a(n) = (n!)^3 * Sum_{i=1..n} 1/i^3.A066989a(n) = (n!)^3 * Sum_{i=1..n} 1/i^3.Terms a(0) =1a(1) =9a(2) =251a(3) =16280a(4) =2048824a(5) =444273984a(6) =152759224512External referencesoeis: A066989