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p-adic number

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*p*-adic number}}Image:3-adic integers with dual colorings.svg|thumb|The 3-adic integers, with selected corresponding characters on their Pontryagin dualPontryagin dualIn mathematics, the

**{{mvar|p}}-adic number system**for any prime number {{mvar|p}} extends the ordinary arithmetic of the rational numbers in a different way from the extension of the rational number system to the real and complex number systems. The extension is achieved by an alternative interpretation of the concept of "closeness" or absolute value. In particular, {{mvar|p}}-adic numbers are considered to be close when their difference is divisible by a high power of {{mvar|p}}: the higher the power, the closer they are. This property enables {{mvar|p}}-adic numbers to encode congruence information in a way that turns out to have powerful applications in number theory â€“ including, for example, in the famous proof of Fermat's Last Theorem by Andrew Wiles.{{Harv|GouvÃªa|1994|pp=203â€“222}}{{mvar|p}}-adic numbers were first described by Kurt Hensel in 1897,{{Harv|Hensel|1897}} though, with hindsight, some of Ernst Kummer's earlier work can be interpreted as implicitly using {{mvar|p}}-adic numbers.Translator's introduction, 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}} The {{mvar|p}}-adic numbers were motivated primarily by an attempt to bring the ideas and techniques of power series methods into number theory. Their influence now extends far beyond this. For example, the field of {{mvar|p}}-adic analysis essentially provides an alternative form of calculus.More formally, for a given prime {{mvar|p}}, the field {{math|

**Q**

*p**}} of {{mvar|p}}-adic numbers is a completion of the rational numbers. The field {{math|**Q**p*}} is also given a topology derived from a metric, which is itself derived from the {{math|

*p*}}-adic order, an alternative valuation on the rational numbers. This metric space is complete in the sense that every Cauchy sequence converges to a point in {{math|

**Q**

*p**}}. This is what allows the development of calculus on {{math|**Q**p*}}, and it is the interaction of this analytic and algebraic structure that gives the {{mvar|p}}-adic number systems their power and utility.The {{mvar|p}} in "{{mvar|p}}-adic" is a variable and may be replaced with a prime (yielding, for instance, "the 2-adic numbers") or another

*placeholder variable*(for expressions such as "the â„“-adic numbers"). The "adic" of "{{mvar|p}}-adic" comes from the ending found in words such as dyadic or triadic.

## Introduction

This section is an informal introduction to {{math|*p*}}-adic numbers, using examples from the ring of 10-adic (decadic) numbers. Although for {{math|

*p*}}-adic numbers {{math|

*p*}} should be a prime, base 10 was chosen to highlight the analogy with decimals. The decadic numbers are generally not used in mathematics: since 10 is not prime or prime power, the decadics are not a field. More formal constructions and properties are given below.In the standard decimal representation, almost allThe number of real numbers with terminating decimal representations is countably infinite, while the number of real numbers without such a representation is uncountably infinite. real numbers do not have a terminating decimal representation. For example, 1/3 is represented as a non-terminating decimal as follows

frac{1}{3}=0.333333ldots.

Informally, non-terminating decimals are easily understood, because it is clear that a real number can be approximated to any required degree of precision by a terminating decimal. If two decimal expansions differ only after the 10th decimal place, they are quite close to one another; and if they differ only after the 20th decimal place, they are even closer.10-adic numbers use a similar non-terminating expansion, but with a different concept of "closeness". Whereas two **decimal**expansions are close to one another if their difference is a large

**negative**power of 10, two

**10-adic**expansions are close if their difference is a large

**positive**power of 10. Thus 4739 and 5739, which differ by 103, are close in the 10-adic world, and 72694473 and 82694473 are even closer, differing by 107.More precisely, a positive rational number {{mvar|r}} can be uniquely expressed as {{math|

*r*=: {{sfrac|

*p*|

*q*}}Â·10

*d*}}, where {{mvar|p}}, {{mvar|q}} and 10 are positive integers and are all relatively prime with respect to each other. Let the {{nowrap|10-adic}} "absolute value"The so defined function is not really an absolute value, because the requirement of multiplicativity is violated: |2|_{10} = |2 cdot 10^0|_{10} = frac1{10^0} = |5cdot 10^0|_{10} = |5|_{10} , but |2 cdot 5|_{10} = |10^1|_{10} = frac1{10^1} ne frac1{10^0} = |2|_{10} cdot |5|_{10} . It suffices, however, for establishing a metric, because this does not need multiplicativity. of 10^d be

|10^d|_{10} := frac {1} {10^d} .

