# Math Help - Quadratic Residues

Determine the set of primes for which 11 is a quadratic residue.

2. First, note that 11 is a quadratic residue mod 2 (e.g. $1^2\equiv11\,(\bmod\,2)).$

Let $p$ and $q$ be odd primes. There is a result (equivalent to the law of quadratic reciprocity) that says that $(11\,|\,p)=(11\,|\,q)$ if and only if $p\equiv\pm q\,(\bmod\,44).$ So you only have to determine all odd primes $p < 44$ for which $(11\,|\,p)=1$ All other odd primes for which 11 is a quadratic residue will congruent to plus or miinus one of these.

3. ## Thanks

Thanks.

Do you know where I can find more information on the result that you quoted? I've never seen this before.

4. Originally Posted by Cairo
Do you know where I can find more information on the result that you quoted? I've never seen this before.
I came across it in H.E. Rose, A Course in Number Theory, p.67:

Theorem 2.2 Let $p$ and $q$ be distinct odd primes and $a\ge1.$ The quadratic reciprocity law is equivalent to

if $p\equiv\pm q\,(\bmod\,4a)$ then $(a/p)=(a/q).$