a(n) = Sum_{i=(n*n-n+2)/2..(n*n+n)/2} i!.A175411a(n) = Sum_{i=(n*n-n+2)/2..(n*n+n)/2} i!.Terms a(0) =1a(1) =8a(2) =864a(3) =4037040External referencesoeis: A175411