Diffusion of Dopant in Crystalline Silicon: An Asymptotic Analysis

An existing model of the diffusion of dopant in crystallinesilicon is presented. The model is used in predicting semiconductordevice properties. For small concentrations of dopant, the diffusionis linear. However, at large concentrations, enhanced diffusionoccurs. This may be modelled by a concentration-dependent diffusioncoefficient that is approximately constant at small concentrationsbut increases linearly at higher concentrations. The initialdistribution of implanted dopant is confined to a small regionof the crystal, and has a high peak concentration. This initialdistribution develops into a high-concentration region withadvancing steep fronts and decaying tail regions. In the enhancedregion, the dominant balance is given by the filtration equation,and the tail regions are dominated by quasi-steady convection—diffusion.These two regions are analysed and matched together, using singularperturbation theory.     

Extremely High Concentration Dopant Diffusion in Silicon

Over a fairly wide range of concentrations the diffusion coefficientsof arsenic and boron in silicon grow linearly with concentration.At very high concentrations, however, precipitation or clusteringeffects cause a levelling off or drop in the diffusivity. Asymptoticmethods are used to analyse these effects both as small perturbationsand when they dominate.     

硅体中磷反应扩散系统解的整体存在性及渐近性

本文用上下解方法，研究了一类带混合非齐次边界条件的化学反应──硅体中磷反应扩散系统，采用避开△的特征函数构造上下解的方法，得到一类充分条件，以使其整体解渐近趋向于Ｓｔｅａｄｙ－ｓｔａｔｅ解．     

流动环境中高浓度射流扩散实验研究

通过流动显示和定量测量对浅水流动环境条件下,高浓度流体垂体向抛射入水后形成的射流运动扩散及浓度分布特征进行了水槽实验研究。实验分析了射流入水后与环境水流条件的相互作用,通过数据分析给出了射流中心线着底点与横向扩散角的拟合公式。实验结果表明高浓度射流在近区呈现出不同于一般淹没射流的复杂流动形态及扩散特征,以异重流的形式向下游推移。     

An Asymptotic Analysis of a 2-D Model of Dynamically Active Compartments Coupled by Bulk Diffusion

A class of coupled cell–bulk ODE–PDE models is formulated and analyzed in a two-dimensional domain, which is relevant to studying quorum-sensing behavior on thin substrates. In this model, spatially segregated dynamically active signaling cells of a common small radius \(\epsilon \ll 1\) are coupled through a passive bulk diffusion field. For this coupled system, the method of matched asymptotic expansions is used to construct steady-state solutions and to formulate a spectral problem that characterizes the linear stability properties of the steady-state solutions, with the aim of predicting whether temporal oscillations can be triggered by the cell–bulk coupling. Phase diagrams in parameter space where such collective oscillations can occur, as obtained from our linear stability analysis, are illustrated for two specific choices of the intracellular kinetics. In the limit of very large bulk diffusion, it is shown that solutions to the ODE–PDE cell–bulk system can be approximated by a finite-dimensional dynamical system. This limiting system is studied both analytically, using a linear stability analysis and, globally, using numerical bifurcation software. For one illustrative example of the theory, it is shown that when the number of cells exceeds some critical number, i.e., when a quorum is attained, the passive bulk diffusion field can trigger oscillations through a Hopf bifurcation that would otherwise not occur without the coupling. Moreover, for two specific models for the intracellular dynamics, we show that there are rather wide regions in parameter space where these triggered oscillations are synchronous in nature. Unless the bulk diffusivity is asymptotically large, it is shown that a diffusion-sensing behavior is possible whereby more clustered spatial configurations of cells inside the domain lead to larger regions in parameter space where synchronous collective oscillations between the small cells can occur. Finally, the linear stability analysis for these cell–bulk models is shown to be qualitatively rather similar to the linear stability analysis of localized spot patterns for activator–inhibitor reaction–diffusion systems in the limit of long-range inhibition and short-range activation.     

