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

Terms

a(0) =0a(1) =0a(2) =1a(3) =2a(4) =3a(5) =6a(6) =10a(7) =20a(8) =33a(9) =66a(10) =109a(11) =218a(12) =360a(13) =720a(14) =1189a(15) =2378a(16) =3927a(17) =7854a(18) =12970a(19) =25940a(20) =42837a(21) =85674a(22) =141481a(23) =282962a(24) =467280a(25) =934560a(26) =1543321a(27) =3086642a(28) =5097243a(29) =10194486