In mathematics, the padic number system for any prime number 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, two padic numbers are considered to be close when their difference is divisible by a high power of p: the higher the power, the closer they are. This property enables padic numbers to encode congruence information in a way that turns out to have powerful applications in number theory  including, for example, in the famous proof of Fermat's Last Theorem by Andrew Wiles.^{[1]}
These numbers were first described by Kurt Hensel in 1897,^{[2]} though, with hindsight, some of Ernst Kummer's earlier work can be interpreted as implicitly using padic numbers.^{[note 1]} The padic 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 padic analysis essentially provides an alternative form of calculus.
More formally, for a given prime p, the field Q_{p} of padic numbers is a completion of the rational numbers. The field Q_{p} is also given a topology derived from a metric, which is itself derived from the padic 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 Q_{p}. This is what allows the development of calculus on Q_{p}, and it is the interaction of this analytic and algebraic structure that gives the padic number systems their power and utility.
The p in "padic" is a variable and may be replaced with a prime (yielding, for instance, "the 2adic numbers") or another placeholder variable (for expressions such as "the ladic numbers"). The "adic" of "padic" comes from the ending found in words such as dyadic or triadic.
This section is an informal introduction to padic numbers, using examples from the ring of 10adic (decadic) numbers. Although for padic numbers 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 all^{[note 2]}real numbers do not have a terminating decimal representation. For example, 1/3 is represented as a nonterminating decimal as follows
Informally, nonterminating 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.
10adic numbers use a similar nonterminating 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 10adic expansions are close if their difference is a large positive power of 10. Thus 4739 and 5739, which differ by 10^{3}, are close in the 10adic world, and 72694473 and 82694473 are even closer, differing by 10^{7}.
More precisely, every positive rational number r can be uniquely expressed as r =: ·10^{d}, where a and b are positive integers and gcd(a,b)=1, gcd(b,10)=1, gcd(a,10)<10. Let the "absolute value"^{[note 3]} of r be
Additionally, we define
Now, taking a/b = 1 and d = 0,1,2,... we have
with the consequence that we have
Closeness in any number system is defined by a metric. Using the 10adic metric the distance between numbers x and y is given by x  y_{10}. An interesting consequence of the 10adic metric (or of a padic 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 10adics can get progressively closer and closer to the number 1:
and taking this sequence to its limit, we can deduce the 10adic expansion of 1
thus
an expansion which clearly is a ten's complement representation.
In this notation, 10adic 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 padic numbers  for alternatives see the Notation section below.
More formally, a 10adic number can be defined as
where each of the a_{i} is a digit taken from the set {0, 1, ... , 9} and the initial index 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 10adic expansions that are identical to their decimal expansions. Other numbers may have nonterminating 10adic expansions.
It is possible to define addition, subtraction, and multiplication on 10adic numbers in a consistent way, so that the 10adic numbers form a commutative ring.
We can create 10adic expansions for "negative" numbers^{[note 4]} as follows
and fractions which have nonterminating decimal expansions also have nonterminating 10adic expansions. For example
Generalizing the last example, we can find a 10adic expansion with no digits to the right of the decimal point for any rational number a/b such that b is coprime to 10; Euler's theorem guarantees that if b is coprime to 10, then there is an n such that 10^{n}  1 is a multiple of b. The other rational numbers can be expressed as 10adic numbers with some digits after the decimal point.
As noted above, 10adic numbers have a major drawback. It is possible to find pairs of nonzero 10adic numbers (which are not rational, thus having an infinite number of digits) whose product is 0.^{[3]}^{[note 5]} This means that 10adic numbers do not always have multiplicative inverses, that is, valid reciprocals, which in turn implies that though 10adic 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 10adic numbers is not an integral domain because they contain zero divisors.^{[note 5]} 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 p or a prime power p^{n} as the base of the number system instead of 10 and indeed for this reason p in padic is usually taken to be prime.
