a(n) = a(n-1) + a(n-2) + 3*a(n-3) with a(0) = 1, a(1) = 2, a(2) = 5.

Terms

a(0) =1a(1) =2a(2) =5a(3) =10a(4) =21a(5) =46a(6) =97a(7) =206a(8) =441a(9) =938a(10) =1997a(11) =4258a(12) =9069a(13) =19318a(14) =41161a(15) =87686a(16) =186801a(17) =397970a(18) =847829a(19) =1806202a(20) =3847941a(21) =8197630a(22) =17464177a(23) =37205630a(24) =79262697a(25) =168860858a(26) =359740445a(27) =766389394a(28) =1632712413a(29) =3478323142