a(n) = n! - Sum_{j=1..n-1} j!.

Terms

a(0) =1a(1) =1a(2) =3a(3) =15a(4) =87a(5) =567a(6) =4167a(7) =34407a(8) =316647a(9) =3219687a(10) =35878887a(11) =435046887a(12) =5704064487a(13) =80428314087