a(n) = 17*3^n.

Terms

a(0) =17a(1) =51a(2) =153a(3) =459a(4) =1377a(5) =4131a(6) =12393a(7) =37179a(8) =111537a(9) =334611a(10) =1003833a(11) =3011499a(12) =9034497a(13) =27103491a(14) =81310473a(15) =243931419a(16) =731794257a(17) =2195382771a(18) =6586148313a(19) =19758444939a(20) =59275334817a(21) =177826004451a(22) =533478013353