Limit cycles for a class of second order differential equations

We study the limit cycles of a wide class of second order differential equations, which can be seen as a particular perturbation of the harmonic oscillator. In particular, by choosing adequately the perturbed function we show, using the averaging theory, that it is possible to obtain as many limit cycles as we want.     

Minimalities and Modules over Dedekind-Like Rings

We are interested in (right) modules M satisfying the following weak divisibility condition: If R is the underlying ring, then for every r ∈ R either Mr = 0 or Mr = M. Over a commutative ring, this is equivalent to say that M is connected with regular generics. Over arbitrary rings, modules which are “minimal” in several model theoretic senses satisfy this condition. In this article, we investigate modules with this weak divisibility property over Dedekind-like rings and over other related classes of rings.     

Equiangular tight frames with centroidal symmetry

An equiangular tight frame (ETF) is a set of unit vectors whose coherence achieves the Welch bound, and so is as incoherent as possible. Though they arise in many applications, only a few methods for constructing them are known. Motivated by the connection between real ETFs and graph theory, we introduce the notion of ETFs that are symmetric about their centroid. We then discuss how well-known constructions, such as harmonic ETFs and Steiner ETFs, can have centroidal symmetry. Finally, we establish a new equivalence between centroid-symmetric real ETFs and certain types of strongly regular graphs (SRGs). Together, these results give the first proof of the existence of certain SRGs, as well as the disproofs of the existence of others.     

Low degree GW invariants of surfaces Ⅱ

We prove a conjectural formula of Maulik-Pandharipande on the degree one and two GW invariants of a surface with a smooth canonical divisor.We use the method of degeneration and the localized GW invariants introduced by the authors.     

On Hopf bifurcations of piecewise planar Hamiltonian systems

In this paper we study the number of limit cycles appearing in Hopf bifurcations of piecewise planar Hamiltonian systems. For the case that the Hamiltonian function is a piecewise polynomials of a general form we obtain lower and upper bounds of the number of limit cycles near the origin respectively. For some systems of special form we obtain the Hopf cyclicity.     

A note on matrics with given diagonal entries

Mirsky and Erdos and Mine have given criteria for the existence of a non-negative square matrix with prescribed diagonal entries and row and column sums; it is shown that this result follows from the supply-demand theorem for flows in networks.     

哈密尔顿平面图最小平衡二部划分的上界

设$G（V，E）$是一个图，$V_{1}，V_{2}$是$V$的一个二部划分，用$e（V_{1}，V_{2}）$表示一条边的两个端点在不同划分里边的总数目，当$||V_{1}|-|V_{2}||\leq 1$时，称$V_{1}，V_{2}$是$V$的一个平衡二部划分。最小平衡二部划分是指寻找$G（V，E）$的一个平衡二部划分使得$e（V_{1}，V_{2}）$最小。对于哈密尔顿平面图$G（V，E）$，研究了当Perfect-内部三角形最大边函数值与最小边函数值之差为$d$时，$e（V_{1}，V_{2}）$最小值的上界与$d$之间的关系。     

Determining the Maximum Excess Heat of Solution Differentials Using the Carnot Cycle Temperature Efficiency

The (T ₁−T ₂)/T ₁ efficiency of a very small temperature differential thermodynamic cycle operating near a solvent’s critical locus, where T ₁ is the higher temperature and T ₂ the lower, is shown to be a simple tool for determining the maximum difference in the excess heat of solution between the cycle’s low-temperature liquid side and its high-temperature supercritical fluid side.