# Chapter 4 - Section 4.3 - Primes and Greatest Common Divisors - Exercises: 21

a) 3 b) 189 c) 479001600

#### Work Step by Step

To find the values of the Euler phi-function, we multiply the integers smaller than the number which are relatively prime to it. a) $ϕ(4)=3$ b) $ϕ(10)=ϕ(2⋅5)=3⋅7⋅9=189$ c) $ϕ(13)=12!=479001600$

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