Analytical study of the antiferromagnetic Heisenberg films

With the variational cumulant expansion (VCE) method, the thermodynamic behaviors of S = 1/2 antiferromagnetic Heisenberg films in simple cubic lattices are studied analytically. From the analytic properties of the free energy, in principle we are able to calculate analytically the critical temperatures T-c(L) and the thermodynamic functions, to any order cumulant as the functions of the number of L (the hyperlayers in the hyperfilm). Explicit expressions for T-c(L) up to the fourth order are given. A comparison with the existing results for 3-dimensional system is given. The effective range of the interaction is obtained from numerical results.

Degenerate four-wave-mixing signals from dc- and ac-driven semiconductor superlattice

Within the one-dimensional tight-binding model;rnd chi-3 approximation, we have calculated four-wave-mixing (FWM) signals for a semiconductor superlattice in the presence of both static and high-frequency electric fields. When the exciton effect is negligible, the time-periodic field dynamically delocalizes the otherwise localized Wannier-Stark states, and accordingly quasienergy band structures are formed, and manifest in the FWM spectra as a series of equally separated continua. The width of each continuum is proportional to the joint width of the valence and conduction minibands and is independent of the Wannier-Stark index. The realistic homogeneous broadening blurs the continua into broad peaks, whose line shapes, far from the Lorentzian, vary with the delay time in the FWM spectra. The swinging range of the peaks is just the quasienergy bandwidth. The dynamical delocalization (DDL) also induces significant FWM signals well beyond the excitation energy window. When the Coulomb interaction is taken into account, the unequal spacing between the excitonic Wannier-Stark levels weakens the DDL effect, and the FWM spectrum is transformed into groups of discrete lines. Strikingly, the groups are evenly spaced by the ac field frequency, reflecting the characteristic of the quasienergy states. The homogeneous broadening again smears out the line structures, leading to the excitonic FWM spectra quite similar to those without the exciton effect. However, all these features predicted by the dynamical theory do not appear in a recent experiment [Phys. Rev. Lett. 79, 301 (1997)], in which, by using the static approximation the observed Wannier-Stark ladder with delay-time-dependent spacing in the FWM spectra is attributed to a temporally periodic dipole field, produced by the Bloch oscillation of electrons in real space. The contradiction between the dynamical theory and the experiments is discussed. In addition, our calculation indicates that the dynamical localization coherently enhances the time-integrated FWM signals. The feasibility of using such a technique to study the dynamical localization phenomena is shown. [S0163-1829(99)10607-6].

Electronic structures of cubic lattice quantum dots

In this article, we give the electronic structure and optical transition matrix elements of coupled quantum dots (QDs) arranged as different cubic lattices: simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc) superlattices. The results indicate that electron and hole energies of bcc, sc, and fcc superlattices are the lowest, the highest, and the middle, respectively, for the same subband under the same QD density or under the same superlattice constant. For a fixed QD density, the confinement effects in sc, fcc, and bcc superlattices are the strongest, the middle, and the weakest, respectively. There are only one, two, and four confined energy bands, with energies lower than the potential barrier for sc, bcc, and fcc QD superlattices, respectively. The results have great significance for researching and making semiconductor quantum dot devices. (C) 1998 American Institute of Physics. [S0021-8979(98)02119-7]

Trace theory

The concept of traces has been introduced for describing non-sequential behaviour of concurrent systems via its sequential observations. Traces represent concurrent processes in the same way as strings represent sequential ones. The theory of traces can be used as a tool for reasoning about nets and it is hoped that applying this theory one can get a calculus of the concurrent processes anologous to that available for sequential systems. The following topics will be discussed: algebraic properties of traces, trace models of some concurrency phenomena, fixed-point calculus for finding the behaviour of nets, modularity, and some applications of the presented theory.

Practical Evaluation of Security against Generalized Interpolation Attack

Interpolation attack was presented by Jakobsen and Knudsen at FSE'97. Interpolation attack is effective against ciphers that have a certain algebraic structure like the PURE cipher which is a prototype cipher, but it is difficult to apply the attack to real-world ciphers. This difficulty is due to the difficulty of deriving a low degree polynomial relation between ciphertexts and plaintexts. In other words, it is difficult to evaluate the security against interpolation attack. This paper generalizes the interpolation attack. The generalization makes easier to evaluate the security against interpolation attack. We call the generalized interpolation attack linear sum attack. We present an algorithm that evaluates the security of byte-oriented ciphers against linear sum attack. Moreover, we show the relationship between linear sum attack and higher order differential attack. In addition, we show the security of CRYPTON, E2, and RIJNDAEL against linear sum attack using the algorithm.

