a(n)=a(n-1)+3a(n-2)+a(n-3).

Terms

a(0) =1a(1) =1a(2) =1a(3) =5a(4) =9a(5) =25a(6) =57a(7) =141a(8) =337a(9) =817a(10) =1969a(11) =4757a(12) =11481a(13) =27721a(14) =66921a(15) =161565a(16) =390049a(17) =941665a(18) =2273377a(19) =5488421a(20) =13250217a(21) =31988857a(22) =77227929a(23) =186444717a(24) =450117361a(25) =1086679441a(26) =2623476241a(27) =6333631925a(28) =15290740089a(29) =36915112105