a(n) = 4^n - n - 1.

Terms

a(0) =2a(1) =13a(2) =60a(3) =251a(4) =1018a(5) =4089a(6) =16376a(7) =65527a(8) =262134a(9) =1048565a(10) =4194292a(11) =16777203a(12) =67108850a(13) =268435441a(14) =1073741808a(15) =4294967279a(16) =17179869166a(17) =68719476717a(18) =274877906924