Asymptotic series in dynamics of fluid flows: Diffusion versus bifurcations

The Brownian motion over the space of fluid velocity configurations driven by the hydrodynamical equations is considered. The Green function is computed in the form of an asymptotic series close to the standard diffusion kernel. The high order asymptotic coefficients are studied. Similarly to the models of quantum field theory, the asymptotic contributions show a factorial growth and are summated by means of Borel’s procedure. The resulting corrected diffusion spectrum has a closed analytical form. The approach provides a possible ground for the optimization of existing numerical simulation algorithms and can be used for the analysis of other asymptotic series in turbulence.     

关于更新计数过程高阶矩的一个等价式

设N(t)为一更新计数过程，本文得到了在时间间隔有有限期望的情形下，关于N(t)的任意阶矩的一个等价式，该结果可以看成是基本更新定理的推广。对于时间间隔为轻尾的（矩母函数存在）或者是一类特殊的重尾族（确切地，ERV族）时，得到了更加精细的结果。     

A Stochastic Model for Transport of Particulate Matter in Air: An Asymptotic Analysis

A branching particle system with changes of size is considered as a model for transport of particulate matter in air. This type of model is motivated by problems arising in the context of air pollution. High-density fluctuation limits for the process recording the positions and sizes of the particles through time are presented. These results allow to compute approximate probabilities for the temporal and spatial concentrations of particles of given sizes (in particular the small-sized pollutant particles which pose a health hazard).     

An Asymptotic Analysis of One-Dimensional Models of Semiconductor Devices

We study boundary-value problems describing the field distributionand the carrier transport in semiconductor devices in the one-dimensionalcase. Asymptotic solutions are derived and questions of theirexistence and their asymptotic validity are studied     

On the Controllability of an Advection-diffusion Equation with Respect to the Diffusion Parameter: Asymptotic Analysis and Numerical Simulations

Acta Mathematicae Applicatae Sinica, English Series - The advection-diffusion equation yεt − εyεxx + Myεx = 0, (x, t) ∈ (0, 1) × (0, T) is null controllable for...     

An Asymptotic Analysis of Localized Solutions for Some Reaction-Diffusion Models in Multidimensional Domains

In the limit ? → 0, a spike-layer solution is constructed for the reaction-diffusion equation where b > 0 and D is a bounded convex domain. Here Q(u) is such that there exists a unique radially symmetric function u_c(?^?1 r) satisfying ?²Δu_c + Q(u_c) = 0 in all of ?^N, with u_c(ρ) decaying exponentially at infinity. The spike-layer solution has the form u ~ u_c [?|x ? x₀|], where the spike-layer location x₀ ? D is to be determined subject to the condition that dist(x₀, ?D) as ? → D. The determination of x₀ is shown to be exponentially ill conditioned and asymptotic estimates for the exponentially small eigenvalues and the corresponding eigenfunctions associated with the linearized problem are obtained. These spectral results are used together with a limiting solvability condition to derive an equation for x₀. For a strictly convex domain, it is shown that there is an x₀ that is located at an O(?) distance away from the point in D that is furthest from ?D. Finally, hot-spot solutions to Bratu's equation are constructed asymptotically in a singularly perturbed limit.     

SR-HARDI: Spatially Regularizing High Angular Resolution Diffusion Imaging

High angular resolution diffusion imaging (HARDI) has recently been of great interest in mapping the orientation of intravoxel crossing fibers, and such orientation information allows one to infer the connectivity patterns prevalent among different brain regions and possible changes in such connectivity over time for various neurodegenerative and neuropsychiatric diseases. The aim of this article is to propose a penalized multiscale adaptive regression model (PMARM) framework to spatially and adaptively infer the orientation distribution function (ODF) of water diffusion in regions with complex fiber configurations. In PMARM, we reformulate the HARDI imaging reconstruction as a weighted regularized least-square regression (WRLSR) problem. Similarity and distance weights are introduced to account for spatial smoothness of HARDI, while preserving the unknown discontinuities (e.g., edges between white matter and gray matter) of HARDI. The L₁ penalty function is introduced to ensure the sparse solutions of ODFs, while a scaled L₁ weighted estimator is calculated to correct the bias introduced by the L₁ penalty at each voxel. In PMARM, we integrate the multiscale adaptive regression models, the propagation-separation method, and Lasso (least absolute shrinkage and selection operator) to adaptively estimate ODFs across voxels. Experimental results indicate that PMARM can reduce the angle detection errors on fiber crossing area and provide more accurate reconstruction than standard voxel-wise methods. Supplementary materials for this article are available online.     

