Denominator of 2*Stirling_2(n,2)/n!.

Terms

a(0) =1a(1) =1a(2) =12a(3) =4a(4) =360a(5) =40a(6) =20160a(7) =12096a(8) =259200a(9) =604800a(10) =239500800a(11) =760320a(12) =43589145600a(13) =217945728000a(15) =697426329600