Exercise
Which of the following statements is true?
All elements in Z/5Z are invertible
All nonzero elements in Z/6Z are invertible
All nonzero elements in Z/5Z are invertible
All odd elements in Z/6Z are invertible
What about 0?
Wrong, there is no integer b such that 2b = 1 mod 6,
which means that 2 has no inverse in Z/6Z
This is correct and follows from 1 · 1 = 1, 2 · 3 = 1,4 · 4 = 1 mod 5
This is not true, because 3 has no inverse in Z/6Z
(by the way, what does odd mean?)