a(n) = binomial(n^2, n*(n+1)/2).A109901a(n) = binomial(n^2, n*(n+1)/2).Terms a(0) =1a(1) =1a(2) =4a(3) =84a(4) =8008a(5) =3268760a(6) =5567902560External referencesoeis: A109901