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ARTICLE ORIGINS Number theory
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{{distinguish|Numerology}}
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- Lehmer sieve.jpg -
A Lehmer sieve, which is a primitive digital computer once used for finding primes and solving simple Diophantine equations.
Number theory (or arithmetic or higher arithmetic in older usage) is a branch of pure mathematics devoted primarily to the study of the integers. German mathematician Carl Friedrich Gauss (1777â€“1855) said, "Mathematics is the queen of the sciencesâ€”and number theory is the queen of mathematics."{{sfn|Long|1972|p=1}} Number theorists study prime numbers as well as the properties of objects made out of integers (for example, rational numbers) or defined as generalizations of the integers (for example, algebraic integers).Integers can be considered either in themselves or as solutions to equations (Diophantine geometry). Questions in number theory are often best understood through the study of analytical objects (for example, the Riemann zeta function) that encode properties of the integers, primes or other number-theoretic objects in some fashion (analytic number theory). One may also study real numbers in relation to rational numbers, for example, as approximated by the latter (Diophantine approximation).The older term for number theory is arithmetic. By the early twentieth century, it had been superseded by "number theory".Already in 1921, T. L. Heath had to explain: "By arithmetic, Plato meant, not arithmetic in our sense, but the science which considers numbers in themselves, in other words, what we mean by the Theory of Numbers." {{harv|Heath|1921|p=13}} (The word "arithmetic" is used by the general public to mean "elementary calculations"; it has also acquired other meanings in mathematical logic, as in Peano arithmetic, and computer science, as in floating point arithmetic.) 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.Take, for example, {{harvnb|Serre|1973}}. In 1952, Davenport still had to specify that he meant The Higher Arithmetic. Hardy and Wright wrote in the introduction to An Introduction to the Theory of Numbers (1938): "We proposed at one time to change [the title] to An introduction to arithmetic, a more novel and in some ways a more appropriate title; but it was pointed out that this might lead to misunderstandings about the content of the book." {{harv|Hardy|Wright|2008}} In particular, arithmetical is preferred as an adjective to number-theoretic.

## History

### Origins

#### Dawn of arithmetic

The first historical find of an arithmetical nature is a fragment of a table: the broken clay tablet Plimpton 322 (Larsa, Mesopotamia, ca. 1800 BCE) contains a list of "Pythagorean triples", that is, integers (a,b,c) such that a^2+b^2=c^2.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..."{{harvnb|Neugebauer & Sachs|1945|p=40}}. The term takiltum is problematic. Robson prefers the rendering "The holding-square of the diagonal from which 1 is torn out, so that the short side comes up...".{{harvnb|Robson|2001|p=192}}
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The Plimpton 322 tablet
The table's layout suggests{{harvnb|Robson|2001|p=189}}. Other sources give the modern formula (p^2-q^2,2pq,p^2+q^2). Van der Waerden gives both the modern formula and what amounts to the form preferred by Robson.{{harv|van der Waerden|1961|p=79}} that it was constructed by means of what amounts, in modern language, to the identity
left(frac{1}{2} left(x - frac{1}{x}right)right)^2 + 1 = left(frac{1}{2} left(x + frac{1}{x} right)right)^2,

