G.f.: 1/( (1-8*x)*(1+x)^2 )^(1/6).

Terms

a(0) =1a(1) =1a(2) =6a(3) =34a(4) =217a(5) =1449a(6) =9996a(7) =70512a(8) =505674a(9) =3672682a(10) =26943748a(11) =199284540a(12) =1483955746a(13) =11113108930a(14) =83628685440a(15) =631963708200