具有扩散率Ayala模型的概周期解

本文研究了非自治Ayala模型的概周期和周期系统,我们得到在一定条件下,其概周期系统存在唯一全局吸引的概周期解且其概周期解在壳扰动下是稳定的。在与概周期情形类似的条件下我们得到其w-周期系统存在唯一全局吸引的w-周期解。     

奇次周期Riccati型微分系统的周期解

本文讨论的是一类奇次周期Riccati型方程的周期解问题,利用数学归纳法,得到了奇次周期Riccati型方程周期多个周期解存在的充分条件,并且给出了定理实现的例子。     

一维六方准晶的两类周期接触问题

利用复变函数方法讨论了一维六方准晶非周期平面的两类周期接触问题,即无摩擦周期接触以及半平面粘结周期接触问题.利用Hilbert核积分公式,得到了两类周期接触问题封闭形式的解.对于无摩擦周期接触问题,给出了3种常见压头(周期直水平基底、周期直倾斜基底、周期圆基底)作用下接触应力的显式表达式;对于半平面粘结周期接触问题,给出了实际工程中常见的边界上有尖劈形周期位移情况下应力的解析表达式.当忽略相位子场的贡献时,结果与正交各向异性材料周期接触问题的相应结果一致.     

关于拟周期情形的Sacker-Sell问题

研究关于拟周期系统能否进行拟周期三角化的拟周期情形Sacker-Sell问题.在系数矩阵无限光滑、频率满足无理性条件时,得到一类不可拟周期三角化的拟周期线性微分系统,即在一般条件下对这个问题给出否定回答;并建立了拟周期系统进行拟周期三角化的一个充分条件.     

一类推广的干摩擦问题周期解的存在性

为了得到干摩擦问题存在周期解的判断方法,利用微分包含理论对系统+k~2x+μSngp(t)+T(t)a.e.进行了讨论.在周期外力p(t)周期等于固有周期2π/R的情形下得到了周期解存在的充要条件,在周期外力p(t)周期不等于固有周期整数倍2nπ/R的情形下得到了周期解存在的充分条件.     

具有脉冲效应和综合害虫控制的捕食系统

本文通过生物控制和化学控制提出了具有周期脉冲效应与害虫控制的捕食系统. 系统保护天敌避免灭绝,在一些条件下可以使害虫灭绝.就是说当脉冲周期小于某一临界值时,存在全局稳定害虫灭绝周期解.脉冲周期增大大于临界值时,平凡害虫灭绝周期解失去稳定性并产生正周期解,利用分支理论来研究正周期解的存在性.进而,利用李雅普诺夫函数和比较定理确定了持续生存的条件.     

不等长周期时间序列的周期划分方法

针对时间序列周期不等长的情况,提出了一种基于周期划分的时间序列周期分析方法.首先将时间序列变换到频域中获取序列的周期特征,其次根据周期特征计算移动平均的项数来对时间序列做移动平均处理,然后计算移动平均处理后序列中的极值点,最后对极值点按条件进行剔除后得到周期划分点.以划分点为界划分得到时间序列的多个周期段,经过分析采用周期段的中位数线来表示时间序列的周期性变化特征.这种周期划分方法更适用于存在随机波动的长序列,实验表明该方法能较好地对序列做出划分,得到的周期段中位数线的变化特点也与原时间序列基本相符.     

圆周上有4周期轨的连续自映射的周期集

讨论了圆周上有4周期轨的连续自映射的周期集.首先按相对共轭以及相对同伦的关系对圆周上所有有4周期轨的连续自映射分类,再利用映射覆盖图来讨论每一类映射的周期集.最后按同伦最小周期集对圆周上所有有4周期轨的连续自映射进行了分类.将此结果与线段上的Sharkovskii定理对比时可以发现,儿乎所有圆周上有4周期轨的连续自映射的周期集都是全体自然数集.     

可定向闭曲面上保定向的周期映射

讨论可定向闭曲面上保定向周期映射的共轭类分类问题.Kulkarni(1997)指出:亏格g大于3时,曲面上任意周期大于或等于4g的周期映射共轭于两类周期映射中某个映射的幂.之后Hirose(2010)得到:亏格g大于12时,曲面上任意周期大于或等于3g的周期映射共轭于4类周期映射中某个映射的幂.本文在此基础上研究了周期大于或等于3(g-1)的情形:当亏格g大于21时,得到了和Hirose相似的结论,且找出了更多不能被包含在前面所讲的4类周期映射中的情形.     

