Let G be a group with a dihedral subgroup H of order 2pn, where p is an odd prime. We show that if there exist H-connected transversals in G, then G is a solvable group. We apply this result to the loop theory and show that if the inner mapping group of a finite loop Q is dihedral of order 2pn, then Q is a solvable loop.1991 Mathematics Subject Classification: 20D10, 20N05 相似文献
In this paper, the solder fatigue model created by Werner Engelmaier is modified so that it takes better into account the different stress levels the solder material experiences. This is accomplished by replacing the fatigue ductility constant by a correction term that is temperature- excursion- range-dependent. In order to obtain the correction term, the effect of the temperature cycling profile on lifetime is studied. The correction term obtained is package type specific. More compliant plastic packages with a small coefficient of expansion mismatch with the organic printed wiring board are handled separately, and stiff ceramic packages assembled on an organic board – resulting in a large CTE mismatch – are considered as another entity.
Correction terms for both lead-free and tin–lead solder materials are given. This is necessary as the creep characteristics of the solders are different. Comparing solder material specific correction terms makes it possible to determine which solder material performs best in a certain environment and a given assembly type. In the case of ceramic packages, the transition where a eutectic tin–lead solder becomes superior to a SnAgCu solder is expected to take place when temperature excursions are smaller than ca. 70 °C. This indicates that although in a highly accelerated stress test, lead-free solders may be inferior compared to tin–lead solders, in the real field environment, the situation may be reversed. 相似文献
In a unifying way, the doorway mechanism explains spectral properties in a rich variety of open mesoscopic quantum systems, ranging from atoms to nuclei. A distinct state and a background of other states couple to each other which sensitively affects the strength function. The recently measured superscars in the barrier billiard provide an ideal model for an in-depth investigation of this mechanism. We introduce two new statistical observables: the full distribution of the maximum coupling coefficient to the doorway and directed spatial correlators. Using random matrix theory and random plane waves, we obtain a consistent understanding of the experimental data. 相似文献
In this paper, we argue for a novel three-dimensionalist (3D'ist) solution to the problem of persistence, i.e. cross-temporal identity. We restrict the discussion of persistence to simple substances, which do not have other substances as their parts. The account of simple substances employed in the paper is a trope-nominalist strong nuclear theory (SNT), which develops Peter Simons' trope nominalism. Regarding the distinction between three dimensionalism (3D) and four dimensionalism (4D), we follow Michael Della Rocca's formulation, in which 3D explains persistence in virtue of same entities and 4D in virtue of distinct entities (temporal parts). SNT is a 3D'ist position because it accounts for the persistence of simple substances in virtue of diachronically identical ‘nuclear’ tropes. The nuclear tropes of a simple substance are necessary for it and mutually rigidly dependent but distinct. SNT explains qualitative change by tropes that are contingent to a simple substance. We show that it avoids the standard problems of 3D: temporal relativization of ontic predication, Bradley's regress and coincidence, fission and fusion cases. The temporal relativization is avoided because of the analysis of temporary parts that SNT gives in terms of temporal sub-location, which is atemporal part–whole relation. 相似文献
Feasibility of a method for finding flow permeability of porous materials, based on combining computerised X-ray micro-tomography and numerical simulations, is assessed. The permeability is found by solving fluid flow through the complex 3D pore structures obtained by tomography for actual material samples. We estimate overall accuracy of the method and compare numerical and experimental results. Factors contributing to uncertainty of the method include numerical error arising from the finite resolution of tomographic images and the rather small sample size available with the present tomographic techniques. The total uncertainty of computed values of permeability is, however, not essentially larger than that of experimental results. We conclude that the method provides a feasible alternative for finding fluid flow properties of the kind of materials studied. It can be used to estimate all components of permeability tensor and is useful in cases where direct measurements are not achievable. Analogous methods can be applied to other modes of transport, such as diffusion and heat conduction. 相似文献
Polyimides are well established as gas separation membranes due to their intrinsically low free volume and correspondingly high H2 selectivity relative to other gases such as CO2. Prior studies have established that H2/CO2 selectivity can be improved by crosslinking polyimides with diamines differing in spacer length. In this work, we follow the evolution of macroscopic and microscopic properties of a commercial polyimide over long crosslinking times (tx) with 1,3‐diaminopropane. According to spectroscopic analysis, the crosslinking reaction saturates after ≈24 h, whereas tensile, nanoindentation and stress relaxation tests reveal that the material stiffens, and possesses a long relaxation time that increases with increasing tx. Although differential scanning calorimetry shows that the glass transition temperature decreases systematically with increasing tx, permeation studies indicate that the permeabilities of H2 and CO2 decrease, while the H2/CO2 selectivity increases markedly, with increasing tx. At long tx, the polyimide becomes impermeable to CO2, suggesting that it could be used as a barrier material.
