a(n)必须为正整数.
a(n)必须单调递增.
a[a(1)] = 3,
若a(1)=k>2,
则2=a[a(1)]=a(k)>a(2),2>a(2)>a(1)与a(1)>2矛盾.
因此,只能a(1)=k