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{{Short description|Number system extending the rational numbers}}
{{DISPLAYTITLE:''p''-adic number}}
[[Image:3-adic integers with dual colorings.svg|thumb|The 3-adic integers, with selected corresponding characters on their [[Pontryagin dual]] group]]

In [[number theory]], given a [[prime number]] {{mvar|p}},{{efn-num|In this article, unless otherwise stated, {{mvar|p}} denotes a prime number that is fixed once for all.}} the '''{{mvar|p}}-adic numbers''' form an extension of the [[rational number]]s that is distinct from the [[real number]]s, though with some similar properties; {{mvar|p}}-adic numbers can be written in a form similar to (possibly [[infinity (mathematics)|infinite]]) [[decimal representation|decimal]]s, but with digits based on a prime number {{mvar|p}} rather than ten, and extending to the left rather than to the right.

For example, comparing the expansion of the rational number <math>\tfrac15</math> in [[Ternary numeral system|base {{math|3}}]] vs. the {{math|3}}-adic expansion,
<math display="block">\begin{alignat}{3}
\tfrac15 &{}= 0.01210121\ldots \ (\text{base } 3)
         &&{}= 0\cdot 3^0 + 0\cdot 3^{-1} + 1\cdot 3^{-2} + 2\cdot 3^{-3} + \cdots \\[5mu]
\tfrac15 &{}= \dots 121012102 \ \ (\text{3-adic})
         &&{}= \cdots + 2\cdot 3^3 + 1 \cdot 3^2 + 0\cdot3^1 + 2 \cdot 3^0.
\end{alignat}</math>

Formally, given a prime number {{mvar|p}}, a {{mvar|p}}-adic number can be defined as a [[series (mathematics)|series]]
<math display="block">s=\sum_{i=k}^\infty a_i p^i = a_k p^k + a_{k+1} p^{k+1} + a_{k+2} p^{k+2} + \cdots</math>
where {{mvar|k}} is an [[integer]] (possibly negative), and each <math>a_i</math> is an integer such that <math>0\le a_i < p.</math> A '''{{mvar|p}}-adic integer''' is a {{mvar|p}}-adic number such that <math>k\ge 0.</math>

In general the series that represents a {{mvar|p}}-adic number is not [[convergent series|convergent]] in the usual sense, but it is convergent for the [[p-adic absolute value|{{mvar|p}}-adic absolute value]] <math>|s|_p=p^{-k},</math> where {{mvar|k}} is the least integer {{mvar|i}} such that <math>a_i\ne 0</math> (if all <math>a_i</math> are zero, one has the zero {{mvar|p}}-adic number, which has {{math|0}} as its {{mvar|p}}-adic absolute value).

Every rational number can be uniquely expressed as the sum of a series as above, with respect to the {{mvar|p}}-adic absolute value. This allows considering rational numbers as special {{mvar|p}}-adic numbers, and alternatively defining the {{mvar|p}}-adic numbers as the [[completion (metric space)|completion]] of the rational numbers for the {{mvar|p}}-adic absolute value, exactly as the real numbers are the completion of the rational numbers for the usual absolute value.

{{mvar|p}}-adic numbers were first described by [[Kurt Hensel]] in 1897,<ref>{{Harv|Hensel|1897}}</ref> though, with hindsight, some of [[Ernst Kummer|Ernst Kummer's]] earlier work can be interpreted as implicitly using {{mvar|p}}-adic numbers.<ref group="note">Translator's introduction, [https://books.google.com/books?id=Qxte2mhlEOYC&pg=PA35 page 35]: "Indeed, with hindsight it becomes apparent that a [[discrete valuation]] is behind Kummer's concept of ideal numbers." {{Harv|Dedekind|Weber|2012|p=35}}</ref>

== Motivation ==

Roughly speaking, [[modular arithmetic]] modulo a positive integer {{mvar|n}} consists of "approximating" every integer by the remainder of its [[Euclidean division|division]] by {{mvar|n}}, called its ''residue modulo'' {{mvar|n}}. The main property of modular arithmetic is that the residue modulo {{mvar|n}} of the result of a succession of operations on integers is the same as the result of the same succession of operations on residues modulo {{mvar|n}}.

When studying [[Diophantine equations]], it's sometimes useful to reduce the equation modulo a prime {{mvar|p}}, since this usually provides more insight about the equation itself. Unfortunately, doing this loses some information because the reduction <math>\mathbb Z\twoheadrightarrow\mathbb Z/p</math> is not injective.

One way to preserve more information is to use larger moduli, such as higher prime powers, {{math|''p''<sup>2</sup>}}, {{math|''p''<sup>3</sup>, ...}}. However, this has the disadvantage of <math>\mathbb Z/p^e</math> not being a field, which loses a lot of the algebraic properties that <math>\mathbb Z/p</math> has.<ref name="Chen">{{Harv|Chen|loc=Chapter 27}}</ref>

[[Kurt Hensel]] discovered a method which consists of using a prime modulus {{mvar|p}}, and applying [[Hensel's lemma]] to lift solutions modulo {{mvar|p}} to modulo {{math|''p''<sup>2</sup>}}, {{math|''p''<sup>3</sup>, ...}}. This process creates an infinite sequence of residues, and a {{mvar|p}}-adic number is defined as the "limit" of such a sequence.

Essentially, {{mvar|p}}-adic numbers allows "taking modulo {{math|''p''<sup>''e''</sup>}} for all {{mvar|e}} at once". A distinguishing feature of {{mvar|p}}-adic numbers from ordinary modulo arithmetic is that the set of {{mvar|p}}-adic numbers <math>\mathbb Q_p</math> forms a [[Field_(mathematics)|field]], making division by {{mvar|p}} possible (unlike when working modulo {{math|''p''<sup>''e''</sup>}}). Furthermore, the mapping <math>\mathbb Z\hookrightarrow\mathbb Z_p</math> is [[Injective_function|injective]], so not much information is lost when reducing to {{mvar|p}}-adic numbers.<ref name="Chen"/>

== Informal description ==

There are multiple ways to understand {{mvar|p}}-adic numbers.

=== As a base-''p'' expansion ===

One way to think about {{mvar|p}}-adic integers is using "base {{mvar|p}}". For example, every integer can be written in base {{mvar|p}},

<math display="block">50=1212_3=1\cdot3^3+2\cdot3^2+1\cdot3^1+2\cdot3^0</math>

Informally, '''{{mvar|p}}-adic integers''' can be thought of as integers in base-{{mvar|p}}, but the digits extend ''infinitely to the left''.<ref name="Chen"/>

<math display="block">\ldots121012102_3=\cdots+2\cdot3^3+1\cdot3^2+0\cdot3^1+2\cdot3^0</math>

Addition and multiplication on {{mvar|p}}-adic integers can be carried out similarly to integers in base-{{mvar|p}}.<ref name="Koc">{{Harv|Koç|2002}}</ref>

When adding together two {{mvar|p}}-adic integers, for example <math>\ldots012102_3+\ldots101211_3</math>, their digits are added with carries being propagated from right to left.

