Expansion of g.f. (1-3*x)/(1-7*x).

Terms

a(0) =1a(1) =4a(2) =28a(3) =196a(4) =1372a(5) =9604a(6) =67228a(7) =470596a(8) =3294172a(9) =23059204a(10) =161414428a(11) =1129900996a(12) =7909306972a(13) =55365148804a(14) =387556041628