In my humble opinion, permutations (n,k) or nPk may be calculated by Pochhammer[n-k+1, k], not by Pochhammer[k,n-k+1]. For example, 5P2 is 5!/(5-2)! = 5!/3! = 5x4=20. Pochhammer[4,2] gives 20. Pochhammer[2,4] results in 120. I am just learning ropes in Mathematica and my math skills are quite rusty. Please, take my words with a grain of salt.
Sang
