a(n) = 3*a(n-1) + 3*a(n-2) - a(n-3); a(0)=1, a(1)=1, a(2)=5.

Terms

a(0) =1a(1) =1a(2) =5a(3) =17a(4) =65a(5) =241a(6) =901a(7) =3361a(8) =12545a(9) =46817a(10) =174725a(11) =652081a(12) =2433601a(13) =9082321a(14) =33895685a(15) =126500417a(16) =472105985a(17) =1761923521a(18) =6575588101a(19) =24540428881a(20) =91586127425a(21) =341804080817