不可能 不动点要求an+1=(aan+b)/(can+d) abcd都不能和n有关
b 2b^2/(b+2) 3b^3/(b^2+2b+4) 4b^4/(b^3+2b^2+4b+8)
所以可以猜测an=nb^n/(b^(n-1)+2b^(n-2)+.)
an+1=(n+1)ban/(an+2n)
=[(n+1)nb^(n+1)/(b^(n-1)+2b^(n-2)+.)]/[nb^n/(b^(n-1)+2b^(n-2)+.)+2n]
=(n+1)b^(n+1)/(b^n+2b^(n-1)+.)
证毕!