# Number theory

**Number theory**is a branch of pure mathematics devoted primarily to the study of the integers and integer-valued functions. German mathematician Carl Friedrich Gauss said, "Mathematics is the queen of the sciences—and number theory is the queen of mathematics." Number theorists study prime numbers as well as the properties of objects made out of integers or defined as generalizations of the integers.

Integers can be considered either in themselves or as solutions to equations. Questions in number theory are often best understood through the study of analytical objects that encode properties of the integers, primes or other number-theoretic objects in some fashion. One may also study real numbers in relation to rational numbers, for example, as approximated by the latter.

The older term for number theory is

*arithmetic*. By the early twentieth century, it had been superseded by "number theory". The use of the term

*arithmetic*for

*number theory*regained some ground in the second half of the 20th century, arguably in part due to French influence. In particular,

*arithmetical*is preferred as an adjective to

*number-theoretic*.

## History

### Origins

#### Dawn of arithmetic

The earliest historical find of an arithmetical nature is a fragment of a table: the broken clay tablet Plimpton 322 contains a list of "Pythagorean triples", that is, integers such that.The triples are too many and too large to have been obtained by brute force. The heading over the first column reads: "The

*takiltum*of the diagonal which has been subtracted such that the width..."

The table's layout suggests that it was constructed by means of what amounts, in modern language, to the identity

which is implicit in routine Old Babylonian exercises. If some other method was used, the triples were first constructed and then reordered by, presumably for actual use as a "table", for example, with a view to applications.

It is not known what these applications may have been, or whether there could have been any; Babylonian astronomy, for example, truly came into its own only later. It has been suggested instead that the table was a source of numerical examples for school problems.

While Babylonian number theory—or what survives of Babylonian mathematics that can be called thus—consists of this single, striking fragment, Babylonian algebra was exceptionally well developed. Late Neoplatonic sources state that Pythagoras learned mathematics from the Babylonians. Much earlier sources state that Thales and Pythagoras traveled and studied in Egypt.

Euclid IX 21–34 is very probably Pythagorean; it is very simple material, but it is all that is needed to prove that square root of 2|

is irrational. Pythagorean mystics gave great importance to the odd and the even.

The discovery that is irrational is credited to the early Pythagoreans. By revealing that numbers could be irrational, this discovery seems to have provoked the first foundational crisis in mathematical history; its proof or its divulgation are sometimes credited to Hippasus, who was expelled or split from the Pythagorean sect. This forced a distinction between

*numbers*, on the one hand, and

*lengths*and

*proportions*, on the other hand.

The Pythagorean tradition spoke also of so-called polygonal or figurate numbers. While square numbers, cubic numbers, etc., are seen now as more natural than triangular numbers, pentagonal numbers, etc., the study of the sums

of triangular and pentagonal numbers would prove fruitful in the early modern period.

We know of no clearly arithmetical material in ancient Egyptian or Vedic sources, though there is some algebra in both. The Chinese remainder theorem appears as an exercise in

*Sunzi Suanjing*

There is also some numerical mysticism in Chinese mathematics, but, unlike that of the Pythagoreans, it seems to have led nowhere. Like the Pythagoreans' perfect numbers, magic squares have passed from superstition into recreation.

#### Classical Greece and the early Hellenistic period

Aside from a few fragments, the mathematics of Classical Greece is known to us either through the reports of contemporary non-mathematicians or through mathematical works from the early Hellenistic period. In the case of number theory, this means, by and large,*Plato*and

*Euclid*, respectively.

While Asian mathematics influenced Greek and Hellenistic learning, it seems to be the case that Greek mathematics is also an indigenous tradition.

