how to proof if p and q are odd prime, and q=2p+1, then -4 is primitive root of q. I think quadratic residue of q is useful, but I cant use it effectively. if for example. p=3, then q=7, and the number -4(mod 7) is the primitive of 7.
my try is: if a is quadratic residue of q then a^((q-1)/2) = a^p = 1 (mod q). and if r is primitive root of q then there exists integer k such that r^2k=a (mod q)
