8774780domain: NAppears in sequencesa(n) = a(1)*a(n-1) + a(2)*a(n-2) + ...+ a(n-1)*a(1) for n >= 5, starting 1,0,1,1.at n=20A025273G.f.: exp( Sum_{n>=1} x^n/n * Sum_{k=0..n} binomial(n,k)^2 * x^k/(1-x)^k ).at n=18A217333Number of valid hook configurations of permutations of [n] that avoid the patterns 231 and 321.at n=19A307788