The problem says show that U(20) does not equal <k> for any k in U(20). [Hence U(20) is not cyclic.] I was trying to understand Example 1 from the text book. where it discusses U(15). I am completely confused about where this comes from. U(15) = {1, 2, 4, 7, 8, 11, 13, 14 } Then it goes on to say $ 7^1 $
