Cho S = 1 + 2 + 2 mu 2 + 2 mu 3 + 2 mu 4 + 2 mu 5 + 2 mu 6 + 2 ^7

cho S = 1 + 2 + 2 mu 2 + 2 mu 3 + 2 mu 4 + 2 mu 5 + 2 mu 6 + 2 mu7
chung to S chia het cho 3