a(n) = 4^n + n.

Terms

a(0) =1a(1) =5a(2) =18a(3) =67a(4) =260a(5) =1029a(6) =4102a(7) =16391a(8) =65544a(9) =262153a(10) =1048586a(11) =4194315a(12) =16777228a(13) =67108877a(14) =268435470a(15) =1073741839a(16) =4294967312a(17) =17179869201a(18) =68719476754a(19) =274877906963