a(n) = Sum_{p|n, p prime} n!/(p!*(n/p)!).

Terms

a(0) =0a(1) =1a(2) =1a(3) =6a(4) =1a(5) =120a(6) =1a(7) =840a(8) =10080a(9) =30240a(10) =1a(11) =3659040a(12) =1a(13) =17297280a(14) =3632428800a(15) =259459200a(16) =1a(18) =1a(22) =1