Proof: Let s=(1, 2,.o, n and x be the set of all permutations of s. Then x=n j〔j+1),p(1,2,…,n-) Q=D,+D▪ Proof: Let S={1,2,…,n} and X be the set of all permutations of S. Then |X|=n!. ▪ j(j+1), pj (1,2,…,n-1) ▪ Aj : pj ▪ Qn=Dn+Dn-1