# Understanding Cryptography by Christof Paar and Jan Pelzl - Chapter 1 Solutions - Ex1.10

- 2 mins

## Exercise 1.10

Find all integers $n$ between $0 \le n \lt m$ that are relatively prime to $m$ for $m = 4,5,9,26$. We denote the number of integers $n$ which fulfill the condition by $\phi(m)$, e.g. $\phi(3) = 2$. This function is called “Euler’s phi function”. What is $\phi(m)$ for $m = 4,5,9,26$?

### Solution

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$m = 4$:

$m = 5$:

$m = 9$:

$m = 26$: