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

Terms

a(0) =0a(1) =2a(2) =3a(3) =7a(4) =16a(5) =38a(6) =91a(7) =219a(8) =528a(9) =1274a(10) =3075a(11) =7423a(12) =17920a(13) =43262a(14) =104443a(15) =252147a(16) =608736a(17) =1469618a(18) =3547971a(19) =8565559a(20) =20679088a(21) =49923734a(22) =120526555a(23) =290976843a(24) =702480240a(25) =1695937322a(26) =4094354883a(27) =9884647087a(28) =23863649056a(29) =57611945198