侧向局部加热对流的周期性

通过流体力学方程组的数值模拟,研究了侧向局部加热条件下Prandtl数Pr=0.0272时流体对流的周期性.结果表明:随着Grashof数Gr的增加,对流按稳态对流、单局部周期对流、双局部周期对流、准周期对流的顺序发展.当Gr<3.6×103时,对流为稳态;在3.6×103相似文献   

极限环的表达式和计算

本文提出一种解析法和数值法相结合的方法，用来计算多项式微分系统的极限环，极限环表示为x=∑k≥0(ak cos kφ bk sin kφ),y=∑k≥0(ck cos kφ dk sin kφ)，先用解析法求出小参数时极限环的初始表达式，然后用增量法和迭代法求出任意参数时极限环满足给定精度的表达式，半稳定极限环和分叉值也可以计算。     

电机工程中一类非线性振动方程的渐近分析

本文采用文［１］中提出的修正的方法，研究电机工程中一类非线性振动方程，定量地给出了存在极限环的参数范围以及极限环的振幅，并判定该极限环是不稳定的，这些结果与已知的定性分析结果完全一致，从而进一步证实了上述渐近方法的有效性。     

Limit cycle bifurcations of planar piecewise differential systems with three zones

This paper considers the limit cycle bifurcation problem of planar piecewise differential systems with three zones. Some computation formulas studied the problem of limit cycle bifurcations are provided by introducing multiple parameters. As an application to the obtained method, the number of limit cycles of a piecewise linear system with three zones studied in Lima et al. (2017) is discussed and some more limit cycles are found.     

ANALYSIS OF THE LIMIT CYCLE OF A GENERALISED VAN DER POL EQUATION BY A TIME TRANSFORMATION METHOD

Abstract

The properties of the limit cycle of a generalised van der Pol equation of the form ü + u = ε (1—u²ⁿ)u, where ε is small and n is any positive integer, are investigated by applying a time transformation perturbation method due to Burton. It is found that as n increases the amplitude of the limit cycle oscillation decreases and its period increases. The time transformation solution is compared with the solution derived using the method of multiple scales and with a numerical solution. It is found that, to first order in ε, the time transformation solution for the limit cycle agrees better with the numerical solution than the multiple scales solution. Both perturbation solutions give the same result for the period of the limit cycle to second order in ε. The accuracy of the time transformation solution decreases as n increases.     

A study of homotopy analysis method for limit cycle of van der Pol equation

The homotopy analysis method for limit cycle of the van der Pol equation is computationally extensive. Aiming at reducing the computational efforts, we construct the deformation equations using different initial conditions from those in the existing literature. The computational efforts are significantly reduced. And, the calculated amplitude and frequency of the limit cycle are in excellent agreement with the exact solutions.     

Limit cycles for a perturbation of a quadratic center with symmetry

To estimate the number of limit cycles appearing under a perturbation of a quadratic system that has a center with symmetry, we use the method of generalized Dulac functions. To this end, we reduce the perturbed system to a Liénard system with a small parameter, for which we construct a Dulac function depending on the parameter. This permits one to estimate the number of limit cycles in the perturbed system for all sufficiently small parameter values. We find the Dulac function by solving a linear programming problem. The suggested method is used to analyze four specific perturbed systems that globally have exactly three limit cycles [i.e., the limit cycle distribution 3 or (3, 0)] and two systems that have the limit cycle distribution (3, 1) (i.e., one nest around each of the two foci).     

Incremental harmonic balance method for nonlinear flutter of an airfoil with uncertain-but-bounded parameters

The limit cycle oscillation of a two-dimensional airfoil with parameter variability in an incompressible flow is investigated using the incremental harmonic balance (IHB) method. The variable parameters, such as the wind speed, the cubic plunge and pitch stiffness coefficients, are modeled as either bounded uncertain or stochastic parameters. In the solution process of the IHB method, the bounded parameters are considered as an active increment. Taking all values over the considered bounded regions of the active parameters provides us with a series of IHB solutions of limit cycle oscillations of the airfoil. With the aid of the attained solutions, the bounds and some statistical properties of the limit cycle oscillations are determined and compared with Monte Carlo simulation (MCS) results. Numerical examples show that the proposed approach is valid and effective for analyzing strongly nonlinear vibration problems with bounded uncertainties.     

The existence of the exponentially stable limit cycle for a class of nonlinear systems

In this paper, the exponentially stable limit cycle phenomenon for a class of nonlinear systems is investigated. Based on the analytic method, the existence and uniqueness of the exponentially stable limit cycle for such systems can be guaranteed. Moreover, the amplitude of oscillation, the period of oscillation, and guaranteed convergence rate can be correctly estimated. Finally, two numerical examples are provided to illustrate the use of the main result.     

推广后继函数法研究第二临界情况下同宿环的稳定性

本文通过灵活选取参照闭曲线,推广了研究闭轨线的后继函数法.通过计算后继函数,本文首先获得了二重极限环的半稳定性判据.在此基础上,运用推广的后继函数法,获得了第二临界情况下同宿环的内稳定性判据,事实上,推广的后继函数法可对以往的结果和本文的结果用统一的方法给予证明,并可向更高临界情况推广.最后本文证明了二重极限环及第二临界情况下的同宿环在一定条件下分支出极限环的唯二性.     

Limit cycle bifurcations by perturbing piecewise smooth Hamiltonian systems with multiple parameters

This paper is concerned with the problem of limit cycle bifurcation for piecewise smooth near-Hamiltonian systems with multiple parameters. By the first Melnikov function, some novel criteria have been established for the existence of multiple limit cycles. Furthermore, an example is included to validate the obtained results by considering the maximum number of limit cycles for a piecewise quadratic system studied in Llibre and Mereu (2014) [12]. Compared with the result in the above reference, one more limit cycle is found by our method.