Characterization of Integer has Square Root in P-adic Integers

Theorem
Let $\Z_p$ be the $p$-adic integers for some prime $p \ne 2$.

Let $a \in Z$ be an integer such that $p \nmid a$.

Then:
 * $\exists x \in \Z_p : x^2 = a$


 * $a$ is a quadratic residue of $p$.

That is, an integer $a$ not divisible by $p$ has a square root in $\Z_p$ ($p \ne 2$) $a$ is a quadratic residue of $p$.

Proof
Let $F \in \Z[X]$ be the polynomial:
 * $\map F X = X^2 - a$

By definition of formal derivative the formal derivative of $F$ is:
 * $\map {F'} X = 2X$

Necessary Condition
Let there exist $x$ such that $x^2 = a$.

From Characterization of Integer Polynomial has Root in P-adic Integers:
 * there exists a coherent sequence $\sequence{a_n}$ of integers:
 * $(1)\quad\ds\lim_{n \mathop \to \infty} {a_n} = a$
 * $(2)\quad\map F {a_n} \equiv 0 \mod {p^{n+1}}$

By definition of coherent sequence:
 * $0 \le a_0 < p-1$

We have:
 * $a_0^2 - a \equiv 0 \pmod p$

That is:
 * $a_0^2 \equiv a \pmod p$

By definition of quadratic residue:
 * $a$ is a quadratic residue of $p$.

Sufficient Condition
Let $a$ be a quadratic residue of $p$.

By definition of quadratic residue of $p$:
 * $\exists b \in {0, 1, \ldots, p-1} : a \equiv b^2 \pmod p$

Then:
 * $\map F b = b^2 - a \equiv 0 \pmod p$

and
 * $\map {F'} b = 2b \not\equiv 0 \pmod p$

From Congruence Modulo Equivalence for Integers in P-adic Integers:
 * $\map F b \equiv 0 \pmod {p\Z}$

and
 * $\map {F'} b \not\equiv 0 \pmod {p\Z}$

From Hensel's Lemma for P-adic Integers:
 * $\exists x \in \Z_p : \map F x = 0$