<math display="block">\begin{array}{cccccccc}
  &        &   &_1 &_1 &   &_1 &   \\
  & \cdots & 0 & 1 & 2 & 1 & 0 & 2 \,_3 \\
+ & \cdots & 1 & 0 & 1 & 2 & 1 & 1 \,_3 \\\hline
  & \cdots & 1 & 2 & 1 & 0 & 2 & 0 \,_3
\end{array}</math>

Multiplication of {{mvar|p}}-adic integers works similarly via [[Multiplication_algorithm|long multiplication]]. Since addition and multiplication can be performed with {{mvar|p}}-adic integers, they form a [[Ring_(mathematics)|ring]], denoted <math>\mathbb Z_p</math> or <math>\mathbf Z_p</math>.

Note that some rational numbers can also be {{mvar|p}}-adic integers, even if they aren't integers in a real sense. For example, the rational number {{sfrac|1|5}} is a 3-adic integer, and has the 3-adic expansion <math>\tfrac{1}{5}=\ldots121012102_3</math>. However, some rational numbers, such as <math>\tfrac{1}{p}</math>, cannot be written as a {{mvar|p}}-adic integer. Because of this, {{mvar|p}}-adic integers are generalized further to {{mvar|p}}-adic numbers:

'''{{mvar|p}}-adic numbers''' can be thought of as {{mvar|p}}-adic integers with ''finitely many digits after the decimal point''. An example of a 3-adic number is

<math display="block">\ldots121012.102_3 = \cdots+1\cdot3^1+2\cdot3^0+1\cdot3^{-1}+0\cdot3^{-2}+2\cdot3^{-3}</math>

Equivalently, every {{mvar|p}}-adic number is of the form <math>\tfrac x{p^k}</math>, where {{mvar|x}} is a {{mvar|p}}-adic integer.

For any {{mvar|p}}-adic number {{mvar|x}}, its [[multiplicative inverse]] <math>\tfrac{1}{x}</math> is also a {{mvar|p}}-adic number, which can be computed using a variant of [[long division]].<ref name="Koc"/> For this reason, the {{mvar|p}}-adic numbers form a [[field]], denoted <math>\mathbb Q_p</math> or <math>\mathbf Q_p</math>.

=== As a sequence of residues mod {{mvar|p}}<sup>''k''</sup> ===

Another way to define {{mvar|p}}-adic integers is by representing it as a sequence of residues <math>x_e</math> mod <math>p^e</math> for each integer <math>e</math>,<ref name="Chen"/>

<math display="block"> x = (x_1 \operatorname{mod} p, ~ x_2 \operatorname{mod} p^2, ~ x_3 \operatorname{mod} p^3, ~ \ldots)</math>

satisfying the compatibility relations <math>x_i \equiv x_j ~ (\operatorname{mod} p^i)</math> for <math>i < j</math>. In this notation, addition and multiplication of {{mvar|p}}-adic integers are defined component-wise:

<math display="block"> x+y = (x_1+y_1 \operatorname{mod} p, ~ x_2+y_2 \operatorname{mod} p^2, ~ x_3+y_3 \operatorname{mod} p^3, ~ \ldots)</math>
<math display="block"> x\cdot y = (x_1\cdot y_1 \operatorname{mod} p, ~ x_2\cdot y_2 \operatorname{mod} p^2, ~ x_3\cdot y_3 \operatorname{mod} p^3, ~ \ldots)</math>

This is equivalent to the base-{{mvar|p}} definition, because the last {{mvar|k}} digits of a base-{{mvar|p}} expansion uniquely define its value mod {{mvar|p}}<sup>''k''</sup>, and vice versa.

This form can also explain why some rational numbers are {{mvar|p}}-adic integers, even if they are not integers. For example, {{sfrac|1|5}} is a 3-adic integer, because its 3-adic expansion consists of the [[Modular multiplicative inverse|multiplicative inverse]]s of 5 mod 3, 3<sup>2</sup>, 3<sup>3</sup>, ...

<math display="block">\begin{align}
\frac15 &= (\tfrac15  \operatorname{mod} 3, ~ \tfrac15  \operatorname{mod} 3^2, ~ \tfrac15  \operatorname{mod} 3^3, ~ \tfrac15 \operatorname{mod} 3^4, ~ \ldots)\\
&= (2 \operatorname{mod} 3, ~ 2 \operatorname{mod} 3^2, ~ 11 \operatorname{mod} 3^3, ~ 65 \operatorname{mod} 3^4, ~ \ldots)
\end{align}</math>

== Definition ==

There are several equivalent definitions of {{mvar|p}}-adic numbers. The two approaches given below are relatively elementary.

=== As formal series in base {{mvar|p}} ===

A '''{{mvar|p}}-adic integer''' is often defined as a [[formal power series]] of the form
<math display="block">r=\sum_{i=0}^\infty a_i p^i = a_0 + a_1 p + a_2 p^2 + a_3 p^3 + \cdots</math>
where each <math>a_i\in\{0,1,\ldots,p-1\}</math> represents a "digit in base {{mvar|p}}".

A '''{{mvar|p}}-adic unit''' is a {{mvar|p}}-adic integer whose first digit is nonzero, i.e. <math>a_0\ne 0</math>. The set of all {{mvar|p}}-adic integers is usually denoted <math>\mathbb Z_p</math>.<ref>{{Harv|Koblitz|1984|p=13}}</ref>

A '''{{mvar|p}}-adic number''' is then defined as a [[formal Laurent series]] of the form
<math display="block">r=\sum_{i=v}^\infty a_i p^i = a_v p^v + a_{v+1} p^{v+1} + a_{v+2} p^{v+2} + a_{v+3} p^{v+3} + \cdots</math>
where {{mvar|v}} is a (possibly negative) integer, and each <math>a_i\in\{0,1,\ldots,p-1\}</math>.<ref name=G18>{{Harv|Gouvêa|1997|p=18}}</ref> Equivalently, a {{mvar|p}}-adic number is anything of the form <math>\tfrac{x}{p^k}</math>, where {{mvar|x}} is a {{mvar|p}}-adic integer.

The first index {{mvar|v}} for which the digit <math>a_v</math> is nonzero in {{mvar|r}} is called the '''[[p-adic valuation|{{mvar|p}}-adic valuation]]''' of {{mvar|r}}, denoted <math>v_p(r)</math>. If <math>r=0</math>, then such an index does not exist, so by convention <math>v_p(0)=\infty</math>.

In this definition, addition, subtraction, multiplication, and division of {{mvar|p}}-adic numbers are carried out similarly to numbers in base {{mvar|p}}, with "carries" or "borrows" moving from left to right rather than right to left.<ref>{{Harv|Koblitz|1984|pp=14–15}}</ref> As an example in <math>\mathbb Q_3</math>,

<math display="block">\begin{array}{lllllllllll}
  &           & & _1        & &           & & _1        & & _1 \\
  & 2\cdot3^0 &+& 0\cdot3^1 &+& 1\cdot3^2 &+& 2\cdot3^3 &+& 1\cdot3^4 &+ \cdots \\
+ & 1\cdot3^0 &+& 1\cdot3^1 &+& 2\cdot3^2 &+& 1\cdot3^3 &+& 0\cdot3^4 &+ \cdots \\\hline
  & 0\cdot3^0 &+& 2\cdot3^1 &+& 0\cdot3^2 &+& 1\cdot3^3 &+& 2\cdot3^4 &+ \cdots
\end{array}</math>

Division of {{mvar|p}}-adic numbers may also be carried out "formally" via [[Formal_power_series#Division|division of formal power series]], with some care about having to "carry".<ref name=G18/>

With these operations, the set of {{mvar|p}}-adic numbers form a [[Field_(mathematics)|field]], denoted <math>\mathbb Q_p</math>.

