a(n) = n^n * (n+1)^(n+1).A090205a(n) = n^n * (n+1)^(n+1).Terms a(0) =1a(1) =4a(2) =108a(3) =6912a(4) =800000a(5) =145800000a(6) =38423222208External referencesoeis: A090205