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

Terms

a(0) =1a(1) =2a(2) =7a(3) =23a(4) =76a(5) =251a(6) =829a(7) =2738a(8) =9043a(9) =29867a(10) =98644a(11) =325799a(12) =1076041a(13) =3553922a(14) =11737807a(15) =38767343a(16) =128039836a(17) =422886851a(18) =1396700389a(19) =4612988018a(20) =15235664443a(21) =50319981347a(22) =166195608484a(23) =548906806799