=== As equivalence classes ===

The {{mvar|p}}-adic numbers may also be defined as equivalence classes, in a similar way as the definition of real numbers as equivalence classes of [[Cauchy sequences]]. It is fundamentally based on the following lemma:

:''Every nonzero rational number {{mvar|r}} can be written <math display=inline>r=p^v\frac{m}{n},</math> where {{mvar|v}}, {{mvar|m}}, and {{mvar|n}} are integers and neither {{mvar|m}} nor {{mvar|n}} is divisible by {{mvar|p}}.''

The exponent {{mvar|v}} is uniquely determined by {{mvar|r}} and is called its '''{{mvar|p}}-adic valuation''', denoted <math>v_p(r)</math>. The proof of the lemma results directly from the [[fundamental theorem of arithmetic]].

A '''{{mvar|p}}-adic series''' is a [[formal Laurent series]] of the form
<math display="block">\sum_{i=v}^\infty r_i p^{i},</math>
where <math>v</math> is a (possibly negative) integer and the <math>r_i</math> are rational numbers that either are zero or have a nonnegative valuation (that is, the denominator of <math>r_i</math> is not divisible by {{mvar|p}}).

Every rational number may be viewed as a {{mvar|p}}-adic series with a single nonzero term, consisting of its factorization of the form <math>p^k\tfrac mn,</math> with {{mvar|m}} and {{mvar|n}} both coprime with {{mvar|p}}.

Two {{mvar|p}}-adic series <math display=inline>\sum_{i=v}^\infty r_i p^{i} </math> and <math display=inline> \sum_{i=w}^\infty s_i p^{i} </math>
are ''equivalent'' if there is an integer {{mvar|N}} such that, for every integer <math>n>N,</math> the rational number
<math display="block">\sum_{i=v}^n r_i p^{i} - \sum_{i=w}^n s_i p^{i} </math>
is zero or has a {{mvar|p}}-adic valuation greater than {{mvar|n}}.

A {{mvar|p}}-adic series <math display=inline>\sum_{i=v}^\infty a_i p^{i} </math> is ''normalized'' if either all <math>a_i</math> are integers such that <math>0\le a_i <p,</math> and <math>a_v >0,</math> or all <math>a_i</math> are zero. In the latter case, the series is called the ''zero series''.

Every {{mvar|p}}-adic series is equivalent to exactly one normalized series. This normalized series is obtained by a sequence of transformations, which are equivalences of series; see [[#Normalization of a p-adic series|§ Normalization of a {{mvar|p}}-adic series]], below.

In other words, the equivalence of {{mvar|p}}-adic series is an [[equivalence relation]], and each [[equivalence class]] contains exactly one normalized {{mvar|p}}-adic series.

The usual operations of series (addition, subtraction, multiplication, division) are compatible with equivalence of {{mvar|p}}-adic series. That is, denoting the equivalence with {{math|~}}, if {{mvar|S}}, {{mvar|T}} and {{mvar|U}} are nonzero {{mvar|p}}-adic series such that <math>S\sim T,</math> one has
<math display="block">\begin{align}
S\pm U&\sim T\pm U,\\
SU&\sim TU,\\
1/S&\sim 1/T.
\end{align}</math>

With this, the '''{{mvar|p}}-adic numbers''' are defined as the ''equivalence classes'' of {{mvar|p}}-adic series.

The uniqueness property of normalization, allows uniquely representing any {{mvar|p}}-adic number by the corresponding normalized {{mvar|p}}-adic series. The compatibility of the series equivalence leads almost immediately to basic properties of {{mvar|p}}-adic numbers:
* ''Addition'', ''multiplication'' and [[multiplicative inverse]] of {{mvar|p}}-adic numbers are defined as for [[formal power series]], followed by the normalization of the result.
* With these operations, the {{mvar|p}}-adic numbers form a [[field (mathematics)|field]], which is an [[extension field]] of the rational numbers.
* The ''valuation'' of a nonzero {{mvar|p}}-adic number {{mvar|x}}, commonly denoted <math>v_p(x)</math> is the exponent of {{mvar|p}} in the first non zero term of the corresponding normalized series; the valuation of zero is <math>v_p(0)=+\infty</math>
* The ''{{mvar|p}}-adic absolute value'' of a nonzero {{mvar|p}}-adic number {{mvar|x}}, is <math>|x|_p=p^{-v(x)};</math> for the zero {{mvar|p}}-adic number, one has <math>|0|_p=0.</math>

==== Normalization of a ''p''-adic series ====
Starting with the series <math display=inline>\sum_{i=v}^\infty r_i p^{i}, </math> we wish to arrive at an equivalent series such that the {{mvar|p}}-adic valuation of <math>r_v</math> is zero. For that, one considers the first nonzero <math>r_i.</math> If its {{mvar|p}}-adic valuation is zero, it suffices to change {{mvar|v}} into {{mvar|i}}, that is to start the summation from {{mvar|v}}. Otherwise, the {{mvar|p}}-adic valuation of <math>r_i</math> is <math>j>0,</math> and <math>r_i= p^js_i</math> where the valuation of <math>s_i</math> is zero; so, one gets an equivalent series by changing <math>r_i</math> to {{math|0}} and <math>r_{i+j}</math> to <math>r_{i+j} + s_i.</math> Iterating this process, one gets eventually, possibly after infinitely many steps, an equivalent series that either is the zero series or is a series such that the valuation of <math>r_v</math> is zero.

Then, if the series is not normalized, consider the first nonzero <math>r_i</math> that is not an integer in the interval <math>[0,p-1].</math> Using [[Bézout's lemma]], write this as <math>r_i=a_i+ps_i</math>, where <math>a_i\in[0,p-1]</math> and <math>s_i</math> has nonnegative valuation. Then, one gets an equivalent series by replacing <math>r_i</math> with <math>a_i,</math> and adding <math>s_i</math> to <math>r_{i+1}.</math> Iterating this process, possibly infinitely many times, provides eventually the desired normalized {{math|p}}-adic series.

=== Other equivalent definitions ===

Other equivalent definitions use [[completion of a ring|completion]] of a [[discrete valuation ring]] (see {{slink||p-adic integers}}), [[completion of a metric space]] (see {{slink||Topological properties}}), or [[inverse limit]]s (see {{slink||Modular properties}}).

A {{mvar|p}}-adic number can be defined as a ''normalized {{mvar|p}}-adic series''. Since there are other equivalent definitions that are commonly used, one says often that a normalized {{mvar|p}}-adic series ''represents'' a {{mvar|p}}-adic number, instead of saying that it ''is'' a {{mvar|p}}-adic number.

