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

Terms

a(0) =0a(1) =1a(2) =3a(3) =10a(4) =33a(5) =109a(6) =360a(7) =1189a(8) =3927a(9) =12970a(10) =42837a(11) =141481a(12) =467280a(13) =1543321a(14) =5097243a(15) =16835050a(16) =55602393a(17) =183642229a(18) =606529080a(19) =2003229469a(20) =6616217487a(21) =21851881930a(22) =72171863277a(23) =238367471761a(24) =787274278560