new vacuum arc controltechnology-self arc diffusion by electrode: SADE

Our new vacuum arc control technology SADE doubles the high current interruption capability of our conventional axial magnetic field technology. First, we describe the vacuum arc motion behavior recorded by a high speed charge-coupled device video camera. This arc behavior is closely related to axial magnetic field intensity. In particular, it depends on the profile of the externally generated axial magnetic field. The anode spot is likely to move to the highest magnetic field intensity. Second, we describe analytical results for concentration of vacuum arc at the anode side contact surface. This analysis implies the possibility of an ideal magnetic field profile and intensity for vacuum arc control. Finally, we describe experimental results for vacuum arc control compared with the physical and theoretical results mentioned above, and we show a practical electrode configuration for vacuum interrupters and its application in a high current interruption experiment     

A problem related to the arithmetic of probability measures on the sphere

The class I_o is described in the convolution semigroup of probability measure on the sphere in Rⁿ which are invariant under rotations about a fixed axis; it is established that the indecomposable measures form a dense set in the topology of weak convergence.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 87, pp. 143–158, 1979.I am deeply grateful to Professor I. V. Ostrovskii for the continual attention he has paid to my work.     

Relation between giant volume magnetostriction and colossal magnetoresistance near the Curie temperature of Sm0.55Sr0.45MnO3

The magnetic, transport, and elastic properties of Sm_0.55Sr_0.45MnO₃ have been established to be interrelated. At the Curie point, one observes a large volume compression ΔV/V≈0.1%, a sharp minimum in the temperature dependence of negative volume magnetostriction ω(T), and a maximum in the temperature dependence of the electrical resistivity. Giant negative volume magnetostriction ω=?5×10^?4 has been found in a magnetic field H=0.9 T, which is accompanied by a colossal negative magnetoresistance of 44% in the same field. The results obtained are discussed in terms of a model of electronic phase separation.