a(n) = Product_{k=1..n-1} (3^k + 1).A290000a(n) = Product_{k=1..n-1} (3^k + 1).Terms a(0) =1a(1) =1a(2) =4a(3) =40a(4) =1120a(5) =91840a(6) =22408960a(7) =16358540800External referencesoeis: A290000