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For each prime number the ring of -adic integers is the formal completion of the ring at the prime ideal . Geometrically this means that is the ring of functions on a formal neighbourhood of inside Spec(Z) (this is discussed in more detail below). Algebraically it means that the elements in look like formal power series where the formal variable is the prime number .
For any prime number , the ring of -adic integers (which, to avoid possible confusion with the ring used in modular arithmetic, is also written as ) may be described in one of several ways:
To the person on the street, it may be described as (the ring of) numbers written in base , but allowing infinite expansions to the left. Thus, numbers of the form
where , added and multiplied with the usual method of carrying familiar from adding and multiplying ordinary integers.
More abstractly, it is the limit , in the category of (unital) rings, of the diagram
This is also a limit in the category of topological rings, taking the rings in the diagram to have discrete topologies.
Alternatively, it is the metric completion of the ring of integers with respect to the -adic absolute value. Since addition and multiplication of integers are uniformly continuous with respect to the -adic absolute value, they extend uniquely to a uniformly continuous addition and multiplication on . Thus is a topological ring.
Also , see at analytic completion.
Hence one also speaks of the -adic completion of the integers. See completion of a ring (which generalizes 2&3).
There is also this characterization:
There is a short exact sequence
Consider the following commuting diagram
Each horizontal sequence is exact. Taking the limit over the vertical sequences yields the sequence in question. Since limits commute over limits, the result follows.
The ring of -adic integers has the following properties:
Relation to profinite completion of the integers
The profinite completion of the integers is
This is isomorphic to the product of the -adic integers for all
(e.g. Lenstra, example 2.2)
The ring of integral adeles is the product of the profinite completion of the integers, example 1, with the real numbers
The group of units of the ring of adeles is called the group of ideles.
Pontrajgin duality to Prüfer -group
Under Pontryagin duality, the abelian group underlying maps to the Prüfer p-group , see at Pontryagin duality for torsion abelian groups.
As the formal neighbourhood of a prime
The formal spectrum of may be understood as the formal neighbourhood of the point corresponding to the prime in the prime spectrum of the integers. The inclusion
is the formal dual of the canonical projection maps .
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.
Examples of sequences of local structures
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 -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 -adic numbers is highlighted for instance in