Simple method the show that a group of order 15 is cyclic How can i show with quite simple methods that a group of order 15 is cyclic?
Group of order 15 is abelian How do I prove group of order 15 is abelian?  Is there any general strategy to prove that a group of particular order(composite order) is abelian?
Doubt related to a theorem in congruences of Chapter 5 of Apostol Introduction to analytic number theory I am self studying number theory from Introduction to Analytic number theory by Apostol.   I have a doubt in one argument of proof of Theorem 5.14 and I am posting its image.      My doubt is in 2nd line of 2 nd paragraph how Apostol writes - If y is a solution of (5) then ay $\equiv $ at ( mod m) . How does y being a solution of (5) implies that?       Can someone please help