Number of subsets of {1, ..., n} that are not double-free.

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =6a(5) =12a(6) =34a(7) =68a(8) =160a(9) =320a(10) =736a(11) =1472a(12) =3136a(13) =6272a(14) =13504a(15) =27008a(16) =56176a(17) =112352a(18) =234064a(19) =468128a(20) =954976a(21) =1909952a(22) =3913504a(23) =7827008a(24) =15878656a(25) =31757312a(26) =64413184a(27) =128826368a(28) =259449856a(29) =518899712