1/1, 1/2, 1/3, 3/4, 3/5, 1/2, 3/7, 5/8, 5/9, ...
0/1, 1/2, 2/3, 3/4, 2/5, 1/2, 4/7, 5/8, 4/9, ...
a(n),第一和第二差异:
0, 1, 2, 3, 4, 5, 2, 7, 8, 9, 2, 11, 12, ...
1, 1, 1, 1, 1, -3, 5, 1, 1, -7, 9, 1, 1, ...
0, 0, 0, 0, -4, 8, -4, 0, -8, 16, -8, 0, -12, ...