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The question came from H.Iwaniec's "Topics in Classical Automorphic Forms" Lemma 4.8

Let $q$ be an odd square-free number, and let $(\frac{c}{d})$ be the extended Jacobi symbol:

  1. If $c,d>0$, then $(\frac{c}{d})$ is the usual Jacobi symbol,
  2. For $c\neq 0$, $(\frac{c}{d})=\frac{c}{|c|}(\frac{c}{-d})$,
  3. $ (\frac{0}{d})=\begin{cases}1,\, d=\pm 1\\ 0, \,\text{otherwise} \end{cases}$

The text claims that $$\#\{x\bmod{q}:x^2\equiv y\pmod{q}=\prod_{-p|q}\Big(1+\big(\frac{y}{p}\big) \Big) $$

Could someone please give more details about why this equal sign holds true? Thanks a lot.

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  • $\begingroup$ In point 2, do you mean $c\neq0$ and $d<0$? $\endgroup$
    – Arthur
    Apr 16 at 16:22
  • $\begingroup$ Also asked here but it is not clear if the single answer has been checked by anyone. $\endgroup$ Apr 16 at 16:24
  • $\begingroup$ @Arthur Yes, when $d<0$, one can transfer to $d>0$ by the formula. $\endgroup$
    – scd
    Apr 16 at 16:25
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By the Chinese Remainder Theorem, if $p_1,\ldots,p_r$ are pairwise distinct primes, then for each choice of integers $a_1,\ldots,a_r$ there exists an $a$ (unique modulo $q=p_1\cdots p_r$) such that $$\begin{align*} a&\equiv a_1\pmod{p_1}\\ a&\equiv a_2\pmod{p_2}\\ &\vdots\\ a&\equiv a_r\pmod{p_r}. \end{align*}$$

Say you are trying to solve $x^2\equiv b\pmod{q}$. Using the Chinese Remainder Theorem, that is equivalent to solving the system of congruences $$\begin{align*} x^2&\equiv b\pmod{p_1}\\ x^2&\equiv b\pmod{p_2}\\ &\vdots\\ x^2&\equiv b\pmod{p_r} \end{align*}$$ because if you find solutions $a_1,\ldots,a_r$ to these $r$ congruences, then you can look for an $a$ that satisfies the system of congruences in my first display, and this $a$ will necessarily solve $x^2\equiv b\pmod{q}$.

Moreover, every tuple $(a_1,\ldots,a_r)$ of solutions to these $r$ congruences will give you a new solution to $x^2\equiv b\pmod{q}$, and different tuples correspond to different solutions.

So to count the solutions to $x^2\equiv b\pmod{q}$ modulo $q$, we can count the number of tuples of solutions to the system of $r$ congruences, instead.

If $\gcd(b,q)=1$, then $x^2\equiv b\pmod{p_i}$ will have either $0$ or $2$ solutions (because $p_i$ is odd and $b\not\equiv 0\pmod{p_i}$). But this is exactly $$1 + \left(\frac{b}{p_i}\right)$$ where $\left(\frac{b}{p_i}\right)$ is the Legendre symbol: because you get $0$ is the symbol is $-1$ (there are no solutions), and $2$ if the symbol is $1$ (yes, there are solutions).

If $\gcd(b,q)\neq 1$, then there will be some primes $p_i$ for which $p_i|b$; in those cases, you have the congruence $x^2\equiv 0\pmod{p_i}$, which has exactly one solution. But again, this is $1+\left(\frac{b}{p_i}\right)$! Because $\left(\frac{b}{p_i}\right)$ is $0$, and you have a total of one solution.

So, how many tuples $(a_1,\ldots,a_r)$ are there? There are the number of possible $a_1$, times the number of possible $a_2$, etc. And this is precisely $$\prod_{i=1}^r\Bigl(\#\text{ solutions to }x^2\equiv b\pmod{p_i}\Bigr) = \prod_{i=1}^r\left( 1+ \left(\frac{b}{p_i}\right)\right).$$

So the number of solutions to $x^2\equiv b\pmod{q}$ modulo $q$ is then exactly the value of this product.

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