a(n) = 3*a(n-1) + a(n-2) - 3*a(n-3).

Terms

a(0) =1a(1) =3a(2) =5a(3) =15a(4) =41a(5) =123a(6) =365a(7) =1095a(8) =3281a(9) =9843a(10) =29525a(11) =88575a(12) =265721a(13) =797163a(14) =2391485a(15) =7174455a(16) =21523361a(17) =64570083a(18) =193710245a(19) =581130735a(20) =1743392201a(21) =5230176603a(22) =15690529805a(23) =47071589415a(24) =141214768241a(25) =423644304723