In a paper it says: "To convince a verifier that a group element is a quadratic residue, the prover executes the following proof with the verifier":
$PK \left\{ (\alpha) : y = \pm g^\alpha \right\}$
Does someone know how such proof is implemented by a signature proof of knowledge? The group is an RSA group $\mathrm{G} = \mathrm{QR}_n$ with $n = p \cdot q$ and two safe primes $p$ and $q$. I have found an implementation as part of a bigger protocol but although the $\pm$ was in the $\mathrm{PK}$ statement, they ignore it in the implementation. Now I am a little bit confused.