# Quantum computing and mathematical functions

I need help with this question.

Compute the period of the function f(x) = 5x mod 128, x >= 0 integer.

• I'm voting to close this question as off-topic because this is a pure math question which belongs to Mathematics. – Yuval Filmus Oct 17 at 20:38
• This doesn't seem to have much to do with quantum computing. – Yuval Filmus Oct 17 at 20:38
• If $5(x+k)=5x \mod 128$ then $5k=0 \mod 128$. What does this tell you about $k$ ? What does this tell you about the period of $5x \mod 128$ ? – gandalf61 Oct 18 at 12:47
• @gandalf61 128 is a power of 2, relatively prime to 5 but i am confused about the period of the function. – Lokesh Pathak Oct 18 at 23:06