Number of subsets of { 1, ..., n } containing an arithmetic progression of length 4.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =8a(7) =25a(8) =64a(9) =148a(10) =356a(11) =826a(12) =1863a(13) =4205a(14) =9246a(15) =19865a(16) =42935a(17) =90872a(18) =190561a(19) =399104a(20) =829883a(21) =1710609a(22) =3523315a(23) =7224223a(24) =14755538a(25) =30092167a(26) =61177910a(27) =124028647a(28) =251168840a(29) =507216174