One can say also that any {{mvar|p}}-adic series represents a {{mvar|p}}-adic number, since every {{mvar|p}}-adic series is equivalent to a unique normalized {{mvar|p}}-adic series. This is useful for defining operations (addition, subtraction, multiplication, division) of {{mvar|p}}-adic numbers: the result of such an operation is obtained by normalizing the result of the corresponding operation on series. This well defines operations on {{mvar|p}}-adic numbers, since the series operations are compatible with equivalence of {{mvar|p}}-adic series.

{{anchor|Field of p-adic numbers}}
With these operations, {{mvar|p}}-adic numbers form a [[field (mathematics)|field]] called the '''field of {{math|''p''}}-adic numbers''' and denoted <math>\Q_p</math> or <math>\mathbf Q_p.</math> There is a unique [[field homomorphism]] from the rational numbers into the {{mvar|p}}-adic numbers, which maps a rational number to its {{mvar|p}}-adic expansion. The [[image (mathematics)|image]] of this homomorphism is commonly identified with the field of rational numbers. This allows considering the {{math|''p''}}-adic numbers as an [[extension field]] of the rational numbers, and the rational numbers as a [[subfield (mathematics)|subfield]] of the {{math|''p''}}-adic numbers.

The ''valuation'' of a nonzero {{mvar|p}}-adic number {{mvar|x}}, commonly denoted <math>v_p(x),</math> is the exponent of {{mvar|p}} in the first nonzero term of every {{mvar|p}}-adic series that represents {{mvar|x}}. By convention, <math>v_p(0)=\infty;</math> that is, the valuation of zero is <math>\infty.</math> This valuation is a [[discrete valuation]]. The restriction of this valuation to the rational numbers is the {{mvar|p}}-adic valuation of <math>\Q,</math> that is, the exponent {{mvar|v}} in the factorization of a rational number as <math dosplay=inline≝>\tfrac nd p^v,</math> with both {{mvar|n}} and {{mvar|d}} [[coprime]] with {{mvar|p}}.

== Notation ==
There are several different conventions for writing {{mvar|p}}-adic expansions. So far this article has used a notation for {{mvar|p}}-adic expansions in which [[exponentiation|powers]] of {{mvar|p}} increase from right to left. With this right-to-left notation the 3-adic expansion of <math>\tfrac15,</math> for example, is written as
<math display="block">\frac15 = \dots 121012102_3.</math>

When performing arithmetic in this notation, digits are [[carry (arithmetic)|carried]] to the left. It is also possible to write {{mvar|p}}-adic expansions so that the powers of {{mvar|p}} increase from left to right, and digits are carried to the right. With this left-to-right notation the 3-adic expansion of <math>\tfrac15</math> is
<math display="block">
\frac15 = 2.01210121\dots_3 \mbox{ or }
\frac1{15} = 20.1210121\dots_3.
</math>

{{mvar|p}}-adic expansions may be written with [[Signed-digit representation|other sets of digits]] instead of {{math|{0,&thinsp;1,&thinsp;...,}}&thinsp;{{math|''p'' − 1}}}. For example, the {{math|3}}-adic expansion of <math>\tfrac15</math> can be written using [[balanced ternary]] digits {{math|{<u>1</u>,&thinsp;0,&thinsp;1}}}, with {{math|<u>1</u>}} representing negative one, as
<math display="block">\frac15 = \dots\underline{1}11\underline{11}11\underline{11}11\underline{1}_{\text{3}} .</math>

In fact any set of {{mvar|p}} integers which are in distinct [[residue class]]es modulo {{mvar|p}} may be used as {{mvar|p}}-adic digits. In number theory, [[Witt vector#Motivation|Teichmüller representatives]] are sometimes used as digits.<ref>{{Harv|Hazewinkel|2009|p=342}}</ref>

'''{{vanchor|Quote notation}}''' is a variant of the {{mvar|p}}-adic representation of [[rational number]]s that was proposed in 1979 by [[Eric Hehner]] and [[Nigel Horspool]] for implementing on computers the (exact) arithmetic with these numbers.<ref>{{Harv|Hehner|Horspool|1979|pp=124–134}}</ref> It can be used as a compact way to represent rational numbers, which have an infinite periodic sequence of digits. In this notation, a quote mark (') is used to separate the repeating part from the nonrepeating part.
<math display="block">\frac15=1210\,'2_3</math>

== ''p''-adic expansion of rational numbers ==

The [[decimal expansion]] of a positive [[rational number]] <math>r</math> is its representation as a [[series (mathematics)|series]]
<math display="block">r = \sum_{i=k}^\infty a_i 10^{-i},</math>
where <math>k</math> is an integer and each <math>a_i</math> is also an [[integer]] such that <math>0\le a_i <10.</math> This expansion can be computed by [[long division]] of the numerator by the denominator, which is itself based on the following theorem: If <math>r=\tfrac n d</math> is a rational number such that <math>0\le r <1,</math> there is an integer <math>a</math> such that <math>0\le a <10,</math> and <math>10r = a+r',</math> with <math>0\le r'<1.</math> The decimal expansion is obtained by repeatedly applying this result to the remainder <math>r'</math> which in the iteration assumes the role of the original rational number <math>r</math>.

The {{mvar|p}}-''adic expansion'' of a rational number can be computed similarly, but with a different division step. Suppose that <math>r=\tfrac{n}{d}</math> is a rational number with nonnegative valuation (that is, {{mvar|d}} is not divisible by {{mvar|p}}). The division step consists of writing
{{anchor|division_step}}<math display="block">r = a + p\,r'</math>
where <math>a</math> is an integer such that <math>0\le a <p,</math> and <math>r'</math> has nonnegative valuation.

The integer {{mvar|a}} can be computed as a [[modular multiplicative inverse]]: <math>a=nd^{-1}\operatorname{mod}p</math>. Because of this, writing {{mvar|r}} in this way is always possible, and such a representation is unique.

The {{mvar|p}}-adic expansion of a rational number is eventually [[periodic function|periodic]]. [[Converse (logic)|Conversely]], a series <math display=inline>\sum_{i=k}^\infty a_i p^i,</math> with <math>0\le a_i <p</math> converges (for the {{mvar|p}}-adic absolute value) to a rational number [[if and only if]] it is eventually periodic; in this case, the series is the {{mvar|p}}-adic expansion of that rational number. The [[mathematical proof|proof]] is similar to that of the similar result for [[repeating decimal]]s.

=== Example ===
Let us compute the 5-adic expansion of <math>\tfrac 13.</math> We can write this number as <math>\tfrac13 = 2 + 5 \cdot \tfrac{-1}3</math>. Thus we use <math>a=2</math> for the first step.
<math display="block">\frac13 = 2 + 5^1 \cdot \left(\frac{-1}3\right)</math>
For the next step, we can write the "remainder" <math>\tfrac{-1}3</math> as <math>\tfrac{-1}3 = 3 + 5 \cdot \tfrac{-2}3</math>. Thus we use <math>a=3</math>.
<math display="block">\frac13 = 2 + 3\cdot 5^1 + 5^2 \cdot \left(\frac{-2}3\right)</math>
We can write the "remainder" <math>\tfrac{-2}3</math> as <math>\tfrac{-2}3 = 1 + 5 \cdot \tfrac{-1}3</math>. Thus we use <math>a=1</math>.
<math display="block">\frac13 = 2 + 3\cdot 5^1 + 1\cdot5^2 + 5^3\cdot \left(\frac{-1}3\right)</math>
Notice that we obtain the "remainder" <math>\tfrac{-1}3</math> again, which means the digits can only repeat from this point on.
<math display="block">\frac13 = 2 + 3\cdot 5^1 + 1\cdot5^2 + 3\cdot 5^3 + 1\cdot 5^4 + 3\cdot 5^5 + 1\cdot 5^6 + \cdots</math>
In the standard 5-adic notation, we can write this as
<math display="block">\frac 13= \ldots 1313132_5 </math>
with the [[ellipsis]] <math> \ldots </math> on the left hand side.

