>>432
641 = 5 * 2^7 +1

5*2^7 = 641 -1
5^4*2^28 = (641-1)^4
(641-2^4)*2^28 = (641-1)^4
2^32 = 641*2^28 - (641-1)^4 = 641m -1
だから
2^32 + 1は641の倍数。