#### Classical Greece and the early Hellenistic period

{{Further information|Ancient Greek mathematics}}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.{{sfn|Boyer|Merzbach|1991|p=82}} 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 (these are Indian philosophers); 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."WEB,weblink Eusebius of Caesarea: Praeparatio Evangelica (Preparation for the Gospel). Tr. E.H. Gifford (1903) â€“ Book 10, Aristotle claimed that the philosophy of Plato closely followed the teachings of the Pythagoreans,Metaphysics, 1.6.1 (987a) and Cicero repeats this claim: Platonem ferunt didicisse Pythagorea omnia ("They say Plato learned all things Pythagorean").Tusc. Disput. 1.17.39.Plato had a keen interest in mathematics, and distinguished clearly between arithmetic and calculation. (By arithmetic he meant, in part, theorising on number, rather than what arithmetic or number theory have come to mean.) It is through one of Plato's dialoguesâ€”namely, Theaetetusâ€”that we know that Theodorus had proven that sqrt{3}, sqrt{5}, dots, sqrt{17} 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. (Book X of Euclid's Elements is described by Pappus as being largely based on Theaetetus's work.)Euclid devoted part of his Elements to prime numbers and divisibility, topics that belong unambiguously to number theory and are basic to it (Books VII to IX of Euclid's Elements). In particular, he gave an algorithm for computing the greatest common divisor of two numbers (the Euclidean algorithm; Elements, Prop. VII.2) and the first known proof of the infinitude of primes (Elements, Prop. IX.20).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.{{sfn|Vardi|1998|pp=305â€“19}}{{sfn|Weil|1984|pp=17â€“24}} The epigram proposed what has become known asArchimedes's cattle problem; its solution (absent from the manuscript) requires solving an indeterminate quadratic equation (which reduces to what would later be misnamed Pell's 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

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upTitle page of the 1621 edition of Diophantus's Arithmetica, translated into Latin by Claude Gaspard Bachet de MÃ©ziriac.
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 f(x,y)=z^2 or f(x,y,z)=w^2. 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 rationalâ€”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 (say)f(x_1,x_2,x_3)=0, his aim was to find (in essence) three rational functions g_1, g_2, g_3 such that, for all values of r and s, settingx_i = g_i(r,s) for i=1,2,3 gives a solution to f(x_1,x_2,x_3)=0.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 (elliptic curves, as it happens, in what seems to be their first known occurrence) by means of what amounts to a tangent construction: translated into coordinate geometry(which did not exist in Diophantus's time), 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. (Diophantus also resorted to what could be called a special case of a secant construction.)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 (though he never stated as much explicitly).

#### Ä€ryabhaá¹­a, Brahmagupta, BhÄskara

While Greek astronomy probably influenced Indian learning, to the point of introducing trigonometry,{{sfn|Plofker|2008|p=119}} it seems to be the case that Indian mathematics is otherwise an indigenous tradition;Any early contact between Babylonian and Indian mathematics remains conjectural {{harv|Plofker|2008|p=42}}. in particular, there is no evidence that Euclid's Elements reached India before the 18th century.{{sfn|Mumford|2010|p=387}}Ä€ryabhaá¹­a (476â€“550 CE) showed that pairs of simultaneous congruences nequiv a_1 bmod m_1, nequiv a_2 bmod m_2 could be solved by a method he called kuá¹­á¹­aka, or pulveriser;Ä€ryabhaá¹­a, Ä€ryabhatÄ«ya, Chapter 2, verses 32â€“33, cited in: {{harvnb|Plofker|2008|pp=134â€“40}}. See also {{harvnb|Clark|1930|pp=42â€“50}}. A slightly more explicit description of the kuá¹­á¹­aka was later given in Brahmagupta, BrÄhmasphuá¹­asiddhÄnta, XVIII, 3â€“5 (in {{harvnb|Colebrooke|1817|p=325}}, cited in {{harvnb|Clark|1930|p=42}}). this is a procedure close to (a generalisation of) the Euclidean algorithm, which was probably discovered independently in India.{{sfn|Mumford|2010|p=388}} Ä€ryabhaá¹­a seems to have had in mind applications to astronomical calculations.{{sfn|Plofker|2008|p=119}}Brahmagupta (628 CE) started the systematic study of indefinite quadratic equationsâ€”in particular, the misnamed 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 (the chakravala, or "cyclic method") for solving Pell's equation was finally found by Jayadeva (cited in the eleventh century; his work is otherwise lost); the earliest surviving exposition appears in BhÄskara II's BÄ«ja-gaá¹‡ita (twelfth century).{{sfn|Plofker|2008|p=194}}Indian mathematics remained largely unknown in Europe until the late eighteenth century;{{sfn|Plofker|2008|p=283}} Brahmagupta and BhÄskara's work was translated into English in 1817 by Henry Colebrooke.{{sfn|Colebrooke|1817}}

