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

Terms

a(0) =0a(1) =1a(2) =4a(3) =16a(4) =68a(5) =296a(6) =1312a(7) =5888a(8) =26672a(9) =121696a(10) =558464a(11) =2574848a(12) =11917952a(13) =55345408a(14) =257741824a(15) =1203224576a(16) =5629027072a(17) =26383656448a(18) =123868321792a(19) =582414688256