a(n) = n!*Sum_{j=0..n-1} 2^j/(j+1).

Terms

a(0) =0a(1) =1a(2) =4a(3) =20a(4) =128a(5) =1024a(6) =9984a(7) =115968a(8) =1572864a(9) =24477696a(10) =430571520a(11) =8452177920a(12) =183175741440