#### Arithmetic in the Islamic golden age

{{Further information|Mathematics in medieval Islam}}File:Hevelius Selenographia frontispiece.png|upright|left|thumb|Al-Haytham seen by the West: frontispice of SelenographiaSelenographiaIn the early ninth century, the caliph Al-Ma'mun ordered translations of many Greek mathematical works and at least one Sanskrit work (the Sindhind,which may {{harvnb|Colebrooke|1817|p=lxv}}, cited in {{harvnb|Hopkins|1990|p=302}}. See also the preface in{{harvnb|Sachau|1888}} cited in {{harvnb|Smith|1958|pp=168}} or may not{{harvnb|Pingree|1968|pp=97â€“125}}, and {{harvnb|Pingree|1970|pp=103â€“23}}, cited in {{harvnb|Plofker|2008|p=256}}. be Brahmagupta's BrÄhmasphuá¹­asiddhÄnta).Diophantus's main work, the Arithmetica, was translated into Arabic by Qusta ibn Luqa (820â€“912).Part of the treatise al-Fakhri (by al-KarajÄ«, 953 â€“ ca. 1029) builds on it to some extent. According to Rashed Roshdi, Al-KarajÄ«'s contemporary Ibn al-Haytham knew{{sfn|Rashed|1980|pp=305â€“21}} what would later be called Wilson's theorem.

#### Western Europe in the Middle Ages

Other than a treatise on squares in arithmetic progression by Fibonacciâ€”who traveled and studied in north Africa and Constantinopleâ€”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.Bachet, 1621, following a first attempt by Xylander, 1575

