E.g.f. sqrt(1-t^2)*exp(t*(1+arctanh(t))).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =12a(5) =36a(6) =160a(7) =680a(8) =4368a(9) =24976a(10) =219616a(11) =1599104a(12) =18145600a(13) =160805568a(14) =2245960704a(15) =23437238656a(16) =389201754368