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

Terms

a(0) =1a(1) =3a(2) =1a(3) =-3a(4) =-3a(5) =3a(6) =9a(7) =9a(8) =9a(9) =27a(10) =81a(11) =189a(12) =405a(13) =891a(14) =2025a(15) =4617a(16) =10449a(17) =23571a(18) =53217a(19) =120285a(20) =271917a(21) =614547a(22) =1388745a(23) =3138345a(24) =7092441a(25) =16028523a(26) =36223281a(27) =81861597a(28) =185000517a(29) =418086603