E.g.f.: Product_{m>0} (1 - x^m + x^(2m)/2).

Terms

a(0) =1a(1) =-1a(2) =-1a(3) =0a(4) =0a(5) =0a(6) =180a(7) =0a(8) =10080a(9) =90720a(10) =0a(11) =0a(12) =179625600a(13) =-1556755200a(14) =-10897286400a(15) =326918592000a(21) =0