Number of subsets of {1..n} not forming a finite arithmetic progression.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =2a(5) =9a(6) =30a(7) =79a(8) =190a(9) =425a(10) =913a(11) =1909a(12) =3927a(13) =7987a(14) =16141a(15) =32483a(16) =65205a(17) =130690a(18) =261709a(19) =523794a(20) =1048021a(21) =2096530a(22) =4193611a(23) =8387840a(24) =16776371a(25) =33553502a(26) =67107846a(27) =134216618a(28) =268434250a(29) =536869604