a(n) = 3*a(n-1) - a(n-3) + 3*a(n-4), with initial values 2,4,13,40.

Terms

a(0) =2a(1) =4a(2) =13a(3) =40a(4) =122a(5) =365a(6) =1094a(7) =3280a(8) =9841a(9) =29524a(10) =88574a(11) =265721a(12) =797162a(13) =2391484a(14) =7174453a(15) =21523360a(16) =64570082a(17) =193710245a(18) =581130734a(19) =1743392200a(20) =5230176601a(21) =15690529804a(22) =47071589414a(23) =141214768241a(24) =423644304722