Additionally, we define
|0|_{10} := 0 .

Now, taking {{math|*p*/

*q*= 1}} and {{math|

*d*= 0,1,2,...}} we have

{{math||100|10 = 100}}, {{math||101|10 = 10âˆ’1}}, {{math||102|10 = 10âˆ’2, ...}},

with the consequence that we have
lim_{d rightarrow +infty} |10^d|_{10} = 0 .

Closeness in any number system is defined by a metric. Using the 10-adic metric the distance between numbers {{mvar|x}} and {{mvar|y}} is given by {{math||{{mvar|x}}â€‰âˆ’â€‰{{mvar|y}}|10}}. An interesting consequence of the 10-adic metric (or of a {{mvar|p}}-adic metric) is that there is no longer a need for the negative sign. (In fact, there is no order relation which is compatible with the ring operations and this metric) As an example, by examining the following sequence we can see how unsigned 10-adics can get progressively closer and closer to the number âˆ’1:
9=-1+10 so |9-(-1)|_{10} = frac {1} {10}.
99=-1+10^2 so |99-(-1)|_{10} = frac {1} {100}.
999=-1+10^3 so |999-(-1)|_{10} = frac {1} {1000}.
9999=-1+10^4 so |9999-(-1)|_{10} = frac {1} {10000}.

and taking this sequence to its limit, we can deduce the 10-adic expansion of âˆ’1
|dots 9999 -(-1)|_{10} = 0 ,

thus
dots 9999 =-1 ,

an expansion which clearly is a ten's complement representation.In this notation, 10-adic expansions can be extended indefinitely to the left, in contrast to decimal expansions, which can be extended indefinitely to the right. Note that this is not the only way to write {{mvar|p}}-adic numbers â€“ for alternatives see the *Notation*section below.More formally, a 10-adic number can be defined as

sum_{i=n}^infty a_i 10^i

where each of the {{math|*a*

*i*}} is a digit taken from the set {0,â€‰1,â€‰â€¦â€‰,â€‰9} and the initial index {{mvar|n}} may be positive, negative or 0, but must be finite. From this definition, it is clear that positive integers and positive rational numbers with terminating decimal expansions will have terminating 10-adic expansions that are identical to their decimal expansions. Other numbers may have non-terminating 10-adic expansions.It is possible to define addition, subtraction, and multiplication on 10-adic numbers in a consistent way, so that the 10-adic numbers form a commutative ring.We can create 10-adic expansions for "negative numbers"More precisely: additively inverted numbers, because there are no numbers less than 0. as follows

-100 = -1 times 100 = dots 9999 times 100 = dots 9900
Rightarrow -35 = -100+65 = dots 9900 + 65 = dots 9965
Rightarrow -left(3+dfrac{1}{2}right)=dfrac{-35}{10}= dfrac{dots 9965}{10}=dots 9996.5

and fractions which have non-terminating decimal expansions also have non-terminating 10-adic expansions. For example
dfrac{10^6-1}{7}=142857; qquad

dfrac{10^{12}-1}{7}=142857142857; qquaddfrac{10^{18}-1}{7}=142857142857142857
Rightarrow-dfrac{1}{7}=dots 142857142857142857
Rightarrow-dfrac{6}{7}=dots 142857142857142857 times 6 = dots 857142857142857142
Rightarrowdfrac{1}{7} = -dfrac{6}{7}+1 = dots 857142857142857143.

Generalizing the last example, we can find a 10-adic expansion with no digits to the right of the decimal point for any rational number {{math|*p*/

*q*}} such that {{math|q}} is co-prime to 10; Euler's theorem guarantees that if {{mvar|q}} is co-prime to 10, then there is an {{mvar|n}} such that {{math|10

*n*âˆ’ 1}} is a multiple of {{math|q}}. The other rational numbers can be expressed as 10-adic numbers with some digits after the decimal point.As noted above, 10-adic numbers have a major drawback. {{anchor|zero_div}}It is possible to find pairs of non-zero 10-adic numbers (which are not rational, thus having an infinite number of digits) whose product is 0.See GÃ©rard Michon's article atFor iinN_0 let x_i := 5^{2^i} and y_i := 6^{5^i}. Then, for {{math|

