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

Terms

a(0) =1a(1) =2a(2) =2a(3) =6a(4) =2a(5) =234a(6) =2a(7) =331842a(8) =46658a(9) =24883200258a(10) =2a(12) =2a(16) =2a(18) =2