a(n)= 3*a(n-3) +3*a(n-6) +a(n-9).

Terms

a(0) =0a(1) =1a(2) =3a(3) =1a(4) =2a(5) =12a(6) =2a(7) =9a(8) =45a(9) =9a(10) =34a(11) =174a(12) =34a(13) =131a(14) =669a(15) =131a(16) =504a(17) =2574a(18) =504a(19) =1939a(20) =9903a(21) =1939a(22) =7460a(23) =38100a(24) =7460a(25) =28701a(26) =146583a(27) =28701a(28) =110422a(29) =563952