Eusebius, PE X, chapter 4 mentions of Pythagoras:

"In fact the said Pythagoras, while busily studying the wisdom of each nation, visited Babylon, and Egypt, and all Persia, being instructed by the Magi and the priests: and in addition to these he is related to have studied under the Brahmans ; and from some he gathered astrology, from others geometry, and arithmetic and music from others, and different things from different nations, and only from the wise men of Greece did he get nothing, wedded as they were to a poverty and dearth of wisdom: so on the contrary he himself became the author of instruction to the Greeks in the learning which he had procured from abroad."

Aristotle claimed that the philosophy of Plato closely followed the teachings of the Pythagoreans, and Cicero repeats this claim:

*Platonem ferunt didicisse Pythagorea omnia*.

Plato had a keen interest in mathematics, and distinguished clearly between arithmetic and calculation. It is through one of Plato's dialogues—namely,

*Theaetetus*—that we know that Theodorus had proven that are irrational. Theaetetus was, like Plato, a disciple of Theodorus's; he worked on distinguishing different kinds of incommensurables, and was thus arguably a pioneer in the study of number systems.

Euclid devoted part of his

*Elements*to prime numbers and divisibility, topics that belong unambiguously to number theory and are basic to it. In particular, he gave an algorithm for computing the greatest common divisor of two numbers and the first known proof of the infinitude of primes.

In 1773, Lessing published an epigram he had found in a manuscript during his work as a librarian; it claimed to be a letter sent by Archimedes to Eratosthenes. The epigram proposed what has become known as

Archimedes's cattle problem; its solution requires solving an indeterminate quadratic equation. As far as we know, such equations were first successfully treated by the [|Indian school]. It is not known whether Archimedes himself had a method of solution.

#### Diophantus

Very little is known about Diophantus of Alexandria; he probably lived in the third century CE, that is, about five hundred years after Euclid. Six out of the thirteen books of Diophantus's*Arithmetica*survive in the original Greek; four more books survive in an Arabic translation. The

*Arithmetica*is a collection of worked-out problems where the task is invariably to find rational solutions to a system of polynomial equations, usually of the form or. Thus, nowadays, we speak of

*Diophantine equations*when we speak of polynomial equations to which rational or integer solutions must be found.

One may say that Diophantus was studying rational points, that is, points whose coordinates are rational—on curves and algebraic varieties; however, unlike the Greeks of the Classical period, who did what we would now call basic algebra in geometrical terms, Diophantus did what we would now call basic algebraic geometry in purely algebraic terms. In modern language, what Diophantus did was to find rational parametrizations of varieties; that is, given an equation of the form

, his aim was to find three rational functions such that, for all values of and, setting

for gives a solution to

Diophantus also studied the equations of some non-rational curves, for which no rational parametrisation is possible. He managed to find some rational points on these curves by means of what amounts to a tangent construction: translated into coordinate geometry

, his method would be visualised as drawing a tangent to a curve at a known rational point, and then finding the other point of intersection of the tangent with the curve; that other point is a new rational point.

While Diophantus was concerned largely with rational solutions, he assumed some results on integer numbers, in particular that every integer is the sum of four squares.

#### Āryabhaṭa, Brahmagupta, Bhāskara

While Greek astronomy probably influenced Indian learning, to the point of introducing trigonometry, it seems to be the case that Indian mathematics is otherwise an indigenous tradition; in particular, there is no evidence that Euclid's Elements reached India before the 18th century.Āryabhaṭa showed that pairs of simultaneous congruences, could be solved by a method he called

*kuṭṭaka*, or

*pulveriser*; this is a procedure close to the Euclidean algorithm, which was probably discovered independently in India. Āryabhaṭa seems to have had in mind applications to astronomical calculations.

Brahmagupta started the systematic study of indefinite quadratic equations—in particular, the Pell equation, in which Archimedes may have first been interested, and which did not start to be solved in the West until the time of Fermat and Euler. Later Sanskrit authors would follow, using Brahmagupta's technical terminology. A general procedure for solving Pell's equation was finally found by Jayadeva ; the earliest surviving exposition appears in Bhāskara II's Bīja-gaṇita.

