a(n) = floor(n!/2^n).

Terms

a(0) =0a(1) =0a(2) =0a(3) =1a(4) =3a(5) =11a(6) =39a(7) =157a(8) =708a(9) =3543a(10) =19490a(11) =116943a(12) =760134a(13) =5320940a(14) =39907054a(15) =319256437a(16) =2713679718a(17) =24423117468a(18) =232019615953