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The numbers that have a common prime factor with 16 are the even numbers 2, 4, 6, 8, 10, 12, 14 and 16. They are 8.
So, φ(16)=16−8=8.
If the decomposition of n in a product of prime numbers is known, we can compute φ(n): if n=aibjck, φ(n)=ai−1(a−1)bj−1(b−1)ck−1(c−1) and so on. If n is prime, φ(n)=n−1.
So, φ(24)=23.
The prime numbers that are lesser than 16 are 2, 3, 5, 7, 11 and 13. Note that they are all odd except 2, but odd numbers are all prime.
π(16)=6.