平面应变条件下裂纹尖端附近的弹塑性近似分析

本文给出了平面应变条件下裂纹尖端附近的弹塑性的应力、应变和位移的解析近似解,其特例即Irwin的弹性主项解,从而使弹塑性断裂力学分析所需的一切场量都可以从文中得到.     

高雷诺数下平面Poiseuille流稳定性渐近分析

本文主要讨论高雷诺数下平面Poiseuille流稳定性问题.应用多层结构分析理论,求出了描述该流体稳定性的Orr-Sommerfeld方程特征值的较为合理的逼近式及相应的特征函数的渐近解析解.     

Simplicity v/s Complexity in the Framework of Geometric Asymptotic Analysis and Some New Applications of the Concentration Phenomenon

We introduce a concept of Simplicity which in a sense corresponds to the reverse direction to the concept of Complexity. Many problems of Asymptotic Geometric Analysis rotate around this notion. We also describe the concept of Concentration and suggest a new direction of possible applications of the Concentration Phenomenon. Lecture held in the Seminario Matematico e Fisico on May 12, 2005 Received: April 2006     

Asymptotic expansions of Mellin convolution integrals: An oscillatory case

In a recent paper [J.L. López, Asymptotic expansions of Mellin convolution integrals, SIAM Rev. 50 (2) (2008) 275-293], we have presented a new, very general and simple method for deriving asymptotic expansions of for small x. It contains Watson’s Lemma and other classical methods, Mellin transform techniques, McClure and Wong’s distributional approach and the method of analytic continuation used in this approach as particular cases. In this paper we generalize that idea to the case of oscillatory kernels, that is, to integrals of the form , with c∈R, and we give a method as simple as the one given in the above cited reference for the case c=0. We show that McClure and Wong’s distributional approach for oscillatory kernels and the summability method for oscillatory integrals are particular cases of this method. Some examples are given as illustration.     

Asymptotic behavior of solutions: An application to stochastic NLP

In this article we study the consistency of optimal and stationary (KKT) points of a stochastic non-linear optimization problem involving expectation functionals, when the underlying probability distribution associated with the random variable is weakly approximated by a sequence of random probability measures. The optimization model includes constraints with expectation functionals those are not captured in direct application of the previous results on optimality conditions exist in the literature. We first study the consistency of stationary points of a general NLP problem with convex and locally Lipschitz data and then apply those results to the stochastic NLP problem and stochastic minimax problem. Moreover, we derive an exponential bound for such approximations using a large deviation principle.

     

An Asymptotic Ratio in the Complete Binary Tree

Let T _n be the complete binary tree of height n considered as the Hasse-diagram of a poset with its root 1 _n as the maximum element. For a rooted tree T, define two functions counting the embeddings of T into T _n as follows A(n;T)=|{S T _n  : 1 _n ∈S, S≅T}|, and B(n;T)=|{S T _n :1 _n ∉S, S≅T}|. In this paper we investigate the asymptotic behavior of the ratio A(n;T)/B(n;T), and we show that lim  _n→∞[A(n;T)/B(n;T)]=2^ℓ;−1−1, for any tree T with ℓ leaves. This revised version was published online in June 2006 with corrections to the Cover Date.     

对流扩散问题Crank-Nicolson差分-流线扩散法的高精度分析

讨论了对流扩散问题C rank-N ico lson差分流线扩散格式,利用插值后处理技术提高了特殊网格下该格式在双线性元空间解的精度,从而按Lα(L2(Ω))模达到最优.