a(n) = n! * (1 + 2*Sum_{k=1..n} 1/k).

Terms

a(0) =1a(1) =3a(2) =8a(3) =28a(4) =124a(5) =668a(6) =4248a(7) =31176a(8) =259488a(9) =2416032a(10) =24886080a(11) =281004480a(12) =3451887360a(13) =45832538880a(14) =654109585920