== ''p''-adic integers ==
The '''{{mvar|p}}-adic integers''' are the {{mvar|p}}-adic numbers with a nonnegative valuation.

A <math>p</math>-adic integer can be represented as a sequence
<math display="block"> x = (x_1 \operatorname{mod} p, ~ x_2 \operatorname{mod} p^2, ~ x_3 \operatorname{mod} p^3, ~ \ldots)</math>
of residues <math>x_e</math> mod <math>p^e</math> for each integer <math>e</math>, satisfying the compatibility relations <math>x_i \equiv x_j ~ (\operatorname{mod} p^i)</math> for <math>i < j</math>.

Every [[integer]] is a <math>p</math>-adic integer (including zero, since <math>0<\infty</math>). The rational numbers of the form <math display=inline> \tfrac nd p^k</math> with <math>d</math> coprime with <math>p</math> and <math>k\ge 0</math> are also <math>p</math>-adic integers (for the reason that <math>d</math> has an inverse mod <math>p^e</math> for every <math>e</math>).

The {{mvar|p}}-adic integers form a [[commutative ring]], denoted <math>\Z_p</math>  or <math>\mathbf Z_p</math>, that has the following properties.
* It is an [[integral domain]], since it is a [[subring]] of a field, or since the first term of the series representation of the product of two non zero {{mvar|p}}-adic series is the product of their first terms.
* The [[unit (ring theory)|units]] (invertible elements) of <math>\Z_p</math> are the {{mvar|p}}-adic numbers of valuation zero.
* It is a [[principal ideal domain]], such that each [[ideal (ring theory)|ideal]] is generated by a power of {{mvar|p}}.
* It is a [[local ring]] of [[Krull dimension]] one, since its only [[prime ideal]]s are the [[zero ideal]] and the ideal generated by {{mvar|p}}, the unique [[maximal ideal]].
* It is a [[discrete valuation ring]], since this results from the preceding properties.
* It is the [[completion of a ring|completion]] of the local ring <math>\Z_{(p)} = \bigl\{\tfrac nd \mathbin{\big|} n, d \in \Z,\, d \not\in p\Z \bigr\},</math> which is the [[localization (commutative algebra)|localization]] of <math>\Z</math> at the prime ideal <math>p\Z.</math>

The last property provides a definition of the {{mvar|p}}-adic numbers that is equivalent to the above one: the field of the {{mvar|p}}-adic numbers is the [[field of fractions]] of the completion of the localization of the integers at the prime ideal generated by {{mvar|p}}.

== Topological properties ==
[[File:3-adic integers.png|thumb|Visual depiction of the 3-adic integers <math>\mathbb Z_3</math> as a metric space]]

The {{mvar|p}}-adic valuation allows defining an [[absolute value (algebra)|absolute value]] on {{mvar|p}}-adic numbers: the {{mvar|p}}-adic absolute value of a nonzero {{mvar|p}}-adic number {{mvar|x}} is
<math display=block>|x|_p = p^{-v_p(x)},</math>
where <math>v_p(x)</math> is the {{mvar|p}}-adic valuation of {{mvar|x}}. The {{mvar|p}}-adic absolute value of <math>0</math> is <math>|0|_p = 0.</math> This is an absolute value that satisfies the [[strong triangle inequality]] since, for every {{mvar|x}} and {{mvar|y}}:
* <math>|x|_p = 0</math> if and only if <math>x=0;</math>
* <math>|x|_p\cdot |y|_p = |xy|_p;</math>
* <math>|x+y|_p\le \max\bigl(|x|_p,|y|_p\bigr) \le |x|_p + |y|_p.</math>

Moreover, if <math>|x|_p \ne |y|_p,</math> then <math>|x+y|_p = \max\bigl(|x|_p,|y|_p\bigr).</math>

This makes the {{mvar|p}}-adic numbers a [[metric space]], and even an [[ultrametric space]], with the {{mvar|p}}-adic distance defined by
<math>d_p(x,y)=|x-y|_p.</math>

As a metric space, the {{mvar|p}}-adic numbers form the [[completion (metric space)|completion]] of the rational numbers equipped with the {{mvar|p}}-adic absolute value. This provides another way for defining the {{mvar|p}}-adic numbers.

As the metric is defined from a [[discrete valuation]], every [[open ball]] is also [[closed ball|closed]]. More precisely, the open ball <math>B_r(x) =\{y\mid d_p(x,y)<r\}</math> equals the closed ball <math>\textstyle B_{p^{-v}}[x] =\{y\mid d_p(x,y)\le p^{-v}\},</math>  where {{mvar|v}} is the least integer such that <math>\textstyle p^{-v}< r.</math> Similarly, <math>\textstyle B_r[x] = B_{p^{-w}}(x),</math> where {{mvar|w}} is the greatest integer such that <math>\textstyle p^{-w}>r.</math>

This implies that the {{mvar|p}}-adic numbers <math>\mathbb Q_p</math> form a [[locally compact space]] ([[locally compact field]]), and the {{mvar|p}}-adic integers <math>\mathbb Z_p</math>—that is, the ball <math>B_1[0]=B_p(0)</math>—form a [[compact space]].<ref>{{Harv|Gouvêa|1997|loc=Corollary 4.2.7}}</ref>

The space of 2-adic integers <math>\mathbb Z_2</math> is [[homeomorphic]] to the [[Cantor set]] <math>\mathcal C</math>.<ref>{{Harv|Robert|2000|loc=Chapter 1 Section 2.3}}</ref><ref>{{Harv|Gouvêa|1997|loc=Theorem 4.4.1}}</ref> This can be seen by considering the continuous 1-to-1 mapping <math>\psi:\mathbb Z_2\to \mathcal C</math> defined by
<math display="block">\psi:~a_0+a_12+a_22^2+a_32^3+\cdots~\longmapsto~\frac{2a_0}3+\frac{2a_1}{3^2}+\frac{2a_2}{3^3}+\frac{2a_3}{3^4}+\cdots</math>
Moreover, for any {{mvar|p}}, <math>\mathbb Z_p</math> is homeomorphic to <math>\mathbb Z_2</math>, and therefore also homeomorphic to the Cantor set.<ref>{{Harv|Gouvêa|1997|loc=Theorem 4.4.2}}</ref>

The [[Pontryagin dual]] of the group of {{mvar|p}}-adic integers is the [[Prüfer group|Prüfer {{mvar|p}}-group]] <math>\mathbb Z(p^\infty)</math>, and the Pontryagin dual of the Prüfer {{mvar|p}}-group is the group of {{mvar|p}}-adic integers.<ref>{{Harv|Armacost|Armacost|1972}}</ref>

