Expansion of e.g.f. Sum_{k>=1} arctan(x^k).

Terms

a(0) =1a(1) =2a(2) =4a(3) =24a(4) =144a(5) =480a(6) =4320a(7) =40320a(8) =282240a(9) =4354560a(10) =36288000a(11) =319334400a(12) =6706022400a(13) =74724249600