Expansion of g.f.: (1 + x + x^2)/(1 - 2*x - 2*x^2).

Terms

a(0) =1a(1) =3a(2) =9a(3) =24a(4) =66a(5) =180a(6) =492a(7) =1344a(8) =3672a(9) =10032a(10) =27408a(11) =74880a(12) =204576a(13) =558912a(14) =1526976a(15) =4171776a(16) =11397504a(17) =31138560a(18) =85072128a(19) =232421376a(20) =634987008a(21) =1734816768a(22) =4739607552a(23) =12948848640a(24) =35376912384a(25) =96651522048a(26) =264056868864a(27) =721416781824