fraction  original decimal notation  10adic notation  fraction  original decimal notation  10adic notation  fraction  original decimal notation  10adic notation 
0.5  0.5  0.714285  4285715  0.9  0.9  
0.3  67  0.857142  7142858  0.09  091  
0.6  34  0.125  0.125  0.18  182  
0.25  0.25  0.375  0.375  0.27  273  
0.75  0.75  0.625  0.625  0.36  364  
0.2  0.2  0.875  0.875  0.45  455  
0.4  0.4  0.1  89  0.54  546  
0.6  0.6  0.2  78  0.63  637  
0.8  0.8  0.4  56  0.72  728  
0.16  3.5  0.5  45  0.81  819  
0.83  67.5  0.7  23  0.90  0910  
0.142857  2857143  0.8  12  0.083  6.75  
0.285714  5714286  0.1  0.1  0.416  3.75  
0.428571  8571429  0.3  0.3  0.583  67.25  
0.571428  1428572  0.7  0.7  0.916  34.25 
When dealing with natural numbers, if p is taken to be a fixed prime number, then any positive integer can be written as a base p expansion in the form
where the a_{i} are integers in {0, ... , p  1}.^{[4]} For example, the binary expansion of 35 is 1·2^{5} + 0·2^{4} + 0·2^{3} + 0·2^{2} + 1·2^{1} + 1·2^{0}, often written in the shorthand notation 100011_{2}.
The familiar approach to extending this description to the larger domain of the rationals^{[5]}^{[6]} (and, ultimately, to the reals) is to use sums of the form:
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 padic numbers, on the other hand, we choose to extend the base p expansions in a different way. Unlike traditional integers, where the magnitude is determined by how far they are from zero, the "size" of padic numbers is determined by the padic absolute value, where high positive powers of p are relatively small compared to high negative powers of p.
Consider infinite sums of the form:
where k is some (not necessarily positive) integer, and each coefficient is an integer such that 0 a_{i} < p, which can be called a padic digit.^{[7]} This defines the padic expansions of the padic numbers. Those padic numbers for which a_{i} = 0 for all i < 0 are also called the padic integers, and form a subset of the padic numbers commonly denoted
As opposed to real number expansions which extend to the right as sums of ever smaller, increasingly negative powers of the base p, padic numbers may expand to the left forever, a property that can often be true for the padic integers. For example, consider the padic expansion of 1/3 in base 5. It can be shown to be ...1313132_{5}, that is, the limit of the sequence 2_{5}, 32_{5}, 132_{5}, 3132_{5}, 13132_{5}, 313132_{5}, 1313132_{5}, ... :
Multiplying this infinite sum by 3 in base 5 gives ...0000001_{5}. As there are no negative powers of 5 in this expansion of 1/3 (that is, no numbers to the right of the decimal point), we see that 1/3 satisfies the definition of being a padic integer in base 5.
More formally, the padic expansions can be used to define the field Q_{p} of padic numbers while the padic integers form a subring of Q_{p}, denoted Z_{p}. (Not to be confused with the ring of integers modulo p which is also sometimes written Z_{p}. To avoid ambiguity, Z/pZ or Z/(p) are often used to represent the integers modulo p.)
While it is possible to use the approach above to define padic 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 padic metric. Two different but equivalent solutions to this problem are presented in the Constructions section below.
There are several different conventions for writing padic expansions. So far this article has used a notation for padic expansions in which powers of p increase from right to left. With this righttoleft notation the 3adic expansion of , for example, is written as
When performing arithmetic in this notation, digits are carried to the left. It is also possible to write padic expansions so that the powers of p increase from left to right, and digits are carried to the right. With this lefttoright notation the 3adic expansion of is
padic expansions may be written with other sets of digits instead of {0, 1, ..., p  1}. For example, the 3adic expansion of ^{1}/_{5} can be written using balanced ternary digits {1,0,1} as
In fact any set of p integers which are in distinct residue classes modulo p may be used as padic digits. In number theory, Teichmüller representatives are sometimes used as digits.^{[8]}
p = 2  
^{1}⁄_{2} >  ^{1}⁄_{2} >  
^{1}⁄_{4} >  ^{1}⁄_{4} >  ^{1}⁄_{4} >  ^{1}⁄_{4} >  
^{1}⁄_{8}>  ^{1}⁄_{8}>  ^{1}⁄_{8}>  ^{1}⁄_{8}>  ^{1}⁄_{8}>  ^{1}⁄_{8}>  ^{1}⁄_{8}>  ^{1}⁄_{8}>  
................................................  
