a(n) = 4^n + 4.

Terms

a(0) =5a(1) =8a(2) =20a(3) =68a(4) =260a(5) =1028a(6) =4100a(7) =16388a(8) =65540a(9) =262148a(10) =1048580a(11) =4194308a(12) =16777220a(13) =67108868a(14) =268435460a(15) =1073741828a(16) =4294967300a(17) =17179869188a(18) =68719476740a(19) =274877906948