Indian mathematics remained largely unknown in Europe until the late eighteenth century; Brahmagupta and Bhāskara's work was translated into English in 1817 by Henry Colebrooke.

#### Arithmetic in the Islamic golden age

In the early ninth century, the caliph Al-Ma'mun ordered translations of many Greek mathematical works and at least one Sanskrit work.Diophantus's main work, the

*Arithmetica*, was translated into Arabic by Qusta ibn Luqa.

Part of the treatise

*al-Fakhri*builds on it to some extent. According to Rashed Roshdi, Al-Karajī's contemporary Ibn al-Haytham knew what would later be called Wilson's theorem.

#### Western Europe in the Middle Ages

Other than a treatise on squares in arithmetic progression by Fibonacci—who traveled and studied in north Africa and Constantinople—no number theory to speak of was done in western Europe during the Middle Ages. Matters started to change in Europe in the late Renaissance, thanks to a renewed study of the works of Greek antiquity. A catalyst was the textual emendation and translation into Latin of Diophantus'*Arithmetica*.

### Early modern number theory

#### Fermat

never published his writings; in particular, his work on number theory is contained almost entirely in letters to mathematicians and in private marginal notes. In his notes and letters, he scarcely wrote any proofs - he had no models in the area.Over his lifetime, Fermat made the following contributions to the field:

- One of Fermat's first interests was perfect numbers and amicable numbers; these topics led him to work on integer divisors, which were from the beginning among the subjects of the correspondence that put him in touch with the mathematical community of the day.
- In 1638, Fermat claimed, without proof, that all whole numbers can be expressed as the sum of four squares or fewer.
- Fermat's little theorem : if
*a*is not divisible by a prime*p*, then - If
*a*and*b*are coprime, then is not divisible by any prime congruent to −1 modulo 4; and every prime congruent to 1 modulo 4 can be written in the form. These two statements also date from 1640; in 1659, Fermat stated to Huygens that he had proven the latter statement by the method of infinite descent. - In 1657, Fermat posed the problem of solving as a challenge to English mathematicians. The problem was solved in a few months by Wallis and Brouncker. Fermat considered their solution valid, but pointed out they had provided an algorithm without a proof. He stated that a proof could be found by infinite descent.
- Fermat stated and proved in the appendix to
*Observations on Diophantus*that has no non-trivial solutions in the integers. Fermat also mentioned to his correspondents that has no non-trivial solutions, and that this could also be proven by infinite descent. The first known proof is due to Euler. - Fermat claimed to have shown there are no solutions to for all ; this claim appears in his annotations in the margins of his copy of Diophantus.
#### Euler

*Proofs for Fermat's statements.*This includes Fermat's little theorem ; the fact that if and only if ; initial work towards a proof that every integer is the sum of four squares ; the lack of non-zero integer solutions to .*Pell's equation*, first misnamed by Euler. He wrote on the link between continued fractions and Pell's equation.*First steps towards analytic number theory.*In his work of sums of four squares, partitions, pentagonal numbers, and the distribution of prime numbers, Euler pioneered the use of what can be seen as analysis in number theory. Since he lived before the development of complex analysis, most of his work is restricted to the formal manipulation of power series. He did, however, do some very notable early work on what would later be called the Riemann zeta function.*Quadratic forms*. Following Fermat's lead, Euler did further research on the question of which primes can be expressed in the form, some of it prefiguring quadratic reciprocity.*Diophantine equations*. Euler worked on some Diophantine equations of genus 0 and 1. In particular, he studied Diophantus's work; he tried to systematise it, but the time was not yet ripe for such an endeavour—algebraic geometry was still in its infancy. He did notice there was a connection between Diophantine problems and elliptic integrals, whose study he had himself initiated.#### Lagrange, Legendre, and Gauss

Adrien-Marie Legendre was the first to state the law of quadratic reciprocity. He also

conjectured what amounts to the prime number theorem and Dirichlet's theorem on arithmetic progressions. He gave a full treatment of the equation and worked on quadratic forms along the lines later developed fully by Gauss. In his old age, he was the first to prove "Fermat's last theorem" for .

