Template:Ppp/doc

Usage
Thus  can represent any function f given by a set of pairs (a,fa), (b,fb), .. , except that these cannot include a pair (p,fp). To allow the latter also, we can use.
 * gives
 * gives

Similarly, with Template:Pp we can allow a pair (1,f1) if we use, etc. If   takes integer values with a lower bound we can also apply a shift, see that template.