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

Terms

a(0) =0a(1) =1a(2) =1a(3) =3a(4) =9a(5) =31a(6) =113a(7) =431a(8) =1697a(9) =6847a(10) =28161a(11) =117631a(12) =497665a(13) =2128127a(14) =9183489a(15) =39940863a(16) =174897665a(17) =770452479a(18) =3411959809a(19) =15181264895a(20) =67833868289a(21) =304256253951