a(n) = 4^n - 3*n + 1.

Terms

a(0) =2a(1) =2a(2) =11a(3) =56a(4) =245a(5) =1010a(6) =4079a(7) =16364a(8) =65513a(9) =262118a(10) =1048547a(11) =4194272a(12) =16777181a(13) =67108826a(14) =268435415a(15) =1073741780a(16) =4294967249a(17) =17179869134a(18) =68719476683a(19) =274877906888