a(n) = 2^n - floor((n+3)/2).

Terms

a(0) =0a(1) =0a(2) =2a(3) =5a(4) =13a(5) =28a(6) =60a(7) =123a(8) =251a(9) =506a(10) =1018a(11) =2041a(12) =4089a(13) =8184a(14) =16376a(15) =32759a(16) =65527a(17) =131062a(18) =262134a(19) =524277a(20) =1048565a(21) =2097140a(22) =4194292a(23) =8388595a(24) =16777203a(25) =33554418a(26) =67108850a(27) =134217713a(28) =268435441a(29) =536870896