The function f is defined for all positive integers n by the following rule: f(n) is the number of postive integers each of which is less than n and has no positive factor in common with n other than 1. If p is any prime number then f(p) =
a p - 1
b p - 2
if p = 5, f(p) = 4, 3, 2, and 1. so altogather 4 which is equal to p-1 = 5-1.