Exercise
In Z/8Z the following elements are invertible:
2 and 5
3 and 7
0 and 4
1 and 6
2 is not invertible, because gcd(2,8)=2
This is correct: both 3 and 7 are relatively prime to 8
By definition 0 has no inverse
6 is not invertible, because gcd(6,8)=2