a(n) = 6*a(n-2) + a(n-4), where a(0) = 3, a(1) = 5, a(2) = 19, a(3) = 31.

Terms

a(0) =3a(1) =5a(2) =19a(3) =31a(4) =117a(5) =191a(6) =721a(7) =1177a(8) =4443a(9) =7253a(10) =27379a(11) =44695a(12) =168717a(13) =275423a(14) =1039681a(15) =1697233a(16) =6406803a(17) =10458821a(18) =39480499a(19) =64450159a(20) =243289797a(21) =397159775a(22) =1499219281a(23) =2447408809a(24) =9238605483a(25) =15081612629a(26) =56930852179a(27) =92937084583a(28) =350823718557a(29) =572704120127