*i*â‰¥ 1}} there is xi_i with x_i = x_{i-1}+10^icdotxi_i, so that the sequence left{x_iright}_{iinN} is Cauchy under the metric with lim_{+inftyleftarrow i} x_i ne 0. Same for y_i = y_{i-1}+10^icdoteta_i. However, lim_{+inftyleftarrow e} x_ecdot y_e = 0 . This means that 10-adic numbers do not always have multiplicative inverses i.e. valid reciprocals, which in turn implies that though 10-adic numbers form a ring they do not form a field, a deficiency that makes them much less useful as an analytical tool. Another way of saying this is that the ring of 10-adic numbers is not an integral domain because they contain zero divisors. The reason for this property turns out to be that 10 is a composite number which is not a power of a prime. This problem is simply avoided by using a prime number {{mvar|p}} or a prime power {{mvar|pn}} as the base of the number system instead of 10 and indeed for this reason {{mvar|p}} in {{mvar|p}}-adic is usually taken to be prime.{|class="wikitable"|fraction|original decimal notation|10-adic notation|fraction|original decimal notation|10-adic notation|fraction|original decimal notation|10-adic notation|frac{1}{2}|0.5|0.5|frac{5}{7}

714285}} | 428571}}5|frac{9}{10}|0.9|0.9 |

3}} | 6}}7|frac{6}{7} | 857142}} | 714285}}8|frac{1}{11} | 09}} | 09}}1 |

6}} | 3}}4|frac{1}{8}|0.125|0.125|frac{2}{11} | 18}} | 18}}2 |

27}} | 27}}3 |

36}} | 36}}4 |

45}} | 45}}5 |

1}} | 8}}9|frac{6}{11} | 54}} | 54}}6 |

2}} | 7}}8|frac{7}{11} | 63}} | 63}}7 |

4}} | 5}}6|frac{8}{11} | 72}} | 72}}8 |

6}} | 3}}.5|frac{5}{9} | 5}} | 4}}5|frac{9}{11} | 81}} | 81}}9 |

3}} | 6}}7.5|frac{7}{9} | 7}} | 2}}3|frac{10}{11} | 90}} | 09}}10 |

142857}} | 285714}}3|frac{8}{9} | 8}} | 1}}2|frac{1}{12} | 3}} | 6}}.75 |

285714}} | 571428}}6|frac{1}{10}|0.1|0.1|frac{5}{12} | 6}} | 3}}.75 |

428571}} | 857142}}9|frac{3}{10}|0.3|0.3|frac{7}{12} | 3}} | 6}}7.25 |

571428}} | 142857}}2|frac{7}{10}|0.7|0.7|frac{11}{12} | 6}} | 3}}4.25 |

## *p*-adic expansions

{{needs more references|date=February 2019}}When dealing with natural numbers, if {{mvar|p}} is taken to be a fixed prime number, then any positive integer can be written as a base {{mvar|p}} expansion in the form
sum_{i=0}^n a_i p^i

where the *a*

*i*are integers in {0,â€‰â€¦â€‰,â€‰{{math|

*p*â€‰âˆ’â€‰1}}}.{{Harv|Kelley|2008|pp=22â€“25}} For example, the binary expansion of 35 is 1Â·25 + 0Â·24 + 0Â·23 + 0Â·22 + 1Â·21 + 1Â·20, often written in the shorthand notation 1000112.The familiar approach to extending this description to the larger domain of the rationalsWEB, Bogomolny, Alexander, Alexander Bogomolny, p-adic Expansions,weblink WEB, KoÃ§, Ã‡etin, A Tutorial on p-adic Arithmeticformat=PDF, (and, ultimately, to the reals) is to use sums of the form:

pmsum_{i=-infty}^n a_i p^i.

