a(n) = n! * Sum_{d|n} d^(n - d) / (d! * (n/d)!).

Terms

a(0) =1a(1) =2a(2) =2a(3) =26a(4) =2a(5) =2582a(6) =2a(7) =268802a(8) =7348322a(9) =51120722a(10) =2a(11) =299332756802a(12) =2a(16) =2a(18) =2a(22) =2