随机约束满足问题相变研究综述

随机约束满足问题是经典的NP完全问题,在理论研究和现实生活中有着广泛应用。研究人员发现随机约束满足问题存在相变现象,近几十年来关于此问题相变的研究成果不断涌现。从随机图着色问题和随机可满足问题2个最经典的随机约束满足问题入手,从算法研究、理论物理和数学证明3个方面综述了随机图着色问题和随机可满足问题的相变研究成果。最后对随机约束满足问题相变的研究趋势进行了展望。     

Another look at the dominant point detection of digital curves

We propose a straightforward parameter-free method for detecting dominant points of digital curves. It uses the chain code properties of digital straight lines to compute two vectors of significance. The dominant points are derived from these vectors by a logical function (strategy). The dominant point set depends on the strategy used. Prior knowledge can easily be taken into account to guide the results. The obtained approximating polygon keeps the symmetrical nature of a shape.     

Function-described graphs for modelling objects represented by sets of attributed graphs

We present in this article the model function-described graph (FDG), which is a type of compact representation of a set of attributed graphs (AGs) that borrow from random graphs the capability of probabilistic modelling of structural and attribute information. We define the FDGs, their features and two distance measures between AGs (unclassified patterns) and FDGs (models or classes) and we also explain an efficient matching algorithm. Two applications of FDGs are presented: in the former, FDGs are used for modelling and matching 3D-objects described by multiple views, whereas in the latter, they are used for representing and recognising human faces, described also by several views.     

On the independence number and Hamiltonicity of uniform random intersection graphs

In the uniform random intersection graphs model, denoted by G_n,m,λ, to each vertex v we assign exactly λ randomly chosen labels of some label set M of m labels and we connect every pair of vertices that has at least one label in common. In this model, we estimate the independence number α(G_n,m,λ), for the wide range m=⌊n^α⌋,α<1 and λ=O(m^1/4). We also prove the Hamiltonicity of this model by an interesting combinatorial construction. Finally, we give a brief note concerning the independence number of G_n,m,p random intersection graphs, in which each vertex chooses labels with probability p.     

Expander properties and the cover time of random intersection graphs

We investigate important combinatorial and algorithmic properties of _Gn,m,p$G_{n,m,p}$ random intersection graphs. In particular, we prove that with high probability (a) random intersection graphs are expanders, (b) random walks on such graphs are “rapidly mixing” (in particular they mix in logarithmic time) and (c) the cover time of random walks on such graphs is optimal (i.e. it is Θ(nlogn)$\Theta (nlogn)$). All results are proved for p$p$ very close to the connectivity threshold and for the interesting, non-trivial range where random intersection graphs differ from classical _Gn,p$G_{n,p}$ random graphs.     

On the random generation and counting of weak order extensions of a poset with given class cardinalities

In previous work, we have proposed a simple algorithm to generate random linear extensions of a partially ordered set (poset). A closely related problem is the random generation of so-called weak order extensions of a poset. Such an extension can be informally characterized as a linear order on the equivalence classes of a partition of the poset, not contradicting the underlying poset order. The generation of linear extensions can then be seen as a special case of the generation of weak order extensions where each equivalence class degenerates into a singleton. If no a priori knowledge about the underlying partition is available, time complexity increases tremendously. In first instance, we therefore restrict to the generation of weak order extensions with given class cardinalities, a problem encountered in the context of ranking algorithms. It will be shown that a first random weak order extension can be generated in O(w²(P)·|I(P)|) time, while every subsequent extension with the same class cardinalities can be obtained in O(w(P)·|P|) time, where |I(P)| denotes the number of ideals of the poset P, and w(P) the width of the poset P. Additionally, the number of weak order extensions obeying the specified class cardinalities can also be obtained in the stated O(w²(P)·|I(P)|) time.     

Hysteretic effects in the first-order irreversible phase transition of the ZGB model

The first-order irreversible phase transition (FOIPT) of the ZGB model [Ziff, Gulari, Barshad, Phys. Rev. Lett. 56 (1986) 2553] for the catalytic oxidation of carbon monoxide is studied numerically by using the constant-coverage (CC) ensemble. The CC method allows us to study hysteretic effects close to coexistence, as well as the location of the coexistence point. Also, evidence that the FOIPT exhibits a condensation/evaporation pseudo-transition (in finite samples), as observed in its reversible counterparts, is presented and discussed.     