### Early modern number theory

#### Fermat

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upPierre de Fermat
Pierre de Fermat (1607â€“1665) never published his writings; in particular, his work on number theory is contained almost entirely in letters to mathematicians and in private marginal notes.{{sfn|Weil|1984|pp=45â€“46}} He wrote down nearly no proofs in number theory; he had no models in the area.{{harvnb|Weil|1984|p=118}}. This was more so in number theory than in other areas (remark in {{harvnb|Mahoney|1994|p=284}}). Bachet's own proofs were "ludicrously clumsy" {{harv|Weil|1984|p=33}}. One of Fermat's first interests was perfect numbers (which appear in Euclid, Elements IX) and amicable numbers;Perfect and especially amicable numbers are of little or no interest nowadays. The same was not true in medieval timesâ€”whether in the West or the Arab-speaking worldâ€”due in part to the importance given to them by the Neopythagorean (and hence mystical) Nicomachus (ca. 100 CE), who wrote a primitive but influential "Introduction to Arithmetic". See {{harvnb|van der Waerden|1961|loc=Ch. IV}}. these topics led him to work on integer divisors, which were from the beginning among the subjects of thecorrespondence (1636 onwards) that put him in touch with the mathematical community of the day.{{harvnb|Mahoney|1994|pp=48, 53â€“54}}. The initial subjects of Fermat's correspondence included divisors ("aliquot parts") and many subjects outside number theory; see the list in the letter from Fermat to Roberval, 22.IX.1636, {{harvnb|Tannery|Henry|1891|loc=Vol. II, pp. 72, 74}}, cited in {{harvnb|Mahoney|1994|p=54}}. Fermat's work in arithmetic includes the following.
• Fermat's little theorem (1640),{{harvnb|Tannery|Henry|1891|loc=Vol. II, p. 209}}, Letter XLVI from Fermat to Frenicle, 1640,
cited in {{harvnb|Weil|1984|p=56}} stating that, if a is not divisible by a prime p, then a^{p-1} equiv 1 bmod p.Here, as usual, given two integers a and b and a non-zero integer m, we write a equiv b bmod m (read "a is congruent to b modulo m") to mean that m divides a âˆ’ b, or, what is the same, a and b leave the same residue when divided by m. This notation is actually much later than Fermat's; it first appears in section 1 of Gauss's Disquisitiones Arithmeticae. Fermat's little theorem is a consequence of the fact that the order of an element of a group divides the order of the group. The modern proof would have been within Fermat's means (and was indeed given later by Euler), even though the modern concept of a group came long after Fermat or Euler. (It helps to know that inverses exist modulo p, that is, given a not divisible by a prime p, there is an integer x such that x a equiv 1 bmod p); this fact (which, in modern language, makes the residues mod p into a group, and which was already known to Ä€ryabhaá¹­a; see (#Indian school: Ä€ryabhaá¹­a, Brahmagupta, BhÄskara|above)) was familiar to Fermat thanks to its rediscovery by Bachet {{harv|Weil|1984|p=7}}. Weil goes on to say that Fermat would have recognised that Bachet's argument is essentially Euclid's algorithm.
• If a and b are coprime, then a^2 + b^2 is not divisible by any prime congruent to âˆ’1 modulo 4;{{harvnb|Tannery|Henry|1891|loc=Vol. II, p. 204}}, cited in {{harvnb|Weil|1984|p=63}}. All of the following citations from Fermat's Varia Opera are taken from {{harvnb|Weil|1984|loc=Chap. II}}. The standard Tannery & Henry work includes a revision of Fermat's posthumous Varia Opera Mathematica originally prepared by his son {{harv|Fermat|1679}}. and every prime congruent to 1 modulo 4 can be written in the form a^2 + b^2.{{sfn|Tannery|Henry|1891|loc=Vol. II, p. 213}} 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.{{sfn|Tannery|Henry|1891|loc=Vol. II, p. 423}}
• Fermat posed the problem of solving x^2 - N y^2 = 1 as a challenge to English mathematicians (1657). The problem was solved in a few months by Wallis and Brouncker.{{sfn|Weil|1984|p=92}} Fermat considered their solution valid, but pointed out they had provided an algorithm without a proof (as had Jayadeva and Bhaskara, though Fermat would never know this). He states that a proof can be found by descent.
• Fermat states and proves (by descent) in the appendix to Observations on Diophantus (Obs. XLV){{sfn |Tannery|Henry|1891|loc=Vol. I, pp. 340â€“41}} that x^{4} + y^{4} = z^{4} has no non-trivial solutions in the integers. Fermat also mentioned to his correspondents that x^3 + y^3 = z^3 has no non-trivial solutions, and that this could be proven by descent.{{sfn|Weil|1984|p=115}} The first known proof is due to Euler (1753; indeed by descent).{{sfn|Weil|1984|pp=115â€“16}}
Fermat's claim ("Fermat's last theorem") to have shown there are no solutions tox^n + y^n = z^n for all ngeq 3 appears only in his annotations on the margin of his copy of Diophantus.

