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

Terms

a(0) =1a(1) =2a(2) =5a(3) =11a(4) =26a(5) =59a(6) =137a(7) =314a(8) =725a(9) =1667a(10) =3842a(11) =8843a(12) =20369a(13) =46898a(14) =108005a(15) =248699a(16) =572714a(17) =1318811a(18) =3036953a(19) =6993386a(20) =16104245a(21) =37084403a(22) =85397138a(23) =196650347a(24) =452841761a(25) =1042792802a(26) =2401318085a(27) =5529696491a(28) =12733650746a(29) =29322740219