A new look at the robust control of discrete-time Markov jump linear systems

In this paper, we make a foray in the role played by a set of four operators on the study of robust H₂ and mixed H₂/H_∞ control problems for discrete-time Markov jump linear systems. These operators appear in the study of mean square stability for this class of systems. By means of new linear matrix inequality (LMI) characterisations of controllers, which include slack variables that, to some extent, separate the robustness and performance objectives, we introduce four alternative approaches to the design of controllers which are robustly stabilising and at the same time provide a guaranteed level of H₂ performance. Since each operator provides a different degree of conservatism, the results are unified in the form of an iterative LMI technique for designing robust H₂ controllers, whose convergence is attained in a finite number of steps. The method yields a new way of computing mixed H₂/H_∞ controllers, whose conservatism decreases with iteration. Two numerical examples illustrate the applicability of the proposed results for the control of a small unmanned aerial vehicle, and for an underactuated robotic arm.     

How smart are our environments? An updated look at the state of the art

In this paper we take a look at the state of the art in smart environments research. The survey is motivated by the recent dramatic increase of activity in the field, and summarizes work in a variety of supporting disciplines. We also discuss the application of smart environments research to health monitoring and assistance, followed by ongoing challenges for continued research.     

On the connection between the phase transition of the covering test and the learning success rate in ILP

It is well-known that heuristic search in ILP is prone to plateau phenomena. An explanation can be given after the work of Giordana and Saitta: the ILP covering test is NP-complete and therefore exhibits a sharp phase transition in its coverage probability. As the heuristic value of a hypothesis depends on the number of covered examples, the regions “yes” and “no” represent plateaus that need to be crossed during search without an informative heuristic value. Several subsequent works have extensively studied this finding by running several learning algorithms on a large set of artificially generated problems and argued that the occurrence of this phase transition dooms every learning algorithm to fail to identify the target concept. We note however that only generate-and-test learning algorithms have been applied and that this conclusion has to be qualified in the case of data-driven learning algorithms. Mostly building on the pioneering work of Winston on near-miss examples, we show that, on the same set of problems, a top-down data-driven strategy can cross any plateau if near-misses are supplied in the training set, whereas they do not change the plateau profile and do not guide a generate-and-test strategy. We conclude that the location of the target concept with respect to the phase transition alone is not a reliable indication of the learning problem difficulty as previously thought. Editors: Stephen Muggleton, Ramon Otero, Simon Colton.     

A factor graph-based iterative detection of faster-than-Nyquist signaling in the presence of phase noise and carrier frequency offset

With the increasing demand for higher spectral efficiency in wireless communications, faster-than-Nyquist (FTN) signaling has been rediscovered to increase transmission rate without expanding signaling bandwidth. Most existing studies focus on low-complexity FTN receiver design by assuming perfect synchronization. In practice, however, phase noise (PHN) and carrier frequency offset (CFO) may degrade the performance of FTN detector significantly. In this paper, we develop iterative FTN detector in the presence of PHN and CFO in a factor graph framework. Wiener process is employed to model the time evolution of nonstationary channel phase. The colored noise imposed by sampling of FTN signaling is approximated by autoregressive model. Based on the factor graph constructed, messages are derived on the two subgraphs, i.e., PHN and CFO estimation subgraph and the FTN symbol detection subgraph. We propose two methods to update the messages between subgraphs, namely, Gaussian approximation via Kullback–Leibler divergence (KLD) minimization and the combined sum-product and variational message passing (SP-VMP), both of which enable low-complexity parametric message passing. The proposed SP-VMP algorithm can provide closed-form expressions for parameters updating. Moreover, conjugate gradient (CG) method is adopted to solve the maximum a posteriori probability (MAP) estimation of CFO with fast convergence speed. Simulation results show the superior performance of the proposed algorithm compared with the existing methods and verify the advantage of FTN signaling compared with the Nyquist counterpart.     

