'p' is a mathematical binary infix operator. (`n` p `r`) is the number of strings of length `r` which can be made with `n` different characters, and no character appears twice in the string. (It can also be written P(`n`, `r`), or _{n}P_{r}, or even ^{n}P_{r}.)

Both `n` and `r` must be nonnegative integers, and `r` must be less than or equal to `n`. For example, (4 p 2) is the number of strings with 2 different elements which can be made from 4 characters. Say the 4 characters are `w`, `x`, `y`, and `z`. The possible strings are: `w``x`, `w``y`, `w``z`, `x``w`, `x``y`, `x``z`, `y``w`, `y``x`, `y``z`, `z``w`, `z``x`, and `z``y`; (4 p 2) is 12. (`n` p `r`) equals (`n`! / (`n` - `r`)!).

N-wing tells me maybe the infix 'p' should be capitalized, (`n` P `r`). He also says it stands for 'permutation', like c (or C) stands for 'combination'.