a(n) = a(n-1) + 3*a(n-2) + a(n-3) with a(0) = 1, a(1) = 3, a(2) = 6.

Terms

a(0) =1a(1) =3a(2) =6a(3) =16a(4) =37a(5) =91a(6) =218a(7) =528a(8) =1273a(9) =3075a(10) =7422a(11) =17920a(12) =43261a(13) =104443a(14) =252146a(15) =608736a(16) =1469617a(17) =3547971a(18) =8565558a(19) =20679088a(20) =49923733a(21) =120526555a(22) =290976842a(23) =702480240a(24) =1695937321a(25) =4094354883a(26) =9884647086a(27) =23863649056a(28) =57611945197a(29) =139087539451