G.f.: sqrt( (1 + sqrt(1+8*x)) / (1 + sqrt(1-8*x)) ).

Terms

a(0) =1a(1) =2a(2) =2a(3) =28a(4) =54a(5) =860a(6) =2004a(7) =33720a(8) =86054a(9) =1492908a(10) =4019452a(11) =71101832a(12) =198310460a(13) =3555617432a(14) =10168382696a(15) =184127171952a(16) =536496907782