Expansion of g.f.: (1+4*x-4*x^2)/(1-3*x-4*x^2+4*x^3).

Terms

a(0) =1a(1) =7a(2) =21a(3) =87a(4) =317a(5) =1215a(6) =4565a(7) =17287a(8) =65261a(9) =246671a(10) =931909a(11) =3521367a(12) =13305053a(13) =50272991a(14) =189953717a(15) =717732903a(16) =2711921613a(17) =10246881583a(18) =38717399589a(19) =146292038647