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

Terms

a(0) =1a(1) =0a(2) =1a(3) =4a(4) =5a(5) =12a(6) =29a(7) =56a(8) =121a(9) =264a(10) =553a(11) =1180a(12) =2525a(13) =5364a(14) =11429a(15) =24368a(16) =51889a(17) =110544a(18) =235537a(19) =501748a(20) =1068917a(21) =2277276a(22) =4851437a(23) =10335464a(24) =22018729a(25) =46908504a(26) =99933625a(27) =212898316a(28) =453557453a(29) =966256644