Speaking of factorials, this post I had written a few years ago may be of interest: Permutation facts: https://jugad2.blogspot.in/2016/10/by-vasudev-ram-nicom

Speaking of factorials, this post I had written a few years ago may be of interest: Permutation facts: https://jugad2.blogspot.in/2016/10/by-vasudev-ram-nicomachus-theorem-3d.html https://jugad2.blogspot.in/2016/10/by-vasudev-ram-nicomachus... It mentions many kinds of factorials and other interesting types of numbers too.