Modification of instability processes by multiplicative noises

We study two dynamical systems submitted to white and Gaussian random noise acting multiplicatively. The first system is an imperfect pitchfork bifurcation with a noisy departure from onset. The second system is a pitchfork bifurcation in which the noise acts multiplicatively on the non-linear term of lowest order. In both cases noise suppresses some solutions that exist in the deterministic regime. Besides, for the first system, the imperfectness of the bifurcation reduces the regime of on-off intermittency. For the second system, the unstable mode can achieve a jump of finite amplitude at instability but without hysteresis. We finally identify a generic property that is verified by the stationary probability density function of the dynamical variable when a control parameter is varied.     

关联对的结构

利用了类似于H–F的方法来自洽地确定S对(和D对)的结构,此外,为了避免因辛弱数混合而产生的伪态困难,改进了S对的定义并用不成对粒子算符来构造D对,以使它们相互独立.     

黑洞周围的运动层对霍金辐射探测的影响

本文论述了黑洞周围运动层对霍金辐射恒定速度的破坏，是后者至今仍无法探测到的重要原因。     

Radiation Reaction of a Nonrelativistic Quantum Charged Particle

An alternative approach to analyze the nonrelativistic quantum dynamics of a rigid and extended charged particle taking into account the radiation reaction is discussed with detail. Interpretation of the field operators as annihilation and creation ones, theory of perturbations and renormalization are not used. The analysis is carried out in the Heisenberg picture with the electromagnetic field expanded in a complete orthogonal basis set of functions which allows the electromagnetic field to satisfy arbitrary boundary conditions. The corresponding coefficients are the field operators which satisfy the usual commutation relations. A nonlinear equation of motion for the charged particle is obtained. A careful consideration of the quantum effects allows the derivation of a linear equation of motion which is free of both runaway solutions and preacceleration, even for a point charge. Also, the electromagnetic mass, which is defined as the coefficient of the acceleration operator, vanishes for a point particle. However, this does not mean that the results are free of ambiguities which are exhibited and discussed.     

牛顿运动定律和机翼举力

将空气简化为理想流体，由牛顿运动定律推出气体做曲线运动时的切向伯努利方程和法向压强梯度表达式，并解释了机翼举力。     

不规则星系中星体随机运动的几率密度与天体不确定关系

在这篇论文中，作者用Langevin随机微分方程来描写不规则星系中任何一个作准布朗型运动的星体的随机运动．并用相空间中的表示点（X．X）来描写其每一随机态.用Fokker－Planck方程计算出了这些随机态的稳态几率密度fs（X．X）；同时，用此fs（X．X）计算出了随机运动星体的速度和位置的方差与涨落．进而作者发现：随机运动的星体满足一种天体的不确定关系．     

用单一循环LDA测量法研究四气门汽油机缸内滚流运动的湍流结构

本研究在一台四气门汽油机上对缸内滚流运动进行了LDA测量，通过改进测试系统提高了数据率，用低通滤波的单一循环分析法计算了湍流结构参数。结果发现，滚流的湍流结构具有尺度大、频率低的特点；滚流运动在压缩末期破碎时，湍流的自相关时间和积分时间尺度显著增加。     

一类非线性方程的Dirichlet问题

设D为R~d(d≥3)中非空规则开子集.本文利用超布朗运动给出了一类非线性方程Dirichlet问题非负有界解的通式.在一定条件下证明了满足在无穷远处有规定极限非负有界解的存在唯一性,并给出了解的概率表达式,推广了Dynkin中的结果.     

Elastically coupled molecular motors

We study the influence of filament elasticity on the motion of collective molecular motors. It is found that for a backbone flexibility exceeding a characteristic value (motor stiffness divided through the mean displacement between attached motors), the ability of motors to produce force reduces as compared to rigidly coupled motors, while the maximum velocity remains unchanged. The force-velocity-relation in two different analytic approximations is calculated and compared with Monte-Carlo simulations. Finally, we extend our model by introducing motors with a strain-dependent detachment rate. A remarkable crossover from the nearly hyperbolic shape of the Hill curve for stiff backbones to a linear force-velocity relation for very elastic backbones is found. With realistic model parameters we show that the backbone flexibility plays no role under physiological conditions in muscles, but it should be observable in certain in vitro assays. Received: 27 November 1997 / Revised: 20 February 1998 / Accepted: 27 March 1998