It is generally known that the solutions of deterministic and stochastic differential equations （SDEs） usually grow linearly at such a rate that they may become unbounded after a small lapse of time and may eventual...It is generally known that the solutions of deterministic and stochastic differential equations （SDEs） usually grow linearly at such a rate that they may become unbounded after a small lapse of time and may eventually blow up or explode in finite time. If the drift and diffusion functions are globally Lipschitz, linear growth may still be experienced, as well as a possible blow-up of solutions in finite time. In this paper, a nonlinear scalar delay differential equation with a constant time lag is perturbed by a multiplicative Ito-type time - space white noise to form a stochastic Fokker-Planck delay differential equation. It is established that no explosion is possible in the presence of any intrinsically slow time - space white noise of Ito - type as manifested in the resulting stochastic Fokker- Planck delay differential equation. Time - space white noise has a role to play since the solution of the classical nonlinear equation without it still exhibits explosion.展开更多
Time and Space Thinking that is composed of time and space thinking is brought up in civil engineering. This paper makes a detailed analysis on application and role of time and space thinking in knowledge system and l...Time and Space Thinking that is composed of time and space thinking is brought up in civil engineering. This paper makes a detailed analysis on application and role of time and space thinking in knowledge system and learning method of civil engineering and put forward time and space thinking to be similar with philosophical view of time and space. Time and space thinking is a scientific method, therefore, it is suggested to make students active in having such idea and strengthen students＇ understanding on time and space thinking, which will helping students recognize knowledge system and stimulate innovative abilities.展开更多
基于稀疏恢复的空时自适应算法(Sparse Recovery Space Time Adaptive Processing,SR-STAP)能有效改善机载雷达在复杂环境下对杂波的抑制能力,通常是将空时平面均匀离散为若干个网格来构造字典。然而,真实的杂波点往往不能落在预先离散...基于稀疏恢复的空时自适应算法(Sparse Recovery Space Time Adaptive Processing,SR-STAP)能有效改善机载雷达在复杂环境下对杂波的抑制能力,通常是将空时平面均匀离散为若干个网格来构造字典。然而,真实的杂波点往往不能落在预先离散的网格点上,此时会出现离网效应,导致SR-STAP的性能降低。本文针对此问题,提出了一种基于知识辅助的字典校正方法。首先利用载机平台参数等先验知识均匀离散空间频率,然后计算和修正多普勒频率,并根据先验知识修正空间频率,最后利用修正后的空间频率和多普勒频率对应的空时导向矢量来构造超完备稀疏字典。仿真结果表明,与传统字典构造算法相比,该字典校正方法有效克服了离网效应,改善了STAP的性能。展开更多
中国现有耕地数量有限、耕地后备资源严重不足,提高耕地利用效率是保障中国粮食安全的必要条件之一。为综合展现近年来中国耕地利用效率的相关研究现状和发展趋势,系统回顾现有研究展望未来发展。以中国学术期刊全文数据库(CNKI)、Web o...中国现有耕地数量有限、耕地后备资源严重不足,提高耕地利用效率是保障中国粮食安全的必要条件之一。为综合展现近年来中国耕地利用效率的相关研究现状和发展趋势,系统回顾现有研究展望未来发展。以中国学术期刊全文数据库(CNKI)、Web of Science(WoS)核心期刊为数据源,以2002—2022年208篇研究论文为研究对象,通过文献计量分析法,运用CiteSpace等文献计量可视化软件进行可视化图谱分析。研究发现,中国目前在耕地利用效率上的研究,形成了以DEA方法为主体,综合运用Malmquist指数、相关参数模型为测算框架的耕地利用效率测度体系;以GIS技术、ESDA等时空差异分析方法;以Tobit计量回归模型为主的耕地利用效率影响因素评价体系。对比国内外相关研究发现,中国现阶段相关研究主要从效率测算角度出发,探寻耕地利用效率的提升路径,而国外相关研究主要从可持续发展视角出发,力图寻找更合理的耕作制度与农业系统。展开更多
