a(n) = 4^n - n^2.

Terms

a(0) =1a(1) =3a(2) =12a(3) =55a(4) =240a(5) =999a(6) =4060a(7) =16335a(8) =65472a(9) =262063a(10) =1048476a(11) =4194183a(12) =16777072a(13) =67108695a(14) =268435260a(15) =1073741599a(16) =4294967040a(17) =17179868895a(18) =68719476412a(19) =274877906583