k-element permutation of N A list of k distinct elements chosen from a set N is called a k- element permutation of N. The number of k-element permutations of an n-element set is kth falling k-1 factorial power of n 心nk=Π-i)=n0m-1)…m-k+D= n! n-k)刀 i=0 全排列有多少个？ 集合N上的函数有多少个？ 集合N上的单射/满射双射有多少个？k-element permutation of N ◼ A list of k distinct elements chosen from a set N is called a k￾element permutation of N. 全排列有多少个？ 集合N上的函数有多少个？ 集合N上的单射/满射/双射有多少个？ kth falling factorial power of n