a(n) = floor(n!^2 / n^n).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =11a(6) =30a(7) =96a(8) =339a(9) =1316a(10) =5584a(11) =25733a(12) =128025a(13) =683949a(14) =3905083a(15) =23731187a(16) =152934464a(17) =1041782238a(18) =7479469995a(19) =56448098958a(20) =446768591341