Euler's Totient Theorem
From AoPSWiki
Euler's Totient Theorem is a theorem closely related to his function of the same name.
Theorem
Let 
be Euler's totient function. If 
is an integer and 
is a positive integer relatively prime to 
, then 
.
Credit
This theorem is credited to Leonhard Euler. It is a generalization of Fermat's Little Theorem, which specifies that 
is prime. For this reason it is known as Euler's generalization and Fermat-Euler as well.