针对空时自适应处理中样本协方差矩阵受干扰目标污染时检测性能下降的问题,提出了一种基于知识的空时自适应处理(knowledge aided space time adaptive processing,KA-STAP)方法。该方法将待测距离单元杂波的先验知识与广义内积非同态...针对空时自适应处理中样本协方差矩阵受干扰目标污染时检测性能下降的问题,提出了一种基于知识的空时自适应处理(knowledge aided space time adaptive processing,KA-STAP)方法。该方法将待测距离单元杂波的先验知识与广义内积非同态检测器(general inner product nonhomogeneity detector,GIP NHD)结合,对训练样本进行有效选择。通过仿真证明该方法能有效剔除存在干扰目标的样本,提高训练样本被干扰目标污染时空时自适应处理的检测性能。展开更多
文摘It is generally known that the solutions of deterministic and stochastic differential equations （SDEs） usually grow linearly at such a rate that they may become unbounded after a small lapse of time and may eventually blow up or explode in finite time. If the drift and diffusion functions are globally Lipschitz, linear growth may still be experienced, as well as a possible blow-up of solutions in finite time. In this paper, a nonlinear scalar delay differential equation with a constant time lag is perturbed by a multiplicative Ito-type time - space white noise to form a stochastic Fokker-Planck delay differential equation. It is established that no explosion is possible in the presence of any intrinsically slow time - space white noise of Ito - type as manifested in the resulting stochastic Fokker- Planck delay differential equation. Time - space white noise has a role to play since the solution of the classical nonlinear equation without it still exhibits explosion.
文摘Time and Space Thinking that is composed of time and space thinking is brought up in civil engineering. This paper makes a detailed analysis on application and role of time and space thinking in knowledge system and learning method of civil engineering and put forward time and space thinking to be similar with philosophical view of time and space. Time and space thinking is a scientific method, therefore, it is suggested to make students active in having such idea and strengthen students＇ understanding on time and space thinking, which will helping students recognize knowledge system and stimulate innovative abilities.
文摘基于稀疏恢复的空时自适应算法(Sparse Recovery Space Time Adaptive Processing,SR-STAP)能有效改善机载雷达在复杂环境下对杂波的抑制能力,通常是将空时平面均匀离散为若干个网格来构造字典。然而,真实的杂波点往往不能落在预先离散的网格点上,此时会出现离网效应,导致SR-STAP的性能降低。本文针对此问题,提出了一种基于知识辅助的字典校正方法。首先利用载机平台参数等先验知识均匀离散空间频率,然后计算和修正多普勒频率,并根据先验知识修正空间频率,最后利用修正后的空间频率和多普勒频率对应的空时导向矢量来构造超完备稀疏字典。仿真结果表明,与传统字典构造算法相比,该字典校正方法有效克服了离网效应,改善了STAP的性能。
文摘中国现有耕地数量有限、耕地后备资源严重不足,提高耕地利用效率是保障中国粮食安全的必要条件之一。为综合展现近年来中国耕地利用效率的相关研究现状和发展趋势,系统回顾现有研究展望未来发展。以中国学术期刊全文数据库(CNKI)、Web of Science(WoS)核心期刊为数据源,以2002—2022年208篇研究论文为研究对象,通过文献计量分析法,运用CiteSpace等文献计量可视化软件进行可视化图谱分析。研究发现,中国目前在耕地利用效率上的研究,形成了以DEA方法为主体,综合运用Malmquist指数、相关参数模型为测算框架的耕地利用效率测度体系;以GIS技术、ESDA等时空差异分析方法;以Tobit计量回归模型为主的耕地利用效率影响因素评价体系。对比国内外相关研究发现,中国现阶段相关研究主要从效率测算角度出发,探寻耕地利用效率的提升路径,而国外相关研究主要从可持续发展视角出发,力图寻找更合理的耕作制度与农业系统。
