title: Cyclic group generator of [1, 2, 3, 4, 5, 6] under modulo 7 multiplication
body: <p>Find all the generators in the cyclic group [1, 2, 3, 4, 5, 6] under modulo 7 multiplication.
  I got &lt;1> and &lt;5> as generators. The answer is &lt;3> and &lt;5>. Can somebody please tell why is 3 a generator?</p>