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91.
92.
When the maximum likelihood estimator is computationally inconvenient, covariate and Newton–Raphson adjustment often provide algebraically explicit yet still asymptotically efficient estimators. The bivariate normal correlation coefficient with known variances is used to show that these methods may produce singularities that render the adjusted estimators unstable. 相似文献
93.
大学数学教育与当今遍及全球的数学教育改革方向有较大的偏差.教学内容囿于课本,固守形式演绎体系,重视技能,忽略模式建构;传递、接受的观念根深蒂固,教学方法单一,以教材的逻辑线索代替认知线索;导致学生对内容本质缺乏理解,能力发展不均衡,情感态度发展有欠缺.调整对策是:追求全面的教育目标;提供包括知识背景、理论体系、应用、内容本质、思想方法等完整有机整体的学习内容;多种教学方法综合运用,设计合理的教学线索,让学生主动经历学习过程等. 相似文献
94.
本文对古典风险模型中保险公司按单位时间常数率收到保险费的假设做了改进,将每次收到的保险费的次数看作是复合泊松过程,将每次收到的保费和每次的理陪额均看作是服从指数分布的随机变量,并引入带干扰风险的扰动项,从而对古典风险模型进行推广,且给出了相应的破产概率上界,分析了破产概率的上界与准备金,索赔额,净保费和扰动方差之间的关系. 相似文献
95.
带扩散扰动项的广义双Poisson风险模型下的破产概率 总被引:1,自引:0,他引:1
本文首先在[1]-[4]讨论的基础上,将经典的破产模型推广到带扩散扰动项的广义双Po isson风险模型,即将保费收取过程和索赔总额过程同时推广到广义复合Po isson过程,以此解决在同一时刻有两张以上保单到达和两个以上顾客索赔的实际问题;接着运用鞅方法证明了破产概率满足的Lundberg不等式和一般公式在我们所建的模型下同样成立. 相似文献
96.
提出一种"镜外观察"和"只在正面调整"相结合的方法,能够在正面的反射像不丢失的基础上,快速找到反面的反射像,避免了盲目调节,达到快速找到"十"字像的目的。 相似文献
97.
In density-based topological design, one expects that the final result consists of elements either black (solid material) or white (void), without any grey areas. Moreover, one also expects that the optimal topology can be obtained by starting from any initial topology configuration. An improved structural topological optimization method for multidisplacement constraints is proposed in this paper. In the proposed method, the whole optimization process is divided into two optimization adjustment phases and a phase transferring step. Firstly, an optimization model is built to deal with the varied displacement limits, design space adjustments, and reasonable relations between the element stiffness matrix and mass and its element topology variable. Secondly, a procedure is proposed to solve the optimization problem formulated in the first optimization adjustment phase, by starting with a small design space and advancing to a larger deign space. The design space adjustments are automatic when the design domain needs expansions, in which the convergence of the proposed method will not be affected. The final topology obtained by the proposed procedure in the first optimization phase, can approach to the vicinity of the optimum topology. Then, a heuristic algorithm is given to improve the efficiency and make the designed structural topology black/white in both the phase transferring step and the second optimization adjustment phase. And the optimum topology can finally be obtained by the second phase optimization adjustments. Two examples are presented to show that the topologies obtained by the proposed method are of very good 0/1 design distribution property, and the computational efficiency is enhanced by reducing the element number of the design structural finite model during two optimization adjustment phases. And the examples also show that this method is robust and practicable. 相似文献
98.
‘Slipped’ π?π stacking between flexible macrocycle 14+ (cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylene benzene)) and neutral small molecules induce one-dimensional (1D) ‘sandwich’ chain self-assembly. Unlike most of the reported π?π stacking system, the 1D “sandwich” chain expands with the direction parallel to stacking π surfaces on 14+ and that on molecule 2, 3, 4 or 5 (2 = p-xylene, 3 = benzene-1,4-diamine, 4 = 4,4′-bipyridine, 5 = [1,1′-biphenyl]-4,4′-diol). Moreover, the π?π stacking modes of 1D self-assembly are seriously small molecule adduct dependent. Combined with the other weak interactions (e.g. intermolecular hydrogen bonding), the new substrate design and control strategy can expand the 1D ‘sandwich’ chain (e.g. [14+·4]n) into higher order structure (e.g. two-dimensional (2D) network [14+·4·6]n, 6 = hydroquinone) even in large scale (~280 mg). This 2D network structure, which keeps stable under 423 K, shows highly selective gas absorption of CO2 over N2. 相似文献
99.
Graphitic carbon nitride (CN) has been widely used as a photocatalyst. Very few researchers have reported the use of CN in quantum dot-sensitized solar cells (QDSCs). In this study, we prepared nitrogen-rich carbon quantum dot (CQD)-embedded CN nanotubes (CCNTs) with freeze-dried urea and CQD precursors. The prepared CCNTs were used as efficient light harvesters in QDSCs for the first time; their use significantly improved the power conversion efficiency (PCE) of the solar cells compared to those of CQD, CN NT, and bulk CN-sensitized solar cells. The CCNT-sensitized solar cell exhibits 1.01% PCE, which is the highest value among carbon-based QDSCs. Moreover, the CCNTs-sensitized device showed superior photostability over those of CQDs-, CN NTs-, and bulk CN-sensitized devices. The improved performance of the CCNT-sensitized solar cell can be attributed to the facilitated photoelectron transport and suppressed charge recombination. The integration of nitrogen-rich CQDs in CCNTs adjusts the band alignment and maximizes the visible light harvest by reducing the energy barriers, which improves the charge collection efficiency of the device. 相似文献
100.