a(n) = 3*a(n-1) - 3*a(n-3) + a(n-4) for n > 3, with a(0) = 1, a(1) = 3, a(2) = 9, a(3) = 27.

Terms

a(0) =1a(1) =3a(2) =9a(3) =27a(4) =73a(5) =195a(6) =513a(7) =1347a(8) =3529a(9) =9243a(10) =24201a(11) =63363a(12) =165889a(13) =434307a(14) =1137033a(15) =2976795a(16) =7793353a(17) =20403267a(18) =53416449a(19) =139846083a(20) =366121801a(21) =958519323a(22) =2509436169a(23) =6569789187a(24) =17199931393a(25) =45030004995a(26) =117890083593a(27) =308640245787a(28) =808030653769