The function f is defined for all integers n by the following rule: f(n) is the number of positive integers each of which is less than n and has no positive factor in common which n other than 1. If p is any prime number then f(p)=
(1)p-1 (2)p-2 (3)(p+1)/2 (4)(p-1)/2 (5)2
f(n)其实就是与n互质并且小于n的正整数，也就是所有小于n，并且和n的最大公约数是1的正整数。If n is a prime number, then every positive integer less than n has a common factor of 1. 所以总共有n-1个这样的数，f(n)=n-1
