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

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =7a(5) =19a(6) =49a(7) =129a(8) =337a(9) =883a(10) =2311a(11) =6051a(12) =15841a(13) =41473a(14) =108577a(15) =284259a(16) =744199a(17) =1948339a(18) =5100817a(19) =13354113a(20) =34961521a(21) =91530451a(22) =239629831a(23) =627359043a(24) =1642447297a(25) =4299982849a(26) =11257501249a(27) =29472520899a(28) =77160061447a(29) =202007663443