While toying with the factorial function, I stumbled with the following definition:
Let P_k^{\,n} be the sum of all possible products of k different factors, taken from {1,2,3,...n}.
For example, P_3^{\,4} = 1.2.3 + 1.2.4 + 1.3.4 + 2.3.4 = 50
Now, according to the Sloane site, this guys turn...