== Modular properties ==
The [[quotient ring]] <math>\Z_p/p^n\Z_p</math> may be identified with the [[ring (mathematics)|ring]] <math>\Z/p^n\Z</math> of the integers [[modular arithmetic|modulo]] <math>p^n.</math> This can be shown by remarking that every {{mvar|p}}-adic integer, represented by its normalized {{mvar|p}}-adic series, is congruent modulo <math>p^n</math> with its [[partial sum]] <math display = inline>\sum_{i=0}^{n-1}a_ip^i,</math> whose value is an integer in the interval <math>[0,p^n-1].</math> A straightforward verification shows that this defines a [[ring isomorphism]] from <math>\Z_p/p^n\Z_p</math> to <math>\Z/p^n\Z.</math>

The [[inverse limit]] of the rings <math>\Z_p/p^n\Z_p</math> is defined as the ring formed by the sequences <math>a_0, a_1, \ldots</math> such that <math>a_i \in \Z/p^i \Z</math> and <math display = inline>a_i \equiv a_{i+1} \pmod {p^i}</math> for every {{mvar|i}}.

The mapping that maps a normalized {{mvar|p}}-adic series to the sequence of its partial sums is a ring isomorphism from <math>\Z_p</math> to the inverse limit of the <math>\Z_p/p^n\Z_p.</math> This provides another way for defining {{mvar|p}}-adic integers ([[up to]] an isomorphism).

This definition of {{mvar|p}}-adic integers is specially useful for practical computations, as allowing building {{mvar|p}}-adic integers by successive approximations.

For example, for computing the {{mvar|p}}-adic (multiplicative) inverse of an integer, one can use [[Newton's method]], starting from the inverse modulo {{mvar|p}}; then, each Newton step computes the inverse modulo <math display = inline>p^{n^2}</math> from the inverse modulo <math display = inline>p^n.</math>

The same method can be used for computing the {{mvar|p}}-adic [[square root]] of an integer that is a [[quadratic residue]] modulo {{mvar|p}}. This seems to be the fastest known method for testing whether a large integer is a square: it suffices to test whether the given integer is the square of the value found in <math>\Z_p/p^n\Z_p</math>. Applying Newton's method to find the square root requires <math display = inline>p^n</math> to be larger than twice the given integer, which is quickly satisfied.

[[Hensel lifting]] is a similar method that allows to "lift" the factorization modulo {{mvar|p}} of a polynomial with integer coefficients to a factorization modulo <math display = inline>p^n</math> for large values of {{mvar|n}}. This is commonly used by [[polynomial factorization]] algorithms.

== Cardinality ==
Both <math>\Z_p</math> and <math>\Q_p</math> are [[uncountable set|uncountable]] and have the [[cardinality of the continuum]].<ref>{{Harv|Robert|2000|loc=Chapter 1 Section 1.1}}</ref> For <math>\Z_p,</math> this results from the {{mvar|p}}-adic representation, which defines a [[bijection]] of <math>\Z_p</math> on the [[power set]] <math>\{0,\ldots,p-1\}^\N.</math> For <math>\Q_p</math> this results from its expression as a [[countably infinite]] [[union (set theory)|union]] of copies of <math>\Z_p</math>:
<math display="block">\Q_p=\bigcup_{i=0}^\infty \frac 1{p^i}\Z_p.</math>

== Algebraic closure <span class="anchor" id="Complex p-adic number"></span>  ==

<math>\Q_p</math> contains <math>\Q</math> and is a field of [[characteristic (algebra)|characteristic]] {{math|0}}.

{{anchor|not_orderable}}Because {{math|0}} can be written as sum of squares,<ref group="note">According to [[Hensel's lemma#Examples|Hensel's lemma]] <math>\Q_2</math> contains a square root of {{math|−7}}, so that <math>2^2 +1^2+1^2+1^2+\left(\sqrt{-7}\right)^2 = 0 ,</math> and if {{math|''p'' > 2}} then also by Hensel's lemma <math>\Q_p</math> contains a square root of {{math|1 − ''p''}}, thus
<math>(p-1)\times 1^2 +\left(\sqrt{1-p}\right)^2 = 0 .</math></ref> <math>\Q_p</math>  cannot be turned into an [[Ordered field#Orderability of fields|ordered field]].

The field of [[real numbers]] <math>\R</math> has only a single proper [[algebraic extension]]: the [[complex numbers]] <math>\C</math>.  In other words, this [[quadratic extension]] is already [[algebraically closed field|algebraically closed]]. By contrast, the [[algebraic closure]] of <math>\Q_p</math>, denoted <math>\overline{\Q_p},</math> has infinite degree,<ref>{{Harv|Gouvêa|1997|loc=Corollary 5.3.10}}</ref> that is, <math>\Q_p</math> has infinitely many inequivalent algebraic extensions. Also contrasting the case of real numbers, although there is a unique extension of the {{mvar|p}}-adic valuation to <math>\overline{\Q_p},</math> the latter is not (metrically) complete.<ref>{{Harv|Gouvêa|1997|loc=Theorem 5.7.4}}</ref><ref name=C149>{{Harv|Cassels|1986|p=149}}</ref>

Its (metric) completion is denoted <math>\C_p</math> or <math>\Omega_p</math>,<ref name=C149/><ref name=K13>{{Harv|Koblitz|1980|p=13}}</ref> and sometimes called the '''complex {{mvar|p}}-adic numbers''' by analogy to the complex numbers. Here an end is reached, as <math>\C_p</math> is algebraically closed.<ref name=C149/><ref>{{Harv|Gouvêa|1997|loc=Proposition 5.7.8}}</ref> However unlike <math>\C</math> this field is not [[locally compact]].<ref name=K13/>

<math>\C_p</math> and <math>\C</math> are isomorphic as rings,<ref group="note">Two algebraically closed fields are isomorphic if and only if they have the same characteristic and transcendence degree (see, for example Lang’s ''Algebra'' X §1), and both <math>\C_p</math> and <math>\C</math> have characteristic zero and the cardinality of the continuum.</ref> so we may regard <math>\C_p</math> as <math>\C</math> endowed with an exotic metric. The proof of existence of such a field isomorphism relies on the [[axiom of choice]], and does not provide an explicit example of such an isomorphism (that is, it is not [[constructive proof|constructive]]).

If <math>K</math> is any finite [[Galois extension]] of <math>\Q_p,</math> the [[Galois group]] <math>\operatorname{Gal} \left(K/\Q_p \right)</math> is [[solvable group|solvable]]. Thus, the Galois group <math>{\operatorname{Gal}} \bigl(\,\overline{\Q_p}/ \Q_p \bigr)</math> is [[prosolvable]].

== Multiplicative group ==
<math>\Q_p</math> contains the {{mvar|n}}-th [[cyclotomic field]] ({{math|''n'' > 2}}) if and only if {{math|''n''&thinsp;{{!}} ''p'' − 1}}.<ref>{{Harv|Gouvêa|1997|loc=Proposition 3.4.2}}</ref> For instance, the {{mvar|n}}-th cyclotomic field is a subfield of <math>\Q_{13}</math> if and only if {{math|''n'' {{=}} 1, 2, 3, 4, 6}}, or {{math|12}}. In particular, there is no multiplicative {{mvar|p}}-[[torsion (algebra)|torsion]] in <math>\Q_p</math> if {{math|''p'' > 2}}. Also, {{math|−1}} is the only non-trivial torsion element in <math>\Q_2</math>.

