磁共振现代射频脉冲理论在非均匀场成像中的应用

在磁共振非均匀场成像中，传统的射频脉冲导致回波信号的衰减.为了减小和消除这种磁共振信号的衰减，在讨论了经典理论的基础上根据非线性动力学中的逆散射理论和Shinnar-Le Roux方法导出了用于非均匀场成像的射频脉冲设计方法.模拟结果表明，采用逆散射理论和Shinnar-Le Roux方法优化的脉冲序列可以明显提高信号的信噪比. 关键词： 磁共振成像 射频脉冲 非线性系统     

On the dynamics of nonautonomous periodic general dynamical systems and differential inclusions

We establish the existence and stability results for periodic nonautonomous uniform forward attractors of periodic general dynamical systems (set-valued dynamical systems). We also investigate the dynamical behavior of nonautonomous periodic differential inclusion x^′(t)∈f(t,x(t)) on R^m with only upper semi-continuous right-hand side by applying the abstract results. Firstly, we show that if the system has a compact uniformly attracting set, then it has a periodic nonautonomous uniform forward attractor A. Secondly, we prove that A is robust with respect to both internal and external perturbations. Finally, we apply the robustness result to discuss the effects of small time delays to asymptotic stability of the system.     

Higher-order spectral analysis and weak asymptotic stability of convex processes

This paper deals with the asymptotic stability analysis of a discrete dynamical inclusion whose right-hand side is a convex process. We provide necessary and sufficient conditions for weak asymptotic stability, and obtain sharp estimates for the asymptotic null-controllability set. These estimates involve not only standard, but also higher-order spectral information on the convex process and its adjoint.     

Business–Cycle Models and the Dangers of Linearizing

This paper studies the consequences of linearizing nonlinear business–cycle models near their interior steady state. It is shown that dynamic objects, created for example in a Bogdanov-Takens bifurcation, may be lost in the linearization procedure. Sufficient conditions are provided ensuring the absence of various dynamic features in the nonlinear version of the model. The authors thank Andy Atkeson and Hal Cole for useful comments and suggestions. They also thank three anonymous referees who have helped to greatly improve the exposition of this paper. Part of this research was done while the author was affiliated with the University of Cergy Pontoise (THEMA) and during visits at DELTA. Part of this research has been done while the author was working as a Marie Curie Fellow at INSEE/CREST and during visits at the University of Warwick.     

Calculations of the dynamical critical exponent using the asymptotic series summation method

We consider how the Padé-Borel, Padé-Borel-Leroy, and conformal mapping summation methods for asymptotic series can be used to calculate the dynamical critical exponent for homogeneous and disordered Ising-like systems. __________ Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 147, No. 1, pp. 137–154, April, 2006.     

Twisted Semigroup Crossed Products and Twisted Toeplitz Algebras of Ordered Groups

Let г^＋ be the positive cone of a totally ordered abelian group г, and σa cocycle in г. We study the twisted crossed products by actions of г＋ as endomorphisms of C^＊-algebras, and use this to generalize the theorem of Ji.     

动态方法在企业管理中的应用

将自动控制中的动态原理与方法引入到企业管理中，建立了一套动态，交互式的企业运作解决方案，并在应用中取得了良好的效果。     

Duality for Knizhnik–Zamolodchikov and Dynamical Equations

We consider the Knizhnik–Zamolodchikov (KZ) and dynamical equations, both differential and difference, in the context of (gl _k ,gl _n ) duality. We show that the KZ and dynamical equations naturally exchange under the duality.     

Bang-bang control suppression of amplitude damping in a three-level atom

By using quantum bang-bang control technique, we studied the suppression of amplitude damping, or energy dissipation, in a three-level atom in various configurations. We have explicitly given the bang-bang control groups in three different configurations, and the pulse sequences for these bang-bang control operations. Supported by the National Natural Science Foundation of China (Grant No. 10547003), the Key Project of Chinese Ministry of Education (Grant No. 306020) and Chifeng College Scientific Research Fund (Grant No. ZRZD200604)