E.g.f.: Product_{k>=1} 1/(1 - x^(3*k-1)/(3*k-1)).

Terms

a(0) =1a(1) =0a(2) =1a(3) =0a(4) =6a(5) =24a(6) =90a(7) =504a(8) =7560a(9) =18144a(10) =485352a(11) =4626720a(12) =32033232a(13) =516559680a(14) =9142044912a(15) =64700161344