A definite meaning is given to these sums based on Cauchy sequences, using the absolute value as metric. Thus, for example, 1/3 can be expressed in base 5 as the limit of the sequence 0.1313131313...5. In this formulation, the integers are precisely those numbers for which *a*

*i*= 0 for all

*i*< 0.With

*p*-adic numbers, on the other hand, we choose to extend the base {{mvar|p}} expansions in a different way. Unlike traditional integers, where the magnitude is determined by how far they are from zero, the "size" of {{mvar|p}}-adic numbers is determined by the {{mvar|p}}-adic absolute value, where high positive powers of {{mvar|p}} are relatively small compared to high negative powers of {{mvar|p}}. Consider infinite sums of the form:

sum_{i=k}^{infty} a_i p^i

where *k*is some (not necessarily positive) integer, and each coefficient a_i can be called a

**{{mvar|p}}-adic digit**.WEB, Madore, David, A first introduction to p-adic numbers,weblink PDF, With this approach we obtain the

**{{mvar|p}}-adic expansions**of the {{mvar|p}}-adic numbers. Those {{mvar|p}}-adic numbers for which

*a*

**'i****= 0 for all**{{mvar|p}}-adic integers'''.As opposed to real number expansions which extend to the

*i*< 0 are also called the*right*as sums of ever smaller, increasingly negative powers of the base {{mvar|p}}, {{mvar|p}}-adic numbers may expand to the

*left*forever, a property that can often be true for the {{mvar|p}}-adic integers. For example, consider the {{mvar|p}}-adic expansion of 1/3 in base 5. It can be shown to be â€¦1313132{{sub|5}}, i.e., the limit of the sequence 2{{sub|5}}, 32{{sub|5}}, 132{{sub|5}}, 3132{{sub|5}}, 13132{{sub|5}}, 313132{{sub|5}}, 1313132{{sub|5}}, â€¦ :

dfrac{5^2-1}{3}=dfrac{44_5}{3} = 13_5; ,

dfrac{5^4-1}{3}=dfrac{4444_5}{3} = 1313_5
Rightarrow-dfrac{1}{3}=dots 1313_5
Rightarrow-dfrac{2}{3}=dots 1313_5 times 2 = dots 3131_5
Rightarrowdfrac{1}{3} = -dfrac{2}{3}+1 = dots 3132_5.

Multiplying this infinite sum by 3 in base 5 gives â€¦0000001{{sub|5}}. As there are no negative powers of 5 in this expansion of 1/3 (i.e. no numbers to the right of the decimal point), we see that 1/3 satisfies the definition of being a {{mvar|p}}-adic integer in base 5.More formally, the {{mvar|p}}-adic expansions can be used to define the field {{math|**Q**

*p**}} of**{{mvar|p}}-adic numbers***while the {{mvar|p}}-adic integers form a subring of {{math|**Q*p*}}, denoted {{math|

**Z**

*p**}}. (Not to be confused with the ring of integers modulo {{mvar|p}} which is also sometimes written {{math|**Z**p*}}. To avoid ambiguity, {{math|

**Z**/

*p*

**Z**}} or {{math|

**Z**/(

*p*)}} are often used to represent the integers modulo {{mvar|p}}.)While it is possible to use the approach above to define {{mvar|p}}-adic numbers and explore their properties, just as in the case of real numbers other approaches are generally preferred. Hence we want to define a notion of infinite sum which makes these expressions meaningful, and this is most easily accomplished by the introduction of the {{mvar|p}}-adic metric. Two different but equivalent solutions to this problem are presented in the

*Constructions*section below.

## 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 powers of {{mvar|p}} increase from right to left. With this right-to-left notation the 3-adic expansion of {{frac|1|5}}, for example, is written as
dfrac{1}{5}=dots 121012102_3.

When performing arithmetic in this notation, digits are 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 {{frac|1|5}} is
dfrac{1}{5}=2.01210121dots_3mbox{ or }dfrac{1}{15}=20.1210121dots_3.

{{mvar|p}}-adic expansions may be written with other sets of digits instead of {0,â€‰1,â€‰â€¦,â€‰{{math|*p*â€‰âˆ’â€‰1}}}. For example, the 3-adic expansion of 1/5 can be written using balanced ternary digits {1,0,1} as

dfrac{1}{5}=dotsunderline{1}11underline{11}11underline{11}11underline{1}_3.

