A018793 Number of subsets of { 1, ..., n } containing an A.P. of length 8.

0, 0, 0, 0, 0, 0, 0, 0, 1, 3, 8, 20, 48, 112, 256, 684, 1696, 3840, 8575, 18939, 41452, 90046, 206952, 466731, 1037736, 2233540, 4782408, 10194180, 21638844, 47278465, 102437882, 220088444, 470234676