Theoretical study on the improvement of the doping efficiency of Al in 4H-SiC by co-doping group-ⅣB elements

The p-type doping efficiency of 4H silicon carbide(4H-SiC)is rather low due to the large ionization energies of p-type dopants.Such an issue impedes the explora...     

The Effect of a Low-Frequency Structure on Passive Scalar Transport in the Flow Over a Surface-Mounted Rib

The unsteadiness of the flow over a surface-mounted rib involving passive scalar transport is studied using large-eddy simulation (LES) at a Reynolds number of 3000 (based on the rib height, \(h\), and the free-stream velocity, \(U_{0})\). The purpose of the present study is to gain further insight into the physical origin of the flow instability and its effect on passive scalar transport. Fourier spectral analysis of the velocity at different positions suggests that, in addition to the K-H instability in the shear layer (St ≈?0.42), two lower frequencies (St ≈?0.06 and 0.09) also exist. It is observed that the low-frequency instabilities accompany the shedding process of vortical structures. One low frequency, at \(\text {St}\approx 0.06\), is related to the pumping motion of the recirculation bubble, while the other, at \(\text {St}\approx 0.09\), is associated with the flapping mode of the shear layer. Through comparisons of velocity and temperature fields and the spectra of scalar fluctuations, it is found that the passive scalar is transported by the convection of vortical structures.     

液态氧化铝结构的分子动力学模拟

利用分子动力学模拟液态氧化铝从2663 K到过冷温度2223 K的结构变化,通过总双体分布函数得到的液态氧化铝的原子第一、第二近邻距离与Ansell研究小组的实验结果基本一致.通过配位数和偏角分布函数的分析,液态氧化铝的局域结构主要是由氧的三重铝原子配位和铝原子四重(AlO₄)和五重(AlO₅)氧原子配位组成.没有观察到可能因铝原子高、低配位数氧原子的团簇分离而出现的液-液相变.     

半素F—环的结构

如果环R含有一个有限非零元集X,使得任意非零aR与X之交不空,则称R为F-环。文献[1]证明了:无非零幂零元的F-环是有限个除环的直和。 本文的结果是:R是半素F-环,当且仅当R是有限个除环上的全阵环的直和。从而[1]的结论就是我们结果的一个推论。     

Examples of rings and modules as trivial extensions

Abstract

The two main classes of finite dimensional composition algebras, Hurwitz and symmetric, are characterized in terms of the flexible identity, and even of the third power associative identity, over fields of arbitrary characteristic.     

Stationary tail asymptotics of a tandem queue with feedback

Motivated by applications in manufacturing systems and computer networks, in this paper, we consider a tandem queue with feedback. In this model, the i.i.d. interarrival times and the i.i.d. service times are both exponential and independent. Upon completion of a service at the second station, the customer either leaves the system with probability p or goes back, together with all customers currently waiting in the second queue, to the first queue with probability 1−p. For any fixed number of customers in one queue (either queue 1 or queue 2), using newly developed methods we study properties of the exactly geometric tail asymptotics as the number of customers in the other queue increases to infinity. We hope that this work can serve as a demonstration of how to deal with a block generating function of GI/M/1 type, and an illustration of how the boundary behaviour can affect the tail decay rate.     

Variable Selection for Partially Linear Models via Adaptive LASSO

Partially linear model is a class of commonly used semiparametric models, this paper focus on variable selection and parameter estimation for partially linear models via adaptive LASSO method. Firstly, based on profile least squares and adaptive LASSO method, the adaptive LASSO estimator for partially linear models are constructed, and the selections of penalty parameter and bandwidth are discussed. Under some regular conditions, the consistency and asymptotic normality for the estimator are investigated, and it is proved that the adaptive LASSO estimator has the oracle properties. The proposed method can be easily implemented. Finally a Monte Carlo simulation study is conducted to assess the finite sample performance of the proposed variable selection procedure, results show the adaptive LASSO estimator behaves well.     

Identities with Engel conditions on derivations

Let R be a semiprime ring with a derivation D. The focus is on the two identities with Engel condition on ${D: [x^m, D(x^{n_1}),\ldots,D(x^{n_s})]_s=0}$ for all ${x\in R}$ and ${[x^m, D(x)^{n_1},\ldots,D(x)^{n_s}]_s=0}$ for all ${x\in R}$ , where s, m, n ₁, . . . , n _s are fixed positive integers. Our results are natural generalizations of Posner’s theorem on centralizing derivations, Herstein’s theorem on derivations with power-central values and a recent result by A. Fo?ner, M. Fo?ner and Vukman.