Number of subsets of { 1, ..., n } containing an A.P. of length 5.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =1a(6) =3a(7) =8a(8) =20a(9) =56a(10) =144a(11) =332a(12) =751a(13) =1803a(14) =4163a(15) =9401a(16) =20404a(17) =45169a(18) =98753a(19) =213575a(20) =457945a(21) =990624a(22) =2119184a(23) =4493766a(24) =9460916a(25) =19947914a(26) =41567180a(27) =86219409a(28) =178237782a(29) =369985101