a(n) = 2^(n-1) - tau(n) where tau(n) is the number of divisors of n.

Terms

a(0) =0a(1) =0a(2) =2a(3) =5a(4) =14a(5) =28a(6) =62a(7) =124a(8) =253a(9) =508a(10) =1022a(11) =2042a(12) =4094a(13) =8188a(14) =16380a(15) =32763a(16) =65534a(17) =131066a(18) =262142a(19) =524282a(20) =1048572a(21) =2097148a(22) =4194302a(23) =8388600a(24) =16777213a(25) =33554428a(26) =67108860a(27) =134217722a(28) =268435454a(29) =536870904