先熟悉一个数学概念,叫做置换(Permutation),在组合数学的领域也常被称作排列 我们知道,一个集合 A 到它自身的映射 A\rightarrow A 叫做变换(Transform) 如果这个集合 A 本身是有限集合,而且这个变换是个一一变换,此时这个变换就叫做置换 一个 n 元有限集合,能与集合 \left\{ 1,2,\cdots,n ...
JUBA, Feb. 2 (Xinhua) -- The United Nations Mission in South Sudan (UNMISS) on Sunday expressed concern about the escalating violence between cattle keepers and settled farming communities in Eastern ...
Aviva Siegel, the wife of Israeli-American hostage Keith Siegel and herself a former captive, was excited as she left home to meet her husband as he was being handed over by Hamas on Saturday (Februar ...