grandes-ecoles 2022 Q7.10

grandes-ecoles · France · x-ens-maths-d__mp Number Theory Congruence Reasoning and Parity Arguments
Assume that $p$ is a prime number congruent to 1 modulo 4, and fix $j\in\mathbb{Z}/p^\alpha\mathbb{Z}$ such that $j^2=-1$. Let $a\in\mathbb{Z}/p^\alpha\mathbb{Z}$ and $k\leq\alpha$ be the largest integer such that $p^k$ divides $a$. Show that if $k<\frac{\alpha}{2}$, then $(a,b)\in S(p^\alpha)$ if and only if $$b\equiv\pm ja\bmod p^{\alpha-k}$$ and that if $k\geq\frac{\alpha}{2}$, then $(a,b)\in S(p^\alpha)$ if and only if $p^{\lceil\frac{\alpha}{2}\rceil}$ divides $b$.
Assume that $p$ is a prime number congruent to 1 modulo 4, and fix $j\in\mathbb{Z}/p^\alpha\mathbb{Z}$ such that $j^2=-1$. Let $a\in\mathbb{Z}/p^\alpha\mathbb{Z}$ and $k\leq\alpha$ be the largest integer such that $p^k$ divides $a$. Show that if $k<\frac{\alpha}{2}$, then $(a,b)\in S(p^\alpha)$ if and only if
$$b\equiv\pm ja\bmod p^{\alpha-k}$$
and that if $k\geq\frac{\alpha}{2}$, then $(a,b)\in S(p^\alpha)$ if and only if $p^{\lceil\frac{\alpha}{2}\rceil}$ divides $b$.