a(n) = (n!/2)*Sum(1/k!, k=1..n-2).

Terms

a(0) =0a(1) =0a(2) =0a(3) =3a(4) =18a(5) =100a(6) =615a(7) =4326a(8) =34636a(9) =311760a(10) =3117645a(11) =34294150a(12) =411529866a(13) =5349888336a(14) =74898436795