# Closed Ball is Disjoint Union of Smaller Closed Balls in P-adic Numbers/Lemma 1/Necessary Condition

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## Theorem

Let $p$ be a prime number.

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

Let $n, m \in Z$, such that $n < m$.

Let $y \in \Q_p$.

Let $\norm{y}_p \le p^{-n}$.

Then there exists $i \in \Z$:

- $(1)\quad0 \le i \le p^\paren{m-n}-1$
- $(2)\quad\norm{y - i p^n}_p \le p^{-m}$

## Proof

Now:

\(\displaystyle \norm{y}_p\) | \(\le\) | \(\displaystyle p^{-n}\) | |||||||||||

\(\, \displaystyle \leadsto \, \) | \(\displaystyle p^n \norm{y}_p\) | \(\le\) | \(\displaystyle 1\) | Multiply both sides by $p^n$ | |||||||||

\(\, \displaystyle \leadsto \, \) | \(\displaystyle \norm{p^{-n} }_p \norm{y}_p\) | \(\le\) | \(\displaystyle 1\) | Definition of $p$-adic norm | |||||||||

\(\, \displaystyle \leadsto \, \) | \(\displaystyle \norm{p^{-n} y}_p\) | \(\le\) | \(\displaystyle 1\) | Norm axiom (N2) : (Mulitplicativity) | |||||||||

\(\, \displaystyle \leadsto \, \) | \(\displaystyle \map {B_1^-} {p^{-n}y}\) | \(=\) | \(\displaystyle \map {B_1^-} 0\) | Characterization of Closed Ball in P-adic Numbers |

From Integers are Dense in Unit Ball of P-adic Numbers:

- $\exists \mathop k \in \Z : \norm{p^{-n} y - k}_p \le p^\paren{n-m}$

From Residue Classes form Partition of Integers:

- $\exists \mathop 0 \le i \le p^\paren{m-n}-1 : p^\paren{m-n} \divides k - i$

By definition of the $p$-adic norm: $\norm{k - i}_p \le p^\paren{n-m}$

It follows that:

\(\displaystyle \norm{p^{-n} y - i}_p\) | \(\le\) | \(\displaystyle \max \set{\norm{p^{-n} y - k}_p, \norm{i - k}_p}\) | Corollary to P-adic Metric on P-adic Numbers is Non-Archimedean Metric | ||||||||||

\(\displaystyle \) | \(\le\) | \(\displaystyle p^\paren{n-m}\) | |||||||||||

\(\, \displaystyle \leadsto \, \) | \(\displaystyle \norm{p^{-n} }_p \norm{y - i p^n}_p\) | \(\le\) | \(\displaystyle p^\paren{n-m}\) | Norm axiom (N2) : (Mulitplicativity) | |||||||||

\(\, \displaystyle \leadsto \, \) | \(\displaystyle p^n \norm{y - i p^n}_p\) | \(\le\) | \(\displaystyle p^\paren{n-m}\) | Definition of $p$-adic norm | |||||||||

\(\, \displaystyle \leadsto \, \) | \(\displaystyle \norm{y - i p^n}_p\) | \(\le\) | \(\displaystyle p^{-m}\) | Divide both sides by $p^{-n}$ |

$\blacksquare$