In fact any set of {{mvar|p}} integers which are in distinct residue classes modulo {{mvar|p}} may be used as {{mvar|p}}-adic digits. In number theory, 1â€¦111| bgcolor=#AAFF00 rowspan=4 |

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|-

| bgcolor=#BBFFFF |

| colspan=9 style="font-size:small" width=32em |2-adic ({{math|1=â€‰{{mvar|p}} = 2â€‰}}) arrangement of integers, from left to right. This shows a hierarchical subdivision pattern common for ultrametric spaces. Points within a distance 1/8 are grouped in one colored strip. A pair of strips within a distance 1/4 has the same chroma, four strips within a distance 1/2 have the same hue. The hue is determined by the least significant bit, the saturation â€“ by the next (21) bit, and the brightness depends on the value of 22 bit. Bits (digit places) which are less significant for the usual metric are more significant for the {{mvar|p}}-adic distance.

|}

*p-adic absolute value*in {{math|

**Q**}} as follows:for any non-zero rational number {{mvar|x}}, there is a unique integer {{mvar|n}} allowing us to write {{math|1=

*x*=

*p*

*n*(

*a*/

*b*)}}, where neither of the integers

*a*and

*b*is divisible by {{mvar|p}}. Unless the numerator or denominator of {{mvar|x}} in lowest terms contains {{mvar|p}} as a factor, {{mvar|n}} will be 0. Now define {{math|1=|

*x*|

*p*=

*p*âˆ’

*n*}}. We also define {{math|1=|0|

*p*= 0}}.For example with {{math|1=

*x*= 63/550 = 2âˆ’1Â·32Â·5âˆ’2Â·7Â·11âˆ’1}}

*x*|

*p*}} has the effect that high powers of {{mvar|p}} become "small".By the fundamental theorem of arithmetic, for a given non-zero rational number

*x*there is a unique finite set of distinct primes p_1, ldots, p_r and a corresponding sequence of non-zero integers a_1, ldots, a_r such that:

|x| = p_1^{a_1}ldots p_r^{a_r}.

It then follows that |x|_{p_i} = p_i^{-a_i} for all 1leq ileq r , and |x|_p = 1 for any other prime p notin {p_1,ldots, p_r}.The {{mvar|p}}-adic absolute value defines a metric d*p*on {{math|

**Q**}} by setting

d_p(x,y)=|x-y|_p

The field {{math|**Q**

*p**}} of {{mvar|p}}-adic numbers can then be defined as the completion of the metric space ({{math|**Q***}},â€‰d****p***); its elements are equivalence classes of Cauchy sequences, where two sequences are called equivalent if their difference converges to zero. In this way, we obtain a complete metric space which is also a field and contains {{math|**Q***}}. With this absolute value, the field {{math|**Q*p*}} is a local field.It can be shown that in {{math|

**Q**

*p*}}, every element

*x*may be written in a unique way as

sum_{i=k}^{infty} a_i p^i

where *k*is some integer such that

*a*

**'k****â‰**

*0*and each*a**'i*is in {0,â€‰â€¦,â€‰{{math|

*p*â€‰âˆ’â€‰1}}â€‰}. This series converges to

*x*with respect to the metric d

*p*. The {{mvar|p}}-adic integers {{math|

**Z**

*p**}} are the elements where*k*is non-negative. Consequently, {{math|**Q**p*}} is isomorphic to {{math|

**Z**[1/p] +

**Z**

*p*}}.BOOK, Automorphic Forms and Representations, Bump, Daniel, Daniel Bump, Cambridge University Press, 1998, Cambridge Studies in Advanced Mathematics, 55, 9780521658188, 277, Ostrowski's theorem states that each absolute value on {{math|

**Q**}} is equivalent either to the Euclidean absolute value, the trivial absolute value, or to one of the {{mvar|p}}-adic absolute values for some prime {{mvar|p}}. Each absolute value (or metric) leads to a different completion of {{math|

**Q**}}. (With the trivial absolute value, {{math|

**Q**}} is already complete.)

### Algebraic approach

In the algebraic approach, we first define the ring of {{mvar|p}}-adic integers, and then construct the field of fractions of this ring to get the field of {{mvar|p}}-adic numbers.We start with the inverse limit of the rings{{math|**Z**/

*p*Z

**'n***'}} (see modular arithmetic): a {{mvar|p}}-adic integer*m'' is then a sequence(

*an*)