In his

*Disquisitiones Arithmeticae*, Carl Friedrich Gauss proved the law of quadratic reciprocity and developed the theory of quadratic forms. He also introduced some basic notation and devoted a section to computational matters, including primality tests. The last section of the

*Disquisitiones*established a link between roots of unity and number theory:

The theory of the division of the circle...which is treated in sec. 7 does not belong

by itself to arithmetic, but its principles can only be drawn from higher arithmetic.

In this way, Gauss arguably made a first foray towards both Évariste Galois's work and algebraic number theory.

### Maturity and division into subfields

Starting early in the nineteenth century, the following developments gradually took place:- The rise to self-consciousness of number theory as a field of study.
- The development of much of modern mathematics necessary for basic modern number theory: complex analysis, group theory, Galois theory—accompanied by greater rigor in analysis and abstraction in algebra.
- The rough subdivision of number theory into its modern subfields—in particular, analytic and algebraic number theory.

goes back to Euler, who used formal power series and non-rigorous limiting arguments. The use of

*complex*analysis in number theory comes later: the work of Bernhard Riemann on the zeta function is the canonical starting point; Jacobi's four-square theorem, which predates it, belongs to an initially different strand that has by now taken a leading role in analytic number theory.

The history of each subfield is briefly addressed in its own section below; see the main article of each subfield for fuller treatments. Many of the most interesting questions in each area remain open and are being actively worked on.

## Main subdivisions

### Elementary tools

The term*elementary*generally denotes a method that does not use complex analysis. For example, the prime number theorem was first proven using complex analysis in 1896, but an elementary proof was found only in 1949 by Erdős and Selberg. The term is somewhat ambiguous: for example, proofs based on complex Tauberian theorems are often seen as quite enlightening but not elementary, in spite of using Fourier analysis, rather than complex analysis as such. Here as elsewhere, an

*elementary*proof may be longer and more difficult for most readers than a non-elementary one.

and Terence Tao in 1985, when Erdős was 72 and Tao was 10.

Number theory has the reputation of being a field many of whose results can be stated to the layperson. At the same time, the proofs of these results are not particularly accessible, in part because the range of tools they use is, if anything, unusually broad within mathematics.

### Analytic number theory

*Analytic number theory*may be defined

- in terms of its tools, as the study of the integers by means of tools from real and complex analysis; or
- in terms of its concerns, as the study within number theory of estimates on size and density, as opposed to identities.

The following are examples of problems in analytic number theory: the prime number theorem, the Goldbach conjecture, the Waring problem and the Riemann hypothesis. Some of the most important tools of analytic number theory are the circle method, sieve methods and L-functions. The theory of modular forms also occupies an increasingly central place in the toolbox of analytic number theory.

One may ask analytic questions about algebraic numbers, and use analytic means to answer such questions; it is thus that algebraic and analytic number theory intersect. For example, one may define prime ideals and ask how many prime ideals there are up to a certain size. This question can be answered by means of an examination of Dedekind zeta functions, which are generalizations of the Riemann zeta function, a key analytic object at the roots of the subject. This is an example of a general procedure in analytic number theory: deriving information about the distribution of a sequence from the analytic behavior of an appropriately constructed complex-valued function.

### Algebraic number theory

An*algebraic number*is any complex number that is a solution to some polynomial equation with rational coefficients; for example, every solution of is an algebraic number. Fields of algebraic numbers are also called

*algebraic number fields*, or shortly

*number fields*. Algebraic number theory studies algebraic number fields. Thus, analytic and algebraic number theory can and do overlap: the former is defined by its methods, the latter by its objects of study.

It could be argued that the simplest kind of number fields were already studied by Gauss, as the discussion of quadratic forms in

*Disquisitiones arithmeticae*can be restated in terms of ideals and

norms in quadratic fields.

