f k n = { 1 if g c d ( n , k ) = 1 0 o t h e r w i s e {\displaystyle f_{k}{n}={\begin{cases}1\quad {\text{if }}gcd(n,k)=1\\0\quad otherwise\end{cases}}}
∑ d | n f d n = 1 {\displaystyle \sum _{d|n}f_{d}n=1}
∑ d ≤ n f d n = ϕ ( n ) {\displaystyle \sum _{d\leq n}f_{d}n=\phi (n)}
A x + B = C {\displaystyle Ax+B=C}
Test
