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

Terms

a(0) =1a(1) =1a(2) =1a(3) =4a(4) =7a(5) =13a(6) =28a(7) =55a(8) =109a(9) =220a(10) =439a(11) =877a(12) =1756a(13) =3511a(14) =7021a(15) =14044a(16) =28087a(17) =56173a(18) =112348a(19) =224695a(20) =449389a(21) =898780a(22) =1797559a(23) =3595117a(24) =7190236a(25) =14380471a(26) =28760941a(27) =57521884a(28) =115043767a(29) =230087533