2^20—1
= (2^5)^4 - 1
= ((2^5)^2 + 1)((2^5)^2 -1)
= ((2^5)^2 + 1)(2^5 + 1)(2^5 - 1)
= 31((2^5)^2 + 1)(2^5 + 1)
能被31整除