Number of degree-n even permutations of order exactly 2.

Terms

a(0) =0a(1) =0a(2) =0a(3) =3a(4) =15a(5) =45a(6) =105a(7) =315a(8) =1323a(9) =5355a(10) =18315a(11) =63855a(12) =272415a(13) =1264263a(14) =5409495a(15) =22302735a(16) =101343375a(17) =507711375a(18) =2495918223a(19) =11798364735a(20) =58074029055a(21) =309240315615