Okay, first remember that there are primitive roots ( or generators) in
And there are non-quadratic residues.
So we'd like to show that occurs in our case. -remember that the set of generators is included in the set of non-quadratic residues, see here-
We can write: with such that - since p>2, the case p=2 is trivial-
Then: equality occurs iff
Hence if is not of the form (that is ) there's always a non-quadratic residue that is not a generator.
But, if p is of the form , then all non-quadratic residues are also generators.