p-adic order

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In number theory, for a given prime number p, the p-adic order or p-adic additive valuation of a non-zero integer n is the highest exponent ν such that pν divides n. The p-adic valuation of 0 is defined to be \infty. It is commonly abbreviated νp(n). If n/d is a rational number in lowest terms, so that n and d are relatively prime, then νp(n/d) is equal to νp(n) if p divides n, or -νp(d) if p divides d, or to 0 if it divides neither one. The most important application of the p-adic order is in constructing the field of p-adic numbers. It is also applied toward various more elementary topics, such as the distinction between singly and doubly even numbers.[1]

Distribution of natural numbers by their 2-adic order, labeled with corresponding powers of two in decimal. Zero always has an infinite order

Definition and Properties[edit]

Integers[edit]

Let p be a prime in Z. The p-adic order or p-adic valuation for Z is defined as[2] \nu_p:\textbf{Z} \to \textbf{N}


\nu_p(n)=
\begin{cases}
\mathrm{max}\{v\in\mathbb{N}:p^v | n\} & \text{if } n \neq 0\\
\infty & \text{if } n=0 
\end{cases}

Rational Numbers[edit]

The p-adic order can be extended into the rational numbers. We can define[3] \nu_p:\textbf{Q} \to \textbf{Z}


\nu_p\left(\frac{a}{b}\right)=\nu_p(a)-\nu_p(b).

Some properties are:

\nu_p(m\cdot n)= \nu_p(m) + \nu_p(n)~.
\nu_p(m+n)\geq \inf\{ \nu_p(m), \nu_p(n)\}. Moreover, if \nu_p(m)\ne \nu_p(n), then \nu_p(m+n)= \inf\{ \nu_p(m), \nu_p(n)\}.

where \inf is the Infimum (i.e. the smaller of the two)

p-adic Norm[edit]

From our definition of the p-adic order, we can define the p-adic norm. The p-adic norm of Q is defined as |\quad|_p: \textbf{Q} \to \textbf{R}


|x|_p = 
\begin{cases}
	p^{-\nu_p(x)} & \text{if } x \neq 0\\
	0 & \text{if } x=0
\end{cases}

Some properties of the p-adic norm:


\begin{align}
	|a|_p \geq 0 & \quad \text{Non-negativity}\\
	|a|_p = 0 \iff a = 0 & \quad \text{Positive-definiteness}\\
	|ab|_p = |a|_p|b|_p & \quad \text{Multiplicativeness}\\
	|a+b|_p \leq |a|_p + |b|_p & \quad \text{Subadditivity}\\
	|a+b|_p \leq \max\left(|a|_p, |b|_p\right) & \quad \text{it is non-archimedean}\\
	|-a|_p = |a|_p & \quad \text{Symmetry}
\end{align}

A metric space can be formed on the set Q with a (non-archimedean, translation invariant) metric defined by d:\textbf{Q}\times\textbf{Q} \to \textbf{R}

d(x,y)=|x-y|_p.

See also[edit]

References[edit]

  1. ^ David S. Dummit; Richard M. Foote (2003). Abstract Algebra (3rd ed.). Wiley. ISBN 0-471-43334-9. 
  2. ^ Ireland, K., Rosen, M. (2000). A Classical Introduction to Modern Number Theory. Springer-Verlag New York. Inc., p. 3
  3. ^ Khrennikov, A., Nilsson, M. (2004). P-adic Deterministic and Random Dynamics., Kluwer Academic Publishers, p. 9