Characterization of Polynomial has Root in P-adic Integers/Necessary Condition
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Theorem
Let $\Z_p$ be the $p$-adic integers for some prime $p$.
Let $\map F X \in \Z_p \sqbrk X$ be a polynomial over $\Z_p$.
Let $a \in \Z_p$.
Let $\map F a = 0$.
Then:
- there exists a sequence $\sequence {a_n}$ of integers:
- $(1): \quad \ds \lim_{n \mathop \to \infty} {a_n} = a$
- $(2): \quad \map F {a_n} \equiv 0 \mod {p^{n + 1} \Z_p}$
where $\map F {a_n} \equiv 0 \mod {p^{n + 1} \Z_p}$ denotes congruence modulo the ideal $p^{n + 1} \Z_p$
Proof
Let $\map F a = 0$.
Let $a = \ds \sum_{j \mathop = 0}^\infty d_j p^j$ be the $p$-adic expansion of $a$.
For all $n \in \N_{>0}$, let:
- $a_n = \ds \sum_{j \mathop = 0}^{n - 1} d_j p^j$
By definition of $p$-adic expansion:
- $\ds \lim_{n \mathop \to \infty} {a_n} = a$
By definition of $p$-adic expansion of a $p$-adic integer:
- $\forall n \in \N_{>0} : a_n \in \Z$
We have:
\(\ds \forall n \in \N_{>0}: \, \) | \(\ds a_n\) | \(\equiv\) | \(\ds a \pmod {p^n \Z_p}\) | Partial Sum Congruent to P-adic Integer Modulo Power of p | ||||||||||
\(\ds \leadsto \ \ \) | \(\ds \forall n \in \N_{>0}: \, \) | \(\ds \map F {a_n}\) | \(\equiv\) | \(\ds \map F a \pmod {p^n \Z_p}\) | Polynomials of Congruent Ring Elements are Congruent | |||||||||
\(\ds \) | \(\equiv\) | \(\ds 0 \pmod {p^n\Z_p}\) | as $\map F a = 0$ | |||||||||||
\(\ds \leadsto \ \ \) | \(\ds \forall n \in \N_{>0}: \, \) | \(\ds \map F {a_n}\) | \(\equiv\) | \(\ds 0 \pmod {p^n}\) | Congruence Modulo Equivalence for Integers in P-adic Integers |
$\blacksquare$