a(0) = 0, a(1) = 1, a(2)=1; a(n) = 3a(n-1) + 3a(n-2) - 4a(n-3) for n >= 3.

Terms

a(0) =0a(1) =1a(2) =1a(3) =6a(4) =17a(5) =65a(6) =222a(7) =793a(8) =2785a(9) =9846a(10) =34721a(11) =122561a(12) =432462a(13) =1526185a(14) =5385697a(15) =19005798a(16) =67069745a(17) =236683841a(18) =835237566a(19) =2947485241a(20) =10401433057a(21) =36705804630a(22) =129531772097a(23) =457106997953