Monday, 9 February 2026

Exercise (5.1).15

(i) Is it possible to have $\phi(n) \ge n$?

Give reasons for your answer.

(ii) Show that $0 < \frac{\phi (n)}{n} < 1$ for $n > 1$.


(i) $\phi(n)$ is defined to be the quantity of natural numbers from 1 to $n$, inclusive,  that are coprime to $n$. Since there are only $n$ natural numbers in this range, those coprime are less than or equal to $n$. 

However, the natural number 1 is coprime to $n$, and so that quantity, $\phi(n)$, is less than or equal to $(n-1)$, or equivalently, less than $n$.

$$ \phi(n) < n$$

This is equivalent to $\phi(n) \ge n$ being false.


(ii) Since 1 is coprime to any $n>1$ meaning $\phi(n)>0$, and since we're established $ \phi(n) < n$, then

$$ 0 < \phi(n) < n $$

Equivalently

$$ 0 < \frac{\phi (n)}{n} < 1 $$