For that matter, the 11th-century chakravala method amounts—in modern terms—to an algorithm for finding the units of a real quadratic number field. However, neither Bhāskara nor Gauss knew of number fields as such.

The grounds of the subject as we know it were set in the late nineteenth century, when

*ideal numbers*, the

*theory of ideals*and

*valuation theory*were developed; these are three complementary ways of dealing with the lack of unique factorisation in algebraic number fields. The initial impetus for the development of ideal numbers seems to have come from the study of higher reciprocity laws, that is, generalisations of quadratic reciprocity.

Number fields are often studied as extensions of smaller number fields: a field

*L*is said to be an

*extension*of a field

*K*if

*L*contains

*K*.

Classifying the possible extensions of a given number field is a difficult and partially open problem. Abelian extensions—that is, extensions

*L*of

*K*such that the Galois group Gal of

*L*over

*K*is an abelian group—are relatively well understood.

Their classification was the object of the programme of class field theory, which was initiated in the late 19th century and carried out largely in 1900–1950.

An example of an active area of research in algebraic number theory is Iwasawa theory. The Langlands program, one of the main current large-scale research plans in mathematics, is sometimes described as an attempt to generalise class field theory to non-abelian extensions of number fields.

### Diophantine geometry

The central problem of*Diophantine geometry*is to determine when a Diophantine equation has solutions, and if it does, how many. The approach taken is to think of the solutions of an equation as a geometric object.

For example, an equation in two variables defines a curve in the plane. More generally, an equation, or system of equations, in two or more variables defines a curve, a surface or some other such object in

*n*-dimensional space. In Diophantine geometry, one asks whether there are any

*rational points*or

*integral points*on the curve or surface. If there are any such points, the next step is to ask how many there are and how they are distributed. A basic question in this direction is if there are finitely

or infinitely many rational points on a given curve.

In the Pythagorean equation

we would like to study its rational solutions, that is, its solutions

such that

*x*and

*y*are both rational. This is the same as asking for all integer solutions

to ; any solution to the latter equation gives

us a solution, to the former. It is also the

same as asking for all points with rational coordinates on the curve

described by.

, that is, a curve

of genus 1 having at least one rational point.

The rephrasing of questions on equations in terms of points on curves turns out to be felicitous. The finiteness or not of the number of rational or integer points on an algebraic curve—that is, rational or integer solutions to an equation, where is a polynomial in two variables—turns out to depend crucially on the

*genus*of the curve. The

*genus*can be defined as follows: allow the variables in to be complex numbers; then defines a 2-dimensional surface in 4-dimensional space. If we count the number of holes in the surface; we call this number the

*genus*of. Other geometrical notions turn out to be just as crucial.

There is also the closely linked area of Diophantine approximations: given a number, then finding how well can it be approximated by rationals. This question is of special interest if is an algebraic number. If cannot be well approximated, then some equations do not have integer or rational solutions. Moreover, several concepts turn out to be critical both in Diophantine geometry and in the study of Diophantine approximations. This question is also of special interest in transcendental number theory: if a number can be better approximated than any algebraic number, then it is a transcendental number. It is by this argument that Pi| and e have been shown to be transcendental.

Diophantine geometry should not be confused with the geometry of numbers, which is a collection of graphical methods for answering certain questions in algebraic number theory.

*Arithmetic geometry*, however, is a contemporary term

for much the same domain as that covered by the term

*Diophantine geometry*. The term

*arithmetic geometry*is arguably used

most often when one wishes to emphasise the connections to modern algebraic geometry rather than to techniques in Diophantine approximations.

