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

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =4a(5) =16a(6) =46a(7) =106a(8) =316a(9) =1324a(10) =5356a(11) =18316a(12) =63856a(13) =272416a(14) =1264264a(15) =5409496a(16) =22302736a(17) =101343376a(18) =507711376a(19) =2495918224a(20) =11798364736a(21) =58074029056a(22) =309240315616