a(n) = a(n-1) + 3*a(n-2) - 3*a(n-3), with a(0)=a(1)=1, a(2)=4.

Terms

a(0) =1a(1) =1a(2) =4a(3) =4a(4) =13a(5) =13a(6) =40a(7) =40a(8) =121a(9) =121a(10) =364a(11) =364a(12) =1093a(13) =1093a(14) =3280a(15) =3280a(16) =9841a(17) =9841a(18) =29524a(19) =29524a(20) =88573a(21) =88573a(22) =265720a(23) =265720a(24) =797161a(25) =797161a(26) =2391484a(27) =2391484a(28) =7174453a(29) =7174453