a(n) = 4^n - n.

Terms

a(0) =1a(1) =3a(2) =14a(3) =61a(4) =252a(5) =1019a(6) =4090a(7) =16377a(8) =65528a(9) =262135a(10) =1048566a(11) =4194293a(12) =16777204a(13) =67108851a(14) =268435442a(15) =1073741809a(16) =4294967280a(17) =17179869167a(18) =68719476718a(19) =274877906925