*n*â‰¥1 such that

*a*

*n*is in{{math|

**Z**/

*p*Z

**'n***'}}, and if*n

*â‰¤*m'', then

*an*â‰¡

*a*

**'m****(mod**

*p**'n*).Every natural number

*m*defines such a sequence (

*a*

**'n****) by**

*a**'n*=

*m*mod

*pn*and can therefore be regarded as a {{mvar|p}}-adic integer. For example, in this case 35 as a 2-adic integer would be written as the sequence (1, 3, 3, 3, 3, 35, 35, 35, â€¦).The operators of the ring amount to pointwise addition and multiplication of such sequences. This is well defined because addition and multiplication commute with the "mod" operator; see modular arithmetic.Moreover, every sequence (

*a*

*n*) where the first element is not 0 has an inverse. In that case, for every

*n*,

*an*and

*p*are coprime, and so

*an*and

*pn*are relatively prime. Therefore, each

*an*has an inverse mod

*pn*, and the sequence of these inverses, (

*bn*), is the sought inverse of (

*an*). For example, consider the {{mvar|p}}-adic integer corresponding to the natural number 7; as a 2-adic number, it would be written (1, 3, 7, 7, 7, 7, 7, ...). This object's inverse would be written as an ever-increasing sequence that begins (1, 3, 7, 7, 23, 55, 55, 183, 439, 439, 1463 ...). Naturally, this 2-adic integer has no corresponding natural number.Every such sequence can alternatively be written as a series. For instance, in the 3-adics, the sequence (2, 8, 8, 35, 35, ...) can be written as {{math|2 + 2Â·3 + 0Â·32 + 1Â·33 + 0Â·34 + ...}} The partial sums of this latter series are the elements of the given sequence.The ring of {{mvar|p}}-adic integers has no zero divisors, so we can take the field of fractions to get the field {{math|

**Q**

*p*}} of {{mvar|p}}-adic numbers. Note that in this field of fractions, every non-integer {{mvar|p}}-adic number can be uniquely written as {{math|

*p*âˆ’

*n*

*â€‰u*}} with a natural number

*n*and a unit {{math|u}} in the {{mvar|p}}-adic integers. This means that

mathbf{Q}_p=operatorname{Quot}left(mathbf{Z}_pright)cong (p^{mathbf{N}})^{-1}mathbf{Z}^times_p.

Note that {{math|*S*âˆ’1â€Š

*A*}}, where S=p^{mathbf{N}}={p^{n}:ninmathbf{N}} is a multiplicative subset (contains the unit and closed under multiplication) of a commutative ring (with unit) A, is an algebraic construction called the

**ring of fractions**or localization of A by S.

## Properties

### Cardinality

{{math|**Z**Z

*p**}} is the inverse limit of the finite rings {{math|**}} is uncountable. The endomorphism ring of the PrÃ¼fer {{mvar|p}}-group of rank {{mvar|n}}, denoted {{math|**Z***/****p**'k*p***Z**}}, which is uncountable{{Harv|Robert|2000|loc=Chapter 1 Section 1.1}}—in fact, has the cardinality of the continuum. Accordingly, the field {{math|**Q****(**Z'p''}}; this is sometimes referred to as the Tate module.The number of {{mvar|p}}-adic numbers with terminating {{mvar|p}}-adic representations is countably infinite. And, if the standard digits {0,ldots, p-1} are taken, their value and representation coincides in {{math|

*p*âˆž)*n*}}, is the ring of {{math|*n*Ã—*n*}} matrices over {{math|**Z**

*p*}} and {{math|

**R**}}.

### Topology

thumb|right|400px|A scheme showing the topology of the dyadic (or indeed*p*-adic) integers. Each clump is an open set made up of other clumps. The numbers in the left-most quarter (containing 1) are all the odd numbers. The next group to the right is the even numbers not divisible by 4.Define a topology on {{math|

**Z**

*p*}} by taking as a basis of open sets all sets of the form

U_a(n) = left { n+ lambda p^a : lambda in mathbf{Z}_p right }.

where *a*is a non-negative integer and

*n*is an integer in [1,

*pa*]. For example, in the dyadic integers,

*U*1(1) is the set of odd numbers.

