Euler Phi Function of 78

Example of Euler $\phi$ Function of Square-Free Integer

$\map \phi {78} = 24$

where $\phi$ denotes the Euler $\phi$ Function.

Proof

From Euler Phi Function of Square-Free Integer:

$\ds \map \phi n = \prod_{\substack {p \mathop \divides n \\ p \mathop > 2} } \paren {p - 1}$

where $p \divides n$ denotes the primes which divide $n$.

We have that:

$78 = 2 \times 3 \times 13$

and so is square-free.

Thus:

\(\ds \map \phi {78}\)

\(=\)

\(\ds \paren {3 - 1} \paren {13 - 1}\)

\(\ds \)

\(=\)

\(\ds 2 \times 12\)

\(\ds \)

\(=\)

\(\ds 24\)

$\blacksquare$