17  10001  J  
16  10000  J  
15  1111  L  
14  1110  L  
13  1101  L  
12  1100  L  
11  1011  L  
10  1010  L  
9  1001  L  
8  1000  L  
7  111  L  
6  110  L  
5  101  L  
4  100  L  
3  11  L  
2  10  L  
1  1  L  
0  0...000  L  
1  1...111  J  
2  1...110  J  
3  1...101  J  
4  1...100  J  
Dec  Bin  ················································  


2adic ( p = 2 ) 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  by the next (2^{1}) bit, and the brightness depends on the value of 2^{2} bit. Bits (digit places) which are less significant for the usual metric are more significant for the padic distance. 
The real numbers can be defined as equivalence classes of Cauchy sequences of rational numbers; this allows us to, for example, write 1 as 1.000... = 0.999... . The definition of a Cauchy sequence relies on the metric chosen, though, so if we choose a different one, we can construct numbers other than the real numbers. The usual metric which yields the real numbers is called the Euclidean metric.
For a given prime p, we define the padic absolute value in Q as follows: for any nonzero rational number x, there is a unique integer n allowing us to write x = p^{n}(a/b), where neither of the integers a and b is divisible by p. Unless the numerator or denominator of x in lowest terms contains p as a factor, n will be 0. Now define x_{p} = p^{n}. We also define 0_{p} = 0.
For example with x = 63/550 = 2^{1}·3^{2}·5^{2}·7·11^{1}
This definition of x_{p} has the effect that high powers of p become "small". By the fundamental theorem of arithmetic, for a given nonzero rational number x there is a unique finite set of distinct primes and a corresponding sequence of nonzero integers such that:
It then follows that for all , and for any other prime
The padic absolute value defines a metric d_{p} on Q by setting
The field Q_{p} of padic numbers can then be defined as the completion of the metric space (Q, 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 Q. With this absolute value, the field Q_{p} is a local field.
It can be shown that in Q_{p}, every element x may be written in a unique way as
where k is some integer such that a_{k} ? 0 and each a_{i} is in {0, ..., p  1 }. This series converges to x with respect to the metric d_{p}. The padic integers Z_{p} are the elements where k is nonnegative. Consequently, Q_{p} is isomorphic to Z[1/p] + Z_{p}.^{[9]}
Ostrowski's theorem states that each absolute value on Q is equivalent either to the Euclidean absolute value, the trivial absolute value, or to one of the padic absolute values for some prime p. Each absolute value (or metric) leads to a different completion of Q. (With the trivial absolute value, Q is already complete.)
In the algebraic approach, we first define the ring of padic integers, and then construct the field of fractions of this ring to get the field of padic numbers.
We start with the inverse limit of the rings Z/p^{n}Z (see modular arithmetic): a padic integer m is then a sequence (a_{n})_{n>=1} such that a_{n} is in Z/p^{n}Z, and if n l, then a_{n} ? a_{l} (mod p^{n}).
Every natural number m defines such a sequence (a_{n}) by a_{n} ? m (mod p^{n}) and can therefore be regarded as a padic integer. For example, in this case 35 as a 2adic integer would be written as the sequence (1, 3, 3, 3, 3, 35, 35, 35, ...).
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})_{n>=1} with the first element a_{1} ? 0 (mod p) has a multiplicative inverse. In that case, for every n, a_{n} and p are coprime, and so a_{n} and p^{n} are relatively prime. Therefore, each a_{n} has an inverse mod p^{n}, and the sequence of these inverses, (b_{n}), is the sought inverse of (a_{n}). For example, consider the padic integer corresponding to the natural number 7; as a 2adic number, it would be written (1, 3, 7, 7, 7, 7, 7, ...). This object's inverse would be written as an everincreasing sequence that begins (1, 3, 7, 7, 23, 55, 55, 183, 439, 439, 1463 ...). Naturally, this 2adic integer has no corresponding natural number.
Every such sequence can alternatively be written as a series. For instance, in the 3adics, the sequence (2, 8, 8, 35, 35, ...) can be written as 2 + 2·3 + 0·3^{2} + 1·3^{3} + 0·3^{4} + ... The partial sums of this latter series are the elements of the given sequence.
The ring of padic integers has no zero divisors, so we can take the field of fractions to get the field Q_{p} of padic numbers. Note that in this field of fractions, every noninteger padic number can be uniquely written as p^{n} u with a natural number n and a unit u in the padic integers. This means that
Note that S^{1}A, where is a multiplicative subset (contains the unit and closed under multiplication) of a commutative ring (with unit) , is an algebraic construction called the ring of fractions or localization of by .