*Ua*(

*n*) is the set of all

*p*-adic integers whose difference from

*n*has

*p*-adic absolute value less than

*p*1âˆ’

*a*. Then {{math|

**Z**

*p**}} is a compactification of {{math|**Z***}}, under the derived topology (it is****not***a compactification of {{math|**Z***}} with its usual discrete topology). The relative topology on {{math|**Z**}} as a subset of {{math|**Z*p*}} is called the {{mvar|p}}-adic topology on {{math|

**Z**}}.The topology of {{math|

**Z**

*p*}} is that of a Cantor set mathcal{C}.{{Harv|Robert|2000|loc=Chapter 1 Section 2.3}} For instance, we can make a continuous 1-to-1 mapping between the dyadic integers and the Cantor set expressed in base 3 by

mathbf{Z}_2 ni cdots d_2 d_1 d_0 longmapsto 0.e_0 e_1 e_2 cdots_3 in mathcal{C},

where e_n = 2 d_n. The topology of {{math|**Q**

*p**}} is that of a Cantor set minus any point.{{Citation needed|reason=Reliable source needed for the whole sentence|date=September 2017}} In particular, {{math|**Z**p*}} is compact while {{math|

**Q**

*p**}} is not; it is only locally compact. As metric spaces, both {{math|**Z**p*}} and {{math|

**Q**

*p*}} are complete.{{Harv|GouvÃªa|1997|loc=Corollary 3.3.8}}

### Metric completions and algebraic closures

{{math|**Q**

*p*}} contains {{math|

**Q**}} and is a field of characteristic {{math|0}}. {{anchor|not_orderable}}This field cannot be turned into an ordered field.{{math|

**R**}} has only a single proper algebraic extension: {{math|

**C**}}; in other words, this quadratic extension is already algebraically closed. By contrast, the algebraic closure of {{math|

**Q**

*p**}}, denoted overline{mathbf{Q}_p}, has infinite degree,{{Harv|GouvÃªa|1997|loc=Corollary 5.3.10}} i.e. {{math|**Q**p*}} 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 overline{mathbf{Q}_p}, the latter is not (metrically) complete.{{Harv|GouvÃªa|1997|loc=Theorem 5.7.4}}{{Harv|Cassels|1986|p=149}} Its (metric) completion is called {{math|

**C**

*p**}} or {{math|Î©*p*}}.{{Harv|Koblitz|1980|p=13}} Here an end is reached, as {{math|**C**p*}} is algebraically closed.{{Harv|GouvÃªa|1997|loc=Proposition 5.7.8}} However unlike {{math|

**C**}} this field is not locally compact.{{math|

**C**

*p**}} and {{math|**C***}} are isomorphic as rings, so we may regard {{math|**C*p*}} as {{math|

**C**}} 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 (i.e., it is not constructive).If {{math|

**K**}} is a finite Galois extension of {{math|

**Q**

*p*}}, the Galois group text{Gal} left (mathbf{K}/ mathbf{Q}_p right) is solvable. Thus, the Galois group text{Gal} left (overline{mathbf{Q}_p}/ mathbf{Q}_p right) is prosolvable.

### Multiplicative group of {{math|Q*p*}}

{{math|**Q**

*p**}} contains the {{mvar|n}}-th cyclotomic field ({{math|*n*> 2}}) if and only if {{math|*n*{{!}}*p*âˆ’ 1}}.{{Harv|GouvÃªa|1997|loc=Proposition 3.4.2}} For instance, the {{mvar|n}}-th cyclotomic field is a subfield of {{math|**Q***13}} if and only if {{math|****n***{{=}} 1, 2, 3, 4, 6}}, or {{math|12}}. In particular, there is no multiplicative {{mvar|p}}-torsion in {{math|**Q**p*}}, if {{math|

*p*> 2}}. Also, {{math|âˆ’1}} is the only non-trivial torsion element in {{math|

**Q**2}}.Given a natural number {{mvar|k}}, the index of the multiplicative group of the {{mvar|k}}-th powers of the non-zero elements of {{math|

**Q**

*p*}} in mathbf{Q}_p^{times} is finite.The number {{mvar|e}}, defined as the sum of reciprocals of factorials, is not a member of any {{mvar|p}}-adic field; but {{math|

*e p*âˆˆ

**Q**

*p*(

*p*â‰ 2)}}. For {{math|

*p*{{=}} 2}} one must take at least the fourth power.{{Harv|Robert|2000|loc=Section 4.1}} (Thus a number with similar properties as {{mvar|e}} â€” namely a {{mvar|p}}-th root of {{math|

*e p*}} â€” is a member of overline{mathbf{Q}_p} for all {{mvar|p}}.)

