Number of 3-fold-free subsets of {1, 2, ..., n}.

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =12a(5) =24a(6) =36a(7) =72a(8) =144a(9) =240a(10) =480a(11) =960a(12) =1440a(13) =2880a(14) =5760a(15) =8640a(16) =17280a(17) =34560a(18) =57600a(19) =115200a(20) =230400a(21) =345600a(22) =691200a(23) =1382400a(24) =2073600a(25) =4147200a(26) =8294400a(27) =13271040a(28) =26542080a(29) =53084160