a(n) = (5*n)!/((3*n)!*n!*n!).A001451a(n) = (5*n)!/((3*n)!*n!*n!).Terms a(0) =1a(1) =20a(2) =1260a(3) =100100a(4) =8817900a(5) =823727520a(6) =79919739900External referencesoeis: A001451