## Rational arithmetic

Eric Hehner and Nigel Horspool proposed in 1979 the use of a {{mvar|p}}-adic representation for rational numbers on computers{{Harv|Hehner|Horspool|1979|pp=124â€“134}} called Quote notation. The primary advantage of such a representation is that addition, subtraction, and multiplication can be done in a straightforward manner analogous to similar methods for binary integers; and division is even simpler, resembling multiplication. However, it has the disadvantage that representations can be much larger than simply storing the numerator and denominator in binary (for more details see Quote notation#Length of representation).## 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 fields, 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

*P*(

*x*) for the exponent of

*P*in this factorization, and for any choice of number

*c*greater than 1 we can set

|x|_P = c^{-operatorname{ord}_P(x)}.

Completing with respect to this absolute value |.|*P*yields a field

*E*

*P*, 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 non-Archimedean absolute value on

*E*arises as some |.|

*P*. 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**

*p*, thus putting the description of allthe 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 rings and idele groups.

## Localâ€“global principle

Helmut Hasse's localâ€“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 {{mvar|p}}. This principle holds e.g. for equations given by quadratic forms, but fails for higher polynomials in several indeterminates.## See also

{{Div col}}- 1 + 2 + 4 + 8 + ...
- C-minimal theory
- Hensel's lemma
*k*-adic notation- Mahler's theorem
*p*-adic quantum mechanics- Profinite integer
- Volkenborn integral

## Footnotes

### Notes

{{Reflist|group=note}}### Citations

{{Reflist}}## References

- {{Citation |last=Cassels |first=J. W. S. |authorlink=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}}
- {{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}}. — Translation into English by John Stillwell of
*Theorie der algebraischen Functionen einer VerÃ¤nderlichen*(1882). - {{Citation|last=GouvÃªa|first=F. Q.|author-link=F. Q. GouvÃªa|date=March 1994|title=A Marvelous Proof|journal=American Mathematical Monthly|pages=203â€“222|volume=101|issue=3 |jstor=2975598|doi=10.2307/2975598}}
- {{Citation |last=GouvÃªa |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|last=Hehner|first=Eric C. R.|author-link=Eric C. R. Hehner|last2=Horspool|first2=R. Nigel|author2-link=R. Nigel Horspool|year=1979|title=A new representation of the rational numbers for fast easy arithmetic|journal=SIAM Journal on Computing|pages=124â€“134 |volume=8 |issue=2 |doi=10.1137/0208011 |url=https://www.researchgate.net/profile/Eric_Hehner/publication/220617770_A_New_Representation_of_the_Rational_Numbers_for_Fast_Easy_Arithmetic/links/0912f5058d7a114e7f000000.pdf}}
- {{Citation | last = Hensel | first = Kurt | author-link=Kurt Hensel | title = Ãœber eine neue BegrÃ¼ndung der Theorie der algebraischen Zahlen | journal = Jahresbericht der Deutschen Mathematiker-Vereinigung | volume = 6 | year = 1897 | issue = 3 | pages = 83â€“88 | url =weblink}}
- {{Citation|last1=Kelley|first1=John L.|author-link=John Leroy Kelley|title=General Topology|date=2008|origyear=1955|publisher=Ishi Press|location=New York|isbn=978-0-923891-55-8}}
- {{Citation |last=Koblitz |first=Neal |authorlink=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}} - {{Citation |last=Robert |first=Alain M. |year=2000 |title=A Course in
*p*-adic Analysis |publisher=Springer |isbn=0-387-98669-3}}

## Further reading

- {{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|last=Borevich|first=Z. I.|author-link=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 |authorlink=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 | authorlink=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 }} - {{Citation |last=Steen |first=Lynn Arthur |authorlink=Lynn Arthur Steen |year=1978 |title=Counterexamples in Topology |publisher=Dover |isbn=0-486-68735-X}}

## External links

- {{MathWorld|urlname=p-adicNumber|title=p-adic Number}}
- {{planetmath reference|id=3118|title=p-adic integers}}
*p*-adic number at Springer On-line Encyclopaedia of Mathematics- Completion of Algebraic Closure â€“ on-line lecture notes by Brian Conrad
- An Introduction to
*p*-adic Numbers and*p*-adic Analysis - on-line lecture notes by Andrew Baker, 2007 - Efficient p-adic arithmetic (slides)
- Introduction to p-adic numbers

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