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

Terms

a(0) =0a(1) =1a(2) =3a(3) =7a(4) =27a(5) =121a(6) =729a(7) =5041a(8) =40347a(9) =362887a(10) =3628923a(11) =39916801a(12) =479002353a(13) =6227020801a(14) =87178296243