Given a [[natural number]] {{mvar|k}}, the [[index (group theory)|index]] of the multiplicative group of the {{mvar|k}}-th powers of the non-zero elements of <math>\Q_p</math> in <math>\Q_p^\times</math> is finite.

The number {{mvar|[[e (mathematical constant)|e]]}}, defined as the sum of [[reciprocal (mathematics)|reciprocals]] of [[factorial]]s, is not a member of any {{mvar|p}}-adic field; but  <math>e^p \in \Q_p</math> for <math>p \ne 2</math>. For {{math|''p'' {{=}} 2}} one must take at least the fourth power.<ref>{{Harv|Robert|2000|loc=Section 4.1}}</ref> (Thus a number with similar properties as {{mvar|e}} — namely a {{mvar|p}}-th root of {{math|''e<sup>p</sup>''}} — is a member of  <math>\Q_p</math> for all {{mvar|p}}.)

== Local–global principle ==
[[Helmut Hasse]]'s [[Hasse principle|local–global principle]] is said to hold for an equation if it can be solved over the rational numbers [[if and only if]] it can be solved over the real numbers and over the {{mvar|p}}-adic numbers for every prime&nbsp;{{mvar|p}}. This principle holds, for example, for equations given by [[quadratic form]]s, but fails for higher polynomials in several indeterminates.

== Rational arithmetic with Hensel lifting ==
{{main|Hensel lifting}}

== Applications ==
The ''p''-adic numbers have appeared in several fields of mathematics as well as physics.

=== Analysis ===
{{Main|p-adic analysis}}
Similar to the more classical fields of [[real analysis|real]] and [[complex analysis]], which deal, respectively, with functions on the real and complex numbers, '''''p''-adic analysis''' studies functions on ''p''-adic numbers. The theory of complex-valued numerical functions on the ''p''-adic numbers is part of the theory of [[locally compact group]]s ([[abstract harmonic analysis]]). The usual meaning taken for ''p''-adic analysis is the theory of ''p''-adic-valued functions on spaces of interest.

Applications of ''p''-adic analysis have mainly been in number theory, where it has a significant role in [[diophantine geometry]] and [[diophantine approximation]]. Some applications have required the development of ''p''-adic [[functional analysis]] and [[spectral theory]]. In many ways ''p''-adic analysis is less subtle than [[classical analysis]], since the [[ultrametric inequality]] means, for example, that convergence of [[infinite series]] of ''p''-adic numbers is much simpler. [[Topological vector space]]s over ''p''-adic fields show distinctive features; for example aspects relating to [[convex set|convexity]] and the [[Hahn–Banach theorem]] are different.

Two important concepts from ''p''-adic analysis are [[Mahler's theorem]], which characterizes every continuous ''p''-adic function in terms of polynomials, and [[Volkenborn integral]], which provides a method of [[integral|integration]] for ''p''-adic functions.

=== Hodge theory ===
{{Main|p-adic Hodge theory}}
'''''p''-adic Hodge theory''' is a theory that provides a way to classify and study [[Galois representation|''p''-adic Galois representations]] of [[characteristic (algebra)|characteristic 0]] [[local field]]s with residual characteristic ''p'' (such as '''Q'''<sub>''p''</sub>). The theory has its beginnings in [[Jean-Pierre Serre]] and [[John Tate (mathematician)|John Tate]]'s study of [[Tate module]]s of [[abelian variety|abelian varieties]] and the notion of [[Hodge–Tate representation]]. Hodge–Tate representations are related to certain decompositions of ''p''-adic [[cohomology]] theories analogous to the [[Hodge decomposition]], hence the name ''p''-adic Hodge theory. Further developments were inspired by properties of ''p''-adic Galois representations arising from the [[étale cohomology]] of [[Algebraic variety|varieties]]. [[Jean-Marc Fontaine]] introduced many of the basic concepts of the field.

=== Teichmüller theory ===
{{Main|p-adic Teichmüller theory}}
'''''p''-adic Teichmüller theory''' describes the "uniformization" of ''p''-adic curves and their [[Moduli_spaces|moduli]], generalizing the usual [[Teichmüller theory]] that describes the [[Uniformization theorem|uniformization]] of [[Riemann surfaces]] and their moduli. It was introduced and developed by [[Shinichi Mochizuki]].

=== Quantum physics ===
{{Main|p-adic quantum mechanics}}
'''''p''-adic quantum mechanics''' is a collection of related research efforts in [[quantum physics]] that replace real numbers with ''p''-adic numbers. Historically, this research was inspired by the discovery that the [[Veneziano amplitude]] of the open [[bosonic string]], which is calculated using an integral over the real numbers, can be generalized to the ''p''-adic numbers. This observation initiated the study of '''''p''-adic string theory'''.

== Generalizations and related concepts ==
The reals and the {{mvar|p}}-adic numbers are the completions of the rationals; it is also possible to complete other fields, for instance general [[algebraic number field]]s, in an analogous way. This will be described now.

Suppose ''D'' is a [[Dedekind domain]] and ''E'' is its [[field of fractions]]. Pick a non-zero [[prime ideal]] ''P'' of ''D''. If ''x'' is a non-zero element of ''E'', then ''xD'' is a [[fractional ideal]] and can be uniquely factored as a product of positive and negative powers of non-zero prime ideals of ''D''. We write ord<sub>''P''</sub>(''x'') for the exponent of ''P'' in this factorization, and for any choice of number ''c'' greater than 1 we can set
<math display="block">|x|_P = c^{-\!\operatorname{ord}_P(x)}.</math>
Completing with respect to this absolute value {{nowrap begin}}|⋅|<sub>''P''</sub>{{nowrap end}} yields a field ''E''<sub>''P''</sub>, the proper generalization of the field of ''p''-adic numbers to this setting. The choice of ''c'' does not change the completion (different choices yield the same concept of Cauchy sequence, so the same completion). It is convenient, when the [[residue field]] ''D''/''P'' is finite, to take for ''c'' the size of ''D''/''P''.

For example, when ''E'' is a [[number field]], [[Ostrowski's theorem]] says that every non-trivial [[absolute value (algebra)|non-Archimedean absolute value]] on ''E'' arises as some {{nowrap begin}}|⋅|<sub>''P''</sub>{{nowrap end}}. The remaining non-trivial absolute values on ''E'' arise from the different embeddings of ''E'' into the real or complex numbers. (In fact, the non-Archimedean absolute values can be considered as simply the different embeddings of ''E'' into the fields '''C'''<sub>''p''</sub>, thus putting the description of all
the non-trivial absolute values of a number field on a common footing.)

Often, one needs to simultaneously keep track of all the above-mentioned completions when ''E'' is a number field (or more generally a [[global field]]), which are seen as encoding "local" information. This is accomplished by [[adele ring]]s and [[idele group]]s.

