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

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =18a(5) =90a(6) =660a(7) =4620a(8) =42000a(9) =378000a(10) =4142880a(11) =45571680a(12) =586776960a(13) =7628100480a(14) =113020427520