Polynomial Dehn Functions and the Length of Asynchronously Automatic Structures

We extend the range of observed behaviour among length functionsof optimal asynchronously automatic structures. We do so bymeans of a construction that yields asynchronously automaticgroups with finite aspherical presentations where the Dehn functionof the group is polynomial of arbitrary degree. Many of thesegroups can be embedded in the automorphism group of a free group.Moreover, the fact that the groups have aspherical presentationsmakes them useful tools in the search to determine the spectrumof exponents for second order Dehn functions. We contributeto this search by giving the first exact calculations of groupswith quadratic and superquadratic exponents. 2000 Mathematical Subject Classification: 20F06, 20F65, 20F69.     

不同动能的~(129)Xe~(26+)与Au表面作用产生的X射线谱

测量了动能为350—600 keV和1.8—3.9 MeV的129Xe26+入射Au表面产生的X射线谱.结果表明,350—600 keV的Xe26+仅激发出了Au的Mα特征X射线,而1.8—3.9 MeV的Xe26+可激发出Au的Mζ,Mα,Mγ和Mδ特征X射线.分析了X射线强度和产额比与入射离子动能的关系,并估计了Xe26+激发Xe的L-X射线的动能阈值.     

ON THE CONSISTENCY OF CROSS-VALIDATION IN NONLINEAR WAVELET REGRESSION ESTIMATION

1IntroductionConsidernonparametricregressiollwherextli=;E[O,1],En1,.',Ejl1larei.d.d.errorswitl11llea11o,a11dgisanunkl1ownfunctiontobeestimated-Nouparametricregressionestil11ationaimstoesti111atetl1euukllow11regressionfunctiong(x).Tl1enolllinearwaveletesti111atorshavebeenintroducedalldstudiedbyDonoho(1995).DonohoandJohnstoue(1994,1995).Do11oho,Johnstolle-KerkyachariallandPicard(1995)andDelyollandJuditsky(l996).Tl1eseauthorshavedenlo11stratedthevirtuesofllonlineaxwaveletmetl1odsf1o1llthev…     

卫星激光通信滤光膜的研制

为满足卫星激光通信中超高速数据传输的特殊要求,采用电子束和离子辅助沉积技术,制备了532 nm、632 nm和1 064 nm波长处高反射,808 nm和1 550 nm处高透射的多波段滤光膜.选取了H₄和SiO₂作为高低折射率材料,通过对膜系设计曲线的不断优化,减少了灵敏层的个数,得到了相对易于制备的膜系结构;采用电子束加热蒸发方法并加以离子辅助沉积系统制备薄膜, 采用光控与晶控同时监控的方法控制膜厚;通过不断调整工艺,提高了薄膜的抗激光损伤能力,减小了膜厚控制误差,提高了透射波段的透过率及反射波段的反射率,最终得到了光谱性能较好的滤光膜.该薄膜能够承受雨淋、盐雾、高低温等环境测试,满足使用要求.     

液体中光击穿阈值的研究

从等离子体中自由电子密度速率方程出发,考虑到脉冲激光在聚焦区域的特征以及液体中光击穿的实验情况,提出了等离子体椭球模型.通过该模型的建立,对自由电子密度速率方程中电子扩散速率进行了修正,在理论上得到了光击穿的阈值.结果表明,等离子体椭球模型计算出水的击穿阈值更符合实验情况.     

Discrete storage processes and their Poisson flow and fluid flow approximations

Consider discrete storage processes that are modulated by environmental processes. Environmental processes cause interruptions in the input and/or output processes of the discrete storage processes. Due to the difficulties encountered in the exact analysis of such discrete storage systems, often Poisson flow and/or fluid flow models with the same modulating environmental processes are proposed as approximations for these systems. The analysis of Poisson flow and fluid flow models is much easier than that of the discrete storage processes. In this paper we give sufficient conditions under which the content of the discrete storage processes can be bounded by the Poisson flow and the fluid flow models. For example, we show that Poisson flow models and the fluid flow models developed by Kosten (and by Anick, Mitra and Sondhi) can be used to bound the performance of infinite (finite) source packetized voice/data communication systems. We also show that a Poisson flow model and the fluid flow model developed by Mitra can be used to bound the buffer content of a two stage automatic transfer line. The potential use of the bounding techniques presented in this paper, of course, transcends well beyond these examples.Supported in part by NSF grant DMS-9308149.     

一种基于场景的红外图像非均匀性校正算法

提出了一种基于场景的红外图像非均匀性校正算法。该算法结合了两点定标校正算法和基于场景的改进的恒定统计算法,将两点校正算法的校正系数作为恒定统计算法的系数初值,并引入阈值进行运动状态检测,对运动场景和非运动场景分别进行系数更新。实验表明,该算法可以实现对红外图像非均匀性的校正,对于本文实验中的视频图像,在100帧时算法收敛,其收敛时间优于其他传统基于场景的非均匀性校正算法,并一定程度上抑制了"鬼影"现象。     

基于改进中值滤波和提升小波变换的阈值去噪方法研究

对于实际拍摄的一些图像信噪比低,噪声密度大,且含有混合噪声,而现有算法大多只能去除单一噪声的问题。针对混合噪声中含有的脉冲噪声和高斯噪声,提出基于改进中值滤波和提升小波变换去噪相结合的方法。去噪过程中,使用中值滤波器提取脉冲噪声并采用中值滤波算法滤波后,构造提升小波,采用改进阈值函数提升小波阈值去噪方法去除高斯噪声。实验结果表明,当噪声值(,)=(0.4, 20)时,采用本文去噪方法,峰值信噪比（PSNR）为34.002 1,平均绝对误差（MAE）为2.365 3。     

像增强器视场缺陷检测方法研究

在数字式像增强器性能综合检测系统的基础上，研究了像增强器视场缺陷的检测方法。通过数字视频摄像机采集缺陷图像，利用数字图像处理方法确定缺陷性质，缺陷处理算法基于图像灰度特征而不是边缘特征提取。该方法适用于对检测对象有具体量化和定位要求，而不是简单的只有数量和形状特征要求的场合。实验表明，该检测方法能够较准确地检测像增强器的缺陷。     

稀疏阈值的超分辨率图像重建

为了解决基于字典学习的超分辨重构算法耗时过长的问题,提出了基于稀疏阈值模型的图像超分辨率重建方法。首先,将联合字典理论与图像块稀疏阈值方法相结合,训练得到高、低分辨率过完备图像字典对。接着,通过稀疏阈值OMP算法对图像特征块进行稀疏表示。然后,通过高分辨率字典重构出初始的超分辨图像。最后,通过改进迭代反投影算法对初始的超分辨图像进行全局优化,从而进一步提高图像重构质量。实验结果表明,超分辨图像重构平均峰值信噪比(PSNR)为30.1 d B,平均结构自相似度(SSIM)为0.937 9,平均计算时间为10.2 s。有效提高了超分辨重构的速度,改善了重构高分辨图像的质量。