Z_{p} is the inverse limit of the finite rings Z/p^{k}Z, which is uncountable^{[10]}—in fact, has the cardinality of the continuum. Accordingly, the field Q_{p} is uncountable. The endomorphism ring of the Prüfer pgroup of rank n, denoted Z(p^{?})^{n}, is the ring of n × n matrices over Z_{p}; this is sometimes referred to as the Tate module.
The number of padic numbers with terminating padic representations is countably infinite. And, if the standard digits are taken, their value and representation coincides in Z_{p} and R.
Define a topology on Z_{p} by taking as a basis of open sets all sets of the form
where a is a nonnegative integer and n is an integer in [1, p^{a}]. For example, in the dyadic integers, U_{1}(1) is the set of odd numbers. U_{a}(n) is the set of all padic integers whose difference from n has padic absolute value less than p^{1a}. Then Z_{p} is a compactification of Z, under the derived topology (it is not a compactification of Z with its usual discrete topology). The relative topology on Z as a subset of Z_{p} is called the padic topology on Z.
The topology of Z_{p} is that of a Cantor set .^{[11]} For instance, we can make a continuous 1to1 mapping between the dyadic integers and the Cantor set expressed in base 3 by
where
The topology of Q_{p} is that of a Cantor set minus any point.^{[]} In particular, Z_{p} is compact while Q_{p} is not; it is only locally compact. As metric spaces, both Z_{p} and Q_{p} are complete.^{[12]}
Q_{p} contains Q and is a field of characteristic 0. This field cannot be turned into an ordered field.
R has only a single proper algebraic extension: C; in other words, this quadratic extension is already algebraically closed. By contrast, the algebraic closure of Q_{p}, denoted has infinite degree,^{[13]} that is, Q_{p} has infinitely many inequivalent algebraic extensions. Also contrasting the case of real numbers, although there is a unique extension of the padic valuation to the latter is not (metrically) complete.^{[14]}^{[15]} Its (metric) completion is called C_{p} or ?_{p}.^{[15]}^{[16]} Here an end is reached, as C_{p} is algebraically closed.^{[15]}^{[17]} However unlike C this field is not locally compact.^{[16]}
C_{p} and C are isomorphic as rings, so we may regard C_{p} as 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 (that is, it is not constructive).
If K is a finite Galois extension of Q_{p}, the Galois group is solvable. Thus, the Galois group is prosolvable.
Q_{p} contains the nth cyclotomic field (n > 2) if and only if n  p  1.^{[18]} For instance, the nth cyclotomic field is a subfield of Q_{13} if and only if n = 1, 2, 3, 4, 6, or 12. In particular, there is no multiplicative ptorsion in Q_{p}, if p > 2. Also, 1 is the only nontrivial torsion element in Q_{2}.
Given a natural number k, the index of the multiplicative group of the kth powers of the nonzero elements of Q_{p} in is finite.
The number e, defined as the sum of reciprocals of factorials, is not a member of any padic field; but e^{p} ? Q_{p} (p ? 2). For p = 2 one must take at least the fourth power.^{[19]} (Thus a number with similar properties as e  namely a pth root of e^{p}  is a member of for all p.)
Eric Hehner and Nigel Horspool proposed in 1979 the use of a padic representation for rational numbers on computers^{[20]} 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 § Space).
The reals and the padic 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 nonzero prime ideal P of D. If x is a nonzero element of E, then xD is a fractional ideal and can be uniquely factored as a product of positive and negative powers of nonzero 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
Completing with respect to this absolute value ._{P} yields a field E_{P}, the proper generalization of the field of padic 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 nontrivial nonArchimedean absolute value on E arises as some ._{P}. The remaining nontrivial absolute values on E arise from the different embeddings of E into the real or complex numbers. (In fact, the nonArchimedean absolute values can be considered as simply the different embeddings of E into the fields C_{p}, thus putting the description of all the nontrivial absolute values of a number field on a common footing.)
Often, one needs to simultaneously keep track of all the abovementioned 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.
padic integers can be extended to padic solenoids in the same way that integers can be extended to the real numbers, as the direct product of the circle ring and the padic integers
Helmut Hasse's localglobal 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 padic numbers for every prime p. This principle holds, for example, for equations given by quadratic forms, but fails for higher polynomials in several indeterminates.