Number of subsets of {1,2,...,n} such that no two elements differ by 4 or 5.

Terms

a(0) =1a(1) =2a(2) =4a(3) =8a(4) =16a(5) =24a(6) =32a(7) =42a(8) =55a(9) =76a(10) =118a(11) =192a(12) =314a(13) =504a(14) =767a(15) =1120a(16) =1612a(17) =2324a(18) =3412a(19) =5148a(20) =7900a(21) =12169a(22) =18631a(23) =28152a(24) =42024a(25) =62364a(26) =92576a(27) =138141a(28) =207629a(29) =313718