Interesting thread. Apropos of permutations, I had written these two posts a few months ago: Permutation facts: https://jugad2.blogspot.in/2016/10/by-vasudev

Interesting thread. Apropos of permutations, I had written these two posts a few months ago: 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... perm_facts version with list comps and functional programming: https://jugad2.blogspot.in/2016/10/permfacts-version-with-list-comps-and.html https://jugad2.blogspot.in/2016/10/permfacts-version-with-li... The first post above also has some interesting facts about factorials, their occurrences and applications, which was a revelation to me.