What the problem means is that f(n) represents the number of numbers that are coprime with n in a positive integer less than n (considering that n= 1, it is generally nonnegative). This function is called Euler function and expressed by φ(n). If p is a prime number, the number of coprime with p is: 1, 2...p- 1 in a positive integer obviously smaller than p, so there is the number of p- 1, that is, f(p)=p- 1.