Euler Phi Function of 91

Example of Euler $\phi$ Function of Non-Square Semiprime

$\map \phi {91} = 72$

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

Proof

We have that:

$91 = 7 \times 13$

Thus:

\(\ds \map \phi {91}\)

\(=\)

\(\ds \paren {7 - 1} \paren {13 - 1}\)

Euler $\phi$ Function of Non-Square Semiprime

\(\ds \)

\(=\)

\(\ds 6 \times 12\)

\(\ds \)

\(=\)

\(\ds 72\)

$\blacksquare$