Number of nonempty subsets of {1, ..., n} containing no three cyclically successive elements.

Terms

a(0) =0a(1) =1a(2) =3a(3) =6a(4) =10a(5) =20a(6) =38a(7) =70a(8) =130a(9) =240a(10) =442a(11) =814a(12) =1498a(13) =2756a(14) =5070a(15) =9326a(16) =17154a(17) =31552a(18) =58034a(19) =106742a(20) =196330a(21) =361108a(22) =664182a(23) =1221622a(24) =2246914a(25) =4132720a(26) =7601258a(27) =13980894a(28) =25714874a(29) =47297028