a(n) = 2*(Lucas(n)^2 - (-1)^n).

Terms

a(0) =6a(1) =4a(2) =16a(3) =34a(4) =96a(5) =244a(6) =646a(7) =1684a(8) =4416a(9) =11554a(10) =30256a(11) =79204a(12) =207366a(13) =542884a(14) =1421296a(15) =3720994a(16) =9741696a(17) =25504084a(18) =66770566a(19) =174807604a(20) =457652256a(21) =1198149154a(22) =3136795216a(23) =8212236484a(24) =21499914246a(25) =56287506244a(26) =147362604496a(27) =385800307234