21.已知有窮數(shù)列A:a
1,a
2,?,a
n(n≥3)中的每一項(xiàng)都是不大于n的正整數(shù).對(duì)于滿足1≤m≤n的整數(shù)m,令集合A(m)={k|a
k=m,k=1,2,?,n}.記集合A(m)中元素的個(gè)數(shù)為s(m)(約定空集的元素個(gè)數(shù)為0).
(Ⅰ)若A:6,3,2,5,3,7,5,5,求A(5)及s(5);
(Ⅱ)若
,求證:a
1,a
2,?,a
n互不相同;
(Ⅲ)已知a
1=a,a
2=b,若對(duì)任意的正整數(shù)i,j(i≠j,i+j≤n)都有i+j∈A(a
i)或i+j∈A(a
j),求a
1+a
2+?+a
n的值.