Investigation of the hysteresis phenomenon of an a‐Si:H TFT at an elevated temperature for AMOLED displays

Abstract— The temperature dependence of the hysteresis of an a‐Si:H TFT has been investigated. An a‐Si:H TFT pixel driving scheme has been proposed and investigated. This scheme can eliminate changes in the organic light‐emitting diode (OLED) current caused by hysteresis of an a‐Si:H TFT. The VTH of the a‐Si:H TFT was changed according to the gate‐voltage sweep direction because of the hysteresis of the a‐Si:H TFT. The variation of VTH for a a‐Si:H TFT decreased from 0.41 to 0.17 V at an elevated temperature of 60°C because the sub‐threshold slope (s‐slope) of the a‐Si:H TFT, in the reverse voltage sweep direction, increased more than in the forward voltage sweep direction due to a greater increase in the initial electron trapped charges than the hole charges. Although the OLED current variation caused by hysteresis decreased (～14%) as the temperature increased, the error in the OLED current needed to be improved in order to drive the pixel circuit of AMOLED displays. The proposed pixel circuit can apply the reset voltage (?10 V) before the data voltage for the present frame that was written to fix the sweep direction of the data voltage. The variation in the OLED current caused by hysteresis of the a‐Si:H TFT was eliminated by the fixed voltage sweep direction in the proposed pixel circuit regardless of operating temperature.     

Identifying a transition period at the midpoint of an online collaborative activity: a study among adult learners

This case study describes the group development in an online learning group. Adult learners enrolled in a MBA partially delivered at a distance were observed during fifteen weeks to better understand the group development of an online learning group. Both qualitative and quantitative methods were used to capture the dynamic of social interactions within the group and some affective reactions of members. The results revealed a transition period at the midpoint of the collaborative activity showing a decline of task-oriented communications, motivation and positive mood from this period. Results were discussed through models of group development validated in face-to-face environments. Some implications were proposed to facilitate online activities in computer-supported learning groups.     

一种车站危险品检测中的三维图像重建方法

图像重建是车站危险品检测过程中的重要环节,其结果直接关系到后续危险品的识别。为此提出了一种基于FDK算法的三维图像重建方法。该方法首先将X射线探测器得到的物体投影值,利用正弦函数对投影数据进行加权;然后对不同投影角度的投影数据进行水平方向的一维滤波,去除噪声和伪迹;最后沿X射线方向进行三维反投影。实验结果表明,该方法能更清晰地重建出物体。     

高清数字视频监控系统在钻井现场的研究与应用

视频监控系统可广泛应用于油气田勘探生产领域。在油田数字化的大背景下,钻井现场使用的高清数字视频监控系统高度融合了计算机技术、图像视频压缩技术及网络技术,其画面清晰、图像延时短、系统稳定可靠,更有利于功能扩展,而且数据更易完整保存以及网络化远程传输控制,同时系统硬件结构也便于钻井现场的安装维护任务。     

3DS Max中"放样"的使用技巧初探

3DS Max的应用已越来越广泛,其中的"放样"乍看起来不难,但在实际操作中可能会遇到各种各样的问题,本人在长期的设计和教学中,探索出几条规律,可能对指导"放样"有一定的帮助.     

从MOHO实例看国内城域网的现状与应用性前景

MOHO是典型的绿色办公方式,而它的支持环境就是计算机+宽带网络。本文从一般的MOHO(并非特殊行业、特殊应用)实例说明当前国内宽带网络(指小区宽带、固网宽带和无线宽带)的性能、水平。展望不久的将来国内宽带网络与物联网的发展水平。从中看到我国宽带网络的应用性前景。     

Experimental investigation of phase equilibria in the Fe-Hf-Zr system at 1173 K and 1373 K

A series of Fe-Hf-Zr alloys were designed to determine the phase equilibria for the entire composition range by annealing at 1173 K and 1373 K for 440–720 h and 12–360 h respectively, followed by X-ray diffraction (XRD) measurements and electron probe microanalysis (EPMA). At the Fe-rich corner, it was argued that the Fe₂₃Zr₆ phase is a stable phase without oxygen contamination, of which the formation mechanism was discussed. The Fe₂(Hf,Zr)_C15 phase forms a continuous solid solution. The two-phase region of β(Hf,Zr) + Fe(Hf,Zr)₃ was detected at 1173 K. The Fe(Hf,Zr)₃ phase was observed in this ternary system at 1173 K, which is attributed to the increasing thermodynamic stability by dissolving Hf. The solubility of Fe in α(Hf,Zr) was determined to be extremely low. The liquid phase appears at 1373 K in a large composition range. No ternary compound was identified at both temperatures in this system. Based on the experimental results and reasonable extrapolations, the isothermal sections of 1173 K and 1373 K were constructed, which respectively consist of seven and five three-phase regions.