https://www.youtube.com/watch?v=sBGCEQURXdQ
https://www.youtube.com/watch?v=palSdK9P-ns
Euler's theorem, also known as Fermat-Euler theorem, states that if a and n are coprime positive integers, then a^φ(n) ≡ 1 (mod n), where φ(n) is Euler's totient function.