Contents

Idea

For each prime number $p$ the ring of $p$-adic integers $\mathbb{Z}_p$ is the formal completion of the ring $\mathbb{Z}$ at the prime ideal $(p)$. Geometrically this means that $\mathbb{Z}_p$ is the ring of functions on a formal neighbourhood of $p$ inside Spec(Z) (this is discussed in more detail below). Algebraically it means that the elements in $\mathbb{Z}_p$ look like formal power series where the formal variable is the prime number $p$.

Definition

For any prime number $p$, the ring of $p$-adic integers $\mathbb{Z}_p$ (which, to avoid possible confusion with the ring $\mathbb{Z}/(p)$ used in modular arithmetic, is also written as $\widehat{\mathbb{Z}}_p$) may be described in one of several ways:

1. To the person on the street, it may be described as (the ring of) numbers written in base $p$, but allowing infinite expansions to the left. Thus, numbers of the form

   $$\sum_{n \geq 0} a_n p^n$$

   where $0 \leq a_n \lt p$, added and multiplied with the usual method of carrying familiar from adding and multiplying ordinary integers.

2. More abstractly, it is the limit $\underset{\leftarrow}{\lim} \mathbb{Z}/(p^n)$, in the category of (unital) rings, of the diagram

   $$\ldots \to \mathbb{Z}/(p^{n+1}) \to \mathbb{Z}/(p^n) \to \ldots \to \mathbb{Z}/(p) .$$

   This is also a limit in the category of topological rings, taking the rings in the diagram to have discrete topologies.

3. Alternatively, it is the metric completion of the ring of integers $\mathbb{Z}$ with respect to the $p$-adic absolute value. Since addition and multiplication of integers are uniformly continuous with respect to the $p$-adic absolute value, they extend uniquely to a uniformly continuous addition and multiplication on $\mathbb{Z}_p$. Thus $\mathbb{Z}_p$ is a topological ring.

4. Also $\mathbb{Z}[ [ x ] ]/(x-p)\mathbb{Z}[ [ x ] ]$, see at analytic completion.

Hence one also speaks of the $p$-adic completion of the integers. See completion of a ring (which generalizes 2&3).

There is also this characterization:

Properties

Topology

The ring of $p$-adic integers has the following properties:

• As a topological space, it is compact, Hausdorff, and totally disconnected (i.e., is a Stone space). Moreover, every point is an accumulation point, and there is a countable basis of clopen sets – a Stone space with these properties must be homeomorphic to Cantor space.

• As a topological group under addition, it is therefore an almost connected group. As an abelian compact group, it is Pontryagin dual to the Prüfer $p$-group as discrete group.

Relation to profinite completion of the integers

(e.g. Lenstra, example 2.2)

Pontrajgin duality to Prüfer $p$-group

Under Pontryagin duality, the abelian group underlying $\mathbb{Z}_p$ maps to the Prüfer p-group $\mathbb{Z}[p^{-1}]/\mathbb{Z}$, see at Pontryagin duality for torsion abelian groups.

As the formal neighbourhood of a prime

The formal spectrum $Spf(\mathbb{Z}_p)$ of $\mathbb{Z}_p$ may be understood as the formal neighbourhood of the point corresponding to the prime $p$ in the prime spectrum $Spec(\mathbb{Z})$ of the integers. The inclusion

is the formal dual of the canonical projection maps $\mathbb{Z}\to \mathbb{Z}_p\to \mathbb{Z}/(p)$.

This plays a central role for instance in the function field analogy. It is highlighted for instance in (Hartl 06, 1.1, Buium 13, section 1.1.3). See also at arithmetic jet space and at ring of Witt vectors.

Related notions

• $p$-adic number, adele.

• p-adic complex number

• Z-infinity-module

• p-completion

• local-global principle

• p-adic homotopy

• ℓ-adic cohomology

• p-adic geometry

• p-adic physics

• motivic integration

References

Introductions and surveys include

• Dennis Sullivan, pp. 9 of Localization, Periodicity and Galois Symmetry (The 1970 MIT notes) edited by Andrew Ranicki, K-Monographs in Mathematics, Dordrecht: Springer (pdf)

• Bernard Le Stum, One century of $p$-adic geometry – From Hensel to Berkovich and beyond talk notes, June 2012 (pdf)

• Hendrik Lenstra, Profinite groups (pdf)

The synthetic differential geometry-aspect of the $p$-adic numbers is highlighted for instance in

• Urs Hartl, A Dictionary between Fontaine-Theory and its Analogue in Equal Characteristic (arXiv:math/0607182)

• Alexandru Buium, Differential calculus with integers (arXiv:1308.5194)