# Proofs using linear congruences

We have just covered solving linear congruences, and I am confused about how to use them in proofs. I understand that the linear congruence $$cx \equiv b \pmod m$$ has a unique solution $\bmod m$ if $\gcd(c,m) = 1$, but a general approach to problems escapes me.

Sample Questions

Prove that if $x^2 \equiv n \pmod {65}$ has a solution, then so does $x^2 \equiv -n \pmod {65}$.

Show that if $n \equiv 7 \pmod 8$, then $n$ is not the sum of three squares.

My Work

For the first one, $x^2 \equiv n \pmod {65}$ implies that $65 | (x^2 - n)$, so (I believe) that means that $n = b^2$ for some $b$, so $65 | (x^2 - b^2)$. We proved a result that said that if $a^2 \equiv b^2 \pmod p$ for some prime $p$, then $a \equiv b$ or $a \equiv - b \pmod p$. However, I don't think that is the right track to go down here because $65$ is not prime, and I'm unsure about assuming $n = b^2$ for some $b$.

For the second one, suppose to the contrary that $n = a^2 + b^2 + c^2$ for some $a, b, c$. Then $n \equiv 7 \pmod 8$ implies that $8 | (n - 7)$ so $n = 8k + 7$ for some $k$. So substituting in gives me $a^2 + b^2 + c^2 = 8k + 7$, and I'm unsure how to proceed from here.

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+1 for showing your work. – lhf Sep 14 '12 at 12:19

## 2 Answers

Hint $\rm(1)\ \ \ x^2 \equiv n,\,\ y^2\equiv -1\:\Rightarrow\: -n\equiv x^2y^2\equiv (xy)^2.\$ But $\rm\:mod\ 65\!:\ {-}1 \equiv 64\equiv (\_ )^2.$

$\rm(2)\ \ mod\ 4\!:\ x^2\!+\!y^2\!+\!z^2 \equiv\, 3\:\Rightarrow\:x,y,z\:$ odd, by $\rm\:odd^2\equiv 1,\ even^2\equiv 0.\:$ Therefore we deduce $\rm\phantom{(2)\ \ } mod\ 8\!:\ x^2\!+\!y^2\!+\!z^2\equiv\:7\:\Rightarrow\:x,y,z\:$ odd $\rm\:\Rightarrow\:x^2\!+\!y^2\!+\!z^2\equiv 3,\:$ by $\rm\:odd^2\!\equiv\{\pm1,\pm3\}^2\equiv 1.$

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This is exactly what I was hoping for thanks. – Moderat Sep 14 '12 at 13:16

For the first question, note that $65=5 \cdot 13$ and $x^2 \equiv -1$ has solutions mod 5 and mod 13. Note also that solutions of $x^2 \equiv n \bmod 65$ also give solutions mod 5 and mod 13. Combine those.

For the second question, note that squares are congruent to 0, 1, or 4, mod 8. But you cannot sum three numbers from this list to get 7 mod 8.

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