9965204131domain: NAppears in sequencesDouble subfactorials: a(n) = (-1)^floor(n/2) * n!! * Sum_{i=0..floor(n/2)} (-1)^i/(n-2*i)!!.at n=21A334578a(n) is the numerator of Sum_{k=1..n} (-1)^(k+1) / (2*k-1)!!.at n=10A354298a(n) = (2*n-1)!! * Sum_{k=1..n} (-1)^(k-1)/(2*k-1)!!.at n=11A368794