''p''-adic integers can be extended to [[Solenoid (mathematics)#p-adic solenoids|''p''-adic solenoids]] <math>\mathbb{T}_p</math>. There is a map from <math>\mathbb{T}_p</math> to the [[circle group]] whose fibers are the ''p''-adic integers <math>\mathbb{Z}_p</math>, in analogy to how there is a map from <math>\mathbb{R}</math> to the circle whose fibers are <math>\mathbb{Z}</math>.

The ''p''-adic integers can also be extended to [[profinite integers]] <math>\widehat{\mathbb{Z}}</math>, which can be understood as the [[direct product of rings]]
<math display="block">\widehat{\mathbb{Z}} = \prod_p \mathbb{Z}_p.</math>
Unlike the ''p''-adic integers which only generalize the modulo over prime powers ''p''<sup>k</sup>, the profinite integers generalizes the modulo over ''all'' natural numbers ''n''.

== See also ==
{{div col}}
* [[Non-Archimedean (disambiguation)|Non-Archimedean]]
* [[p-adically closed field]]
* [[1 + 2 + 4 + 8 + ⋯]]
* [[Bijective numeration|''k''-adic notation]]
* [[C-minimal theory]]
* [[Two's complement]]
{{div col end}}

== Footnotes ==

=== Notes ===
{{reflist|group=note}}

=== Citations ===
{{reflist}}

== References ==
{{refbegin}}
* {{cite journal |last1=Armacost |first1=D. L. |last2=Armacost |first2=W. L. |title=On ''p''-thetic groups |journal=Pacific Journal of Mathematics |date=1972 |volume=41 |issue=2 |pages=295–301 |url=http://projecteuclid.org/euclid.pjm/1102968274}}
* {{Citation |last=Cassels |first=J. W. S. |author-link=J. W. S. Cassels |title=Local Fields |series=London Mathematical Society Student Texts |volume=3 |publisher=[[Cambridge University Press]] |year=1986 |isbn=0-521-31525-5 |zbl=0595.12006}}
* {{cite web |last1=Chen |first1=Evan |title=An Infinitely Large Napkin |url=https://venhance.github.io/napkin/Napkin.pdf |access-date=23 July 2025}}
* {{citation|title=Theory of Algebraic Functions of One Variable|volume=39|series=History of mathematics|first1=Richard|last1=Dedekind|author1-link=Richard Dedekind|first2=Heinrich|last2=Weber|author2-link=Heinrich Martin Weber|publisher=American Mathematical Society|year=2012|isbn=978-0-8218-8330-3}}. &mdash; Translation into English by [[John Stillwell]] of ''Theorie der algebraischen Functionen einer Veränderlichen'' (1882).
* {{Citation|last=Gouvêa|first=F. Q.|date=March 1994|title=A Marvelous Proof|journal=[[American Mathematical Monthly]]|pages=203–222|volume=101|issue=3 |jstor=2975598|doi=10.2307/2975598}}
* {{Citation |last=Gouvêa |first=Fernando Q. |year=1997 |title=''p''-adic Numbers: An Introduction |edition=2nd |publisher=Springer |isbn=3-540-62911-4 | zbl=0874.11002}}
* {{citation|title=Handbook of Algebra|volume=6|editor-first=M.|editor-last=Hazewinkel|publisher=North Holland|date=2009|isbn=978-0-444-53257-2|page=342|url={{Google books|yimXZ-7L9ZoC|page=342|plainurl=yes}}}}
* {{Citation|last1=Hehner|first1=Eric C. R.|author-link1=Eric C. R. Hehner|last2=Horspool|first2=R. Nigel|year=1979|title=A new representation of the rational numbers for fast easy arithmetic|journal=[[SIAM Journal on Computing]]|pages=124–134 |volume=8 |issue=2 |doi=10.1137/0208011 |url=https://www.researchgate.net/publication/220617770|citeseerx=10.1.1.64.7714}}
* {{Citation | last = Hensel | first = Kurt | author-link=Kurt Hensel | title = Über eine neue Begründung der Theorie der algebraischen Zahlen | journal = Jahresbericht der Deutschen Mathematiker-Vereinigung | volume = 6 | year = 1897 | issue = 3 | pages = 83–88 | url = http://www.digizeitschriften.de/resolveppn/GDZPPN00211612X&L=2}}
* {{Citation|last1=Kelley|first1=John L.|author-link=John Leroy Kelley|title=General Topology|date=2008|orig-year=1955|publisher=Ishi Press|location=New York|isbn=978-0-923891-55-8}}
* {{Citation |last=Koblitz |first=Neal |author-link=Neal Koblitz |title=''p''-adic analysis: a short course on recent work |series=London Mathematical Society Lecture Note Series |volume=46 |publisher=[[Cambridge University Press]] |year=1980 |isbn=0-521-28060-5 |zbl=0439.12011}}
* {{cite web |last1=Koç |first1=Ç. K. |title=A Tutorial on {{mvar|p}}-adic Arithmetic |url=https://cetinkayakoc.net/docs/r09.pdf |access-date=23 July 2025 |location=Electrical & Computer Engineering, Oregon State University |date=2002}}
* {{Citation |last=Robert |first=Alain M. |year=2000 |title=A Course in ''p''-adic Analysis |publisher=Springer |isbn=0-387-98669-3}}
{{refend}}

== Further reading ==
{{refbegin}}
* {{Citation |last=Bachman |first=George |title=Introduction to ''p''-adic Numbers and Valuation Theory |year=1964 |publisher=Academic Press |isbn=0-12-070268-1}}
* {{Citation|last1=Borevich|first1=Z. I.|author-link1=Zenon Ivanovich Borevich|last2=Shafarevich|first2=I. R.|author2-link=Igor Rostislavovich Shafarevich|year=1986|title=Number Theory|publisher=Academic Press|location=Boston, MA|series=Pure and Applied Mathematics|volume=20|isbn=978-0-12-117851-2|url={{Google books|njgVUjjO-EAC|Number Theory|plainurl=yes}}|mr=0195803}}
* {{Citation |last=Koblitz |first=Neal |author-link=Neal Koblitz |year=1984 | series=[[Graduate Texts in Mathematics]] | volume=58 | title=''p''-adic Numbers, ''p''-adic Analysis, and Zeta-Functions | edition=2nd |publisher=Springer |isbn=0-387-96017-1}}
* {{Citation | last=Mahler | first=Kurt | author-link=Kurt Mahler | title=''p''-adic numbers and their functions | edition=2nd | zbl=0444.12013 | series=Cambridge Tracts in Mathematics | volume=76 | location=Cambridge | publisher=[[Cambridge University Press]] | year=1981 | isbn=0-521-23102-7 | url-access=registration | url=https://archive.org/details/padicnumbersthei0000mahl }}
* {{Citation |last=Steen |first=Lynn Arthur |author-link=Lynn Arthur Steen |year=1978 |title=Counterexamples in Topology |publisher=Dover |isbn=0-486-68735-X|title-link=Counterexamples in Topology }}
{{refend}}

== External links ==
{{Commons category|P-adic numbers}}
* {{MathWorld|urlname=p-adicNumber|title=p-adic Number}}
* [http://www.encyclopediaofmath.org/index.php/P-adic_number ''p''-adic number] at [[Encyclopaedia of Mathematics|Springer On-line Encyclopaedia of Mathematics]]

{{Number systems}}
{{Authority control}}

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