a(1) = 2, a(2) = 4, a(3) = 6; for n > 3, a(n) = 3*a(n-1) - 3*a(n-2) + 9*a(n-3).

Terms

a(0) =2a(1) =4a(2) =6a(3) =24a(4) =90a(5) =252a(6) =702a(7) =2160a(8) =6642a(9) =19764a(10) =58806a(11) =176904a(12) =532170a(13) =1595052a(14) =4780782a(15) =14346720a(16) =43053282a(17) =129146724a(18) =387400806a(19) =1162241784a(20) =3486843450a(21) =10460412252a(22) =31380882462a(23) =94143001680a(24) =282430067922a(25) =847289140884