Linearization Method and Linear Complexity

We focus on the relationship between the linearization method and linear complexity and show that the linearization method is another effective technique for calculating linear complexity. We analyze its effectiveness by comparing with the logic circuit method. We compare the relevant conditions and necessary computational cost with those of the Berlekamp-Massey algorithm and the Games-Chan algorithm. The significant property of a linearization method is that it needs no output sequence from a pseudo-random number generator (PRNG) because it calculates linear complexity using the algebraic expression of its algorithm. When a PRNG has n [bit] stages (registers or internal states), the necessary computational cost is smaller than O(2n). On the other hand, the Berlekamp-Massey algorithm needs O(N2) where N ( 2n) denotes period. Since existing methods calculate using the output sequence, an initial value of PRNG influences a resultant value of linear complexity. Therefore, a linear complexity is generally given as an estimate value. On the other hand, a linearization method calculates from an algorithm of PRNG, it can determine the lower bound of linear complexity.

Modelica建模软件形成的高指标微分代数方程系统指标约简算法研究

随着产品设计的复杂化，应用领域中的数学建模和仿真越来越重要，传统建模方法基于赋值语句，主要考虑单一系统，工程人员需要对程序设计语言和算法求解有相当程度的熟悉，这导致了传统建模方法很难满足复杂产品设计的需要。欧洲仿真界的学者在总结现有建模方法的基础上于1997年推出了一种面向对象的、陈述式的基于方程的语言——Modelica，Modelica语言支持多领域建模，Modelica语言得到了仿真领域众多厂商的支持，成为统一建模领域的事实标准，实现Modelica仿真环境具有重大意义。 编译器是Modelica仿真环境中的核心组件，Modelica编译器会产生高指标DAE系统，除了部分特殊结构的高指标DAE，现有求解器一般不能对通用高指标DAE直接求解，现在一般做法是先对高指标DAE进行指标约简，将其转换成等价的低指标DAE，然后进行求解。这就要求Modelica编译器具有指标约简的功能，以便利用现有求解器进行求解。 本文介绍了几种现有的指标约简算法，给出了各个算法指标约简原理和约简步骤，并对这些算法进行了分析和比较，得出各自的优缺点。最后，本文设计并实现了一个指标约简系统，采用三个Modelica模型产生的方程对系统进行了测试，并验证了实验结果的正确性。

Modelica建模软件中求强连通分量与拓扑排序算法优化的研究

Modelica语言仿真建模在科研工作中已经得到了广泛应用。它能方便地对包含机械、电子、液压、控制、热流等领域的复合物理系统进行基于组件的仿真。现有基于Modelica语言的仿真建模软件支持图形化建模和文本建模两种方式，集成了面向对象、陈述式描述、统一建模、组件重用的优势，给科研工作带来了巨大的便利。 Modelica软件仿真的过程可归结为微分代数方程(differential algebraic equation,DAE)系统的求解。在求解DAE系统时，需要对DAE系统进行约简，直到庞大的DAE系统约简为目前自动求解方法成熟的ODE系统，或约简为方程个数不多的、指标较低的DAE系统，才能使Modelica建模仿真软件具有工业上的应用价值。在约简DAE系统之前，需要对之进行预处理，根据方程之间的数据依赖关系进行拓扑排序，确定求解顺序。排序的过程对应着将DAE系统结构关联矩阵进行块状下三角(block lower triangle,BLT)变换。寻找强连通分量和拓扑排序是对DAE系统进行预处理的重要组成部分。 本文剖析了Modelica软件在仿真时的运行机制，使用几个实例来详细描述在仿真过程中，Modelica软件完成的工作。在寻找强连通分量和拓扑排序这一步，本文提出了使用Kosaraju算法的策略，对由模型得到的有向图直接使用Kosaraju算法，得出DAE系统的求解顺序。文章叙述了强连通分量的含义，并阐述了在求强连通分量时的理论依据，由此引出了Tarjan算法和Kosaraju算法，再分析和比较Tarjan算法和Kosaraju算法，对比了两种策略的优劣，并进行了实验。同时，本文分析了OpenModelica软件包的结构，修改了软件包在寻找强连通分量及拓扑排序相关模块的代码。

yet another meta-language for programming language processing

The formal specification language LFC was designed to support formal specification acquisition. However, it is yet suited to be used as a meta-language for specifying programming language processing. This paper introduces LFC as a meta-language, and compares it with ASF+SDF, an algebraic specification formalism that can also be used to programming languages.

欧洲21世纪数据加密标准候选算法简评

简要介绍了欧洲 NESSIE( new European schemes for signatures,integrity,and encryption)大计划最近公布的 17个分组密码算法的基本设计思想、最新分析结果及其有效性 .