## Other subfields

The areas below date from no earlier than the mid-twentieth century, even if they are based on older material. For example, as is explained below, the matter of algorithms in number theory is very old, in some sense older than the concept of proof; at the same time, the modern study of computability dates only from the 1930s and 1940s, and computational complexity theory from the 1970s.### Probabilistic number theory

Much of probabilistic number theory can be seen as an important special case of the study of variables that are almost, but not quite, mutually independent. For example, the event that a random integer between one and a million be divisible by two and the event that it be divisible by three are almost independent, but not quite.It is sometimes said that probabilistic combinatorics uses the fact that whatever happens with probability greater than must happen sometimes; one may say with equal justice that many applications of probabilistic number theory hinge on the fact that whatever is unusual must be rare. If certain algebraic objects can be shown to be in the tail of certain sensibly defined distributions, it follows that there must be few of them; this is a very concrete non-probabilistic statement following from a probabilistic one.

At times, a non-rigorous, probabilistic approach leads to a number of heuristic algorithms and open problems, notably Cramér's conjecture.

### Arithmetic combinatorics

If we begin from a fairly "thick" infinite set, does it contain many elements in arithmetic progression:,, say? Should it be possible to write large integers as sums of elements of ?

These questions are characteristic of

*arithmetic combinatorics*. This is a presently coalescing field; it subsumes

*additive number theory*and, arguably, some of the

*geometry of numbers*,

together with some rapidly developing new material. Its focus on issues of growth and distribution accounts in part for its developing links with ergodic theory, finite group theory, model theory, and other fields. The term

*additive combinatorics*is also used; however, the sets being studied need not be sets of integers, but rather subsets of non-commutative groups, for which the multiplication symbol, not the addition symbol, is traditionally used; they can also be subsets of rings, in which case the growth of and · may be

compared.

### Computational number theory

While the word*algorithm*goes back only to certain readers of al-Khwārizmī, careful descriptions of methods of solution are older than proofs: such methods are as old as any recognisable mathematics—ancient Egyptian, Babylonian, Vedic, Chinese—whereas proofs appeared only with the Greeks of the classical period.

An interesting early case is that of what we now call the Euclidean algorithm. In its basic form it appears as Proposition 2 of Book VII in

*Elements*, together with a proof of correctness. However, in the form that is often used in number theory it first appears in the works of Āryabhaṭa as an algorithm called

*kuṭṭaka*, without a proof of correctness.

There are two main questions: "Can we compute this?" and "Can we compute it rapidly?" Anyone can test whether a number is prime or, if it is not, split it into prime factors; doing so rapidly is another matter. We now know fast algorithms for testing primality, but, in spite of much work, no truly fast algorithm for factoring.

The difficulty of a computation can be useful: modern protocols for encrypting messages depend on functions that are known to all, but whose inverses are known only to a chosen few, and would take one too long a time to figure out on one's own. For example, these functions can be such that their inverses can be computed only if certain large integers are factorized. While many difficult computational problems outside number theory are known, most working encryption protocols nowadays are based on the difficulty of a few number-theoretical problems.

Some things may not be computable at all; in fact, this can be proven in some instances. For instance, in 1970, it was proven, as a solution to Hilbert's 10th problem, that there is no Turing machine which can solve all Diophantine equations. In particular, this means that, given a computably enumerable set of axioms, there are Diophantine equations for which there is no proof, starting from the axioms, of whether the set of equations has or does not have integer solutions.

## Applications

The number-theorist Leonard Dickson said "Thank God that number theory is unsullied by any application". Such a view is no longer applicable to number theory. In 1974, Donald Knuth said "...virtually every theorem in elementary number theory arises in a natural, motivated way in connection with the problem of making computers do high-speed numerical calculations".Elementary number theory is taught in discrete mathematics courses for computer scientists; on the other hand, number theory also has applications to the continuous in numerical analysis. As well as the well-known applications to cryptography, there are also applications to many other areas of mathematics.

## Prizes

The American Mathematical Society awards the*Cole Prize in Number Theory*. Moreover number theory is one of the three mathematical subdisciplines rewarded by the

*Fermat Prize*.