catchmrbharath/sympy — comment on commit d63fbf91d22963cc13ef9030aade6650efa84726
File: sympy/combinatorics/permutations.py line 1619
By pernici:

the link ends with `.htm`

`http://www.cp.eng.chula.ac.th/~piak/teaching/algo/algo2008/count-inv.htm`