Quote:
Originally Posted by MathDoggy
Counterexample to Lehmer's totient problem
2^5211 is prime and when you put in the Phi function it is not equal to (2^5211)1 as Lehmer conjectured

I am confounded. We all know that \(p\in\text{prime}\implies\phi(p)=p1\), recall that the group of units for any prime number p has order (p1).
How so that \(\phi\big(2^{521}1\big)\neq(2^{521}1)1\) but \(2^{521}1\) is a prime?