# Every P-adic Number is Integer Power of p times P-adic Integer

## Theorem

Let $p$ be a prime number.

Let $\struct {\Q_p, \norm {\,\cdot\,}_p}$ be the $p$-adic numbers.

Let $\Z_p$ be the $p$-adic integers.

Then:

$\forall a \in \Q_p: \exists n \in \N: p^n a \in \Z_p$.

## Proof

Let $a \in \Q_p$.

### Case: $\norm{a}_p \le 1$

Let $\norm{a}_p \le 1$.

By definition of the $p$-adic integers:

$a \in \Z_p$.

Hence:

$p^0 a \in \Z_p$.

### Case: $\norm{a}_p > 1$

Let $\norm{a}_p > 1$.

From P-adic Norm of p-adic Number is Power of p, there exists $v \in \Z$ such that $\norm a_p = p^{-v}$.

Hence:

$p^{-v} > 1 = p^0$.
$-v > 0$.

Let $n = -v$.

Then:

 $\displaystyle \norm{p^n a}_p$ $=$ $\displaystyle \norm{p^n }_p \norm a_p$ Norm axiom (N2) (Multiplicativity) $\displaystyle$ $=$ $\displaystyle p^{-n} \norm a_p$ Definition of $p$-adic norm on the rational numbers $\displaystyle$ $=$ $\displaystyle p^{-n} p^{n}$ By definition of $n$ $\displaystyle$ $=$ $\displaystyle 1$ $\displaystyle \leadsto \ \$ $\displaystyle p^n a$ $\in$ $\displaystyle \Z_p$ Definition of $p$-adic integers

The result follows.

$\blacksquare$