a(n) = a(n-1)+2*a(n-2)+a(n-3) with a(0)=1, a(1)=4, a(2) = 6.

Terms

a(0) =1a(1) =4a(2) =6a(3) =15a(4) =31a(5) =67a(6) =144a(7) =309a(8) =664a(9) =1426a(10) =3063a(11) =6579a(12) =14131a(13) =30352a(14) =65193a(15) =140028a(16) =300766a(17) =646015a(18) =1387575a(19) =2980371a(20) =6401536a(21) =13749853a(22) =29533296a(23) =63434538a(24) =136250983a(25) =292653355a(26) =628589859a(27) =1350147552a(28) =2899980625a(29) =6228865588