# Roots of $-1$ quadratic residue modulo$p$

It seems that every root of $$-1$$ quadratic residue modulo $$p$$ prime, $$p = 1 \pmod 4$$ is distinct.

I mean if $$x$$ is a root of $$-1$$ quadratic residue modulo $$p$$, then there is no other prime $$q$$ such $$x$$ is a root of $$-1$$ quadratic residue modulo $$q$$.

Is it correct, and if it is, how can it be demonstrated ?

Best regards.

• How many prime factors $> n$ can $n^2+1$ have? – Daniel Fischer Dec 20 '16 at 11:07
• I tested for every primes < 611953 – BenLaz Dec 20 '16 at 11:09
• Depends on what qualifies as distinct for you. Remember that it is a bit problematic to say that a residue class modulo prime $p_1$ is distinct from a residue class modulo another prime $p_2$. By Chinese remainder theorem they intersect non-trivially. For example, $x=30$ is a square root of $-1$ modulo both $p_1=17$ and $p_2=53$. If you look at integers (as opposed to IMHO slightly more natural residue classes), and place the constraint $0<x<p$, then you can use Daniel's comment to conclude that... – Jyrki Lahtonen Dec 20 '16 at 11:12
• Daniel Fischer : Obviously one, my question was "dumb", so I edit it. – BenLaz Dec 20 '16 at 11:15

This is true, provided that the specific root is in the range $\{ 1, \dots , p-1\}$. In other words: let $a$ be a root of $-1$ mod $p$ and mod $q$ with the conditions $0<a<p$, $0<a<q$. Then necessarily $p=q$.
Indeed, suppose by contradiction that $p\neq q$. If $a^2+1 \equiv 0 \pmod{p}$ and $a^2+1 \equiv 0 \pmod{q}$, then $pq$ divides $a^2+1$. In particular we have $pq \le a^2+1$. But now, $$a^2 \le (p-1)(q-1)=pq -(p+q-1) \le a^2+1-(p+q-1) =$$$$= a^2- (p-1)-(q-1) \le a^2-2a$$ and this implies the contradiction $a \le 0$.
But without our condition, then this is false. For example for $a=8$ we have $a^2+1=65= 5 \cdot 13$, so that $8$ is a square root of $-1$ both mod $5$ and mod $13$. The problem here is that $8 \mod{5}=3$, so $8$ is "hidden behind" $3$.