f(f(1))=3,假设f(1)=1时,f(f(1))=1矛盾假设f(1)>=3,f(f(1))>=f(3)>f(1)>3
所以f(1)=2,f(2)=3*1=3,f(3)=3*2=6,f(6)=3*3=9,f(9)=3*6=18
有f(f(4)=12,f(f(5))=15,所以又在f(6)和f(9)之间只有f(7)和f(8)
所以f(8)=15,f(5)=8
下证f(3k)=3f(k)且f-1(3k)=3f-1(k)
若f(3k)3f-1(k),则有9k=f(f(3k))>f(3f(k)),即3f(k)