806288789375domain: NAppears in sequencesa(n) = n*a(n-1) - a(n-2) with a(0) = 0, a(1) = 1.at n=15A058798a(0) = 1, a(1) = 2; for k>0, a(2*k) = k*a(2*k-1) + a(2*k-2), a(2*k+1) = a(2*k) + a(2*k-1).at n=27A273939