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章志敏 《数学的实践与认识》1980,(4)
<正> 表示任务的完成时间 X 的数学期望和方差,那么 X 应服从什么分布呢?美国“计划评审技术”(PERT)工作者说是β分布,[1]指出是有条件的β分布,华罗庚教授在[2]中指出,经过验算无法接受 PERT 的结论,并提出应用概率论中的极限定理,假设是正态分布,最后希望数学工作者能够进行理论的探讨.本文从获得最大平均信息量的观点,找出了 X应服从截尾的正态分布律. 相似文献
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A novel optical beam splitter constructed on the basis of photonic crystal(PC) with hybrid lattices is proposed in this paper.The band gap of square-lattice PC is so designed that the incident light is divided into several branch beams.Triangular-lattice graded-index PCs are combined for focusing each branch.Computational calculations are carried out on the basis of finite-different time-domain algorithm to prove the feasibility of our design.The waveguide is unnecessary in the design.Thus the device has functions of both splitting and focusing beams.Size of the divided beam at site of full-width at half-maximum is of the order of λ/2.The designed splitter has the advantages that it has a small volume and can be integrated by conventional semiconductor manufacturing process. 相似文献