a(n) = Sum_{k=0..n} (n^2)!/(k! * (n-k)!)^n.A306644a(n) = Sum_{k=0..n} (n^2)!/(k! * (n-k)!)^n.Terms a(0) =1a(1) =2a(2) =36a(3) =94080a(4) =114144030000External referencesoeis: A306644