#### Euler

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upLeonhard Euler
• Proofs for Fermat's statements. This includes Fermat's little theorem (generalised by Euler to non-prime moduli); the fact that p = x^2 + y^2 if and only if pequiv 1 bmod 4; initial work towards a proof that every integer is the sum of four squares (the first complete proof is by Joseph-Louis Lagrange (1770), soon improved by Euler himself{{sfn|Weil|1984|pp=178â€“79}}); the lack of non-zero integer solutions to x^4 + y^4 = z^2 (implying the case n=4 of Fermat's last theorem, the case n=3 of which Euler also proved by a related method).
• Pell's equation, first misnamed by Euler.{{harvnb|Weil|1984|p=174}}. Euler was generous in giving credit to others {{harv|Varadarajan|2006|p=14}}, not always correctly. He wrote on the link between continued fractions and Pell's equation.{{sfn|Weil|1984|p=183}}
• 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 particular, infinite series) 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 (though not fully rigorous) early work on what would later be called the Riemann zeta function.{{harvnb|Varadarajan|2006|pp=45â€“55}}; see also chapter III.
• Quadratic forms. Following Fermat's lead, Euler did further research on the question of which primes can be expressed in the form x^2 + N y^2, some of it prefiguring quadratic reciprocity.{{sfn|Varadarajan|2006|pp=44â€“47}} {{sfn|Weil|1984|pp=177â€“79}}{{sfn|Edwards|1983|pp=285â€“91}}
• Diophantine equations. Euler worked on some Diophantine equations of genus 0 and 1.{{sfn|Varadarajan|2006|pp=55â€“56}}{{sfn|Weil|1984|pp=179â€“81}} In particular, he studied Diophantus's work; he tried to systematise it, but the time was not yet ripe for such an endeavourâ€”algebraic geometry was still in its infancy.{{sfn|Weil|1984|p=181}} He did notice there was a connection between Diophantine problems and elliptic integrals,{{sfn|Weil|1984|p=181}} whose study he had himself initiated.

#### Lagrange, Legendre, and Gauss

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upCarl Friedrich Gauss's Disquisitiones Arithmeticae, first edition
Joseph-Louis Lagrange (1736â€“1813) was the first to give full proofs of some of Fermat's and Euler's work and observationsâ€”for instance, the four-square theorem and the basic theory of the misnamed "Pell's equation" (for which an algorithmic solution was found by Fermat and his contemporaries, and also by Jayadeva and Bhaskara II before them.) He also studied quadratic forms in full generality (as opposed to m X^2 + n Y^2)â€”defining their equivalence relation, showing how to put them in reduced form, etc.Adrien-Marie Legendre (1752â€“1833) was the first to state the law of quadratic reciprocity. He alsoconjectured what amounts to the prime number theorem and Dirichlet's theorem on arithmetic progressions. He gave a full treatment of the equation a x^2 + b y^2 + c z^2 = 0{{sfn|Weil|1984|pp=327â€“28}} and worked on quadratic forms along the lines later developed fully by Gauss.{{sfn|Weil|1984|pp=332â€“34}} In his old age, he was the first to prove "Fermat's last theorem" for n=5 (completing work by Peter Gustav Lejeune Dirichlet, and crediting both him and Sophie Germain).{{sfn|Weil|1984|pp=337â€“38}}(File:10 DM Serie4 Vorderseite.jpg|thumb|left|Carl Friedrich Gauss)In his Disquisitiones Arithmeticae (1798), Carl Friedrich Gauss (1777â€“1855) proved the law of quadratic reciprocity and developed the theory of quadratic forms (in particular, defining their composition). He also introduced some basic notation (congruences) and devoted a section to computational matters, including primality tests.{{sfn|Goldstein|Schappacher|2007|p=14}} 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 belongby itself to arithmetic, but its principles can only be drawn from higher arithmetic.From the preface of Disquisitiones Arithmeticae; the translation is taken from {{harvnb|Goldstein|Schappacher|2007|p=16}}In this way, Gauss arguably made a first foray towards both Ã‰variste Galois's work and algebraic number theory.

### Maturity and division into subfields

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upErnst Kummer
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- Peter Gustav Lejeune Dirichlet.jpg -
upright|Peter Gustav Lejeune Dirichlet
Starting early in the nineteenth century, the following developments gradually took place:
• The rise to self-consciousness of number theory (or higher arithmetic) as a field of study.See the discussion in section 5 of {{harvnb|Goldstein|Schappacher|2007}}. Early signs of self-consciousness are present already in letters by Fermat: thus his remarks on what number theory is, and how "Diophantus's work [...] does not really belong to [it]" (quoted in {{harvnb|Weil|1984|p=25}}).
• The development of much of modern mathematics necessary for basic modern number theory: complex analysis, group theory, Galois theoryâ€”accompanied by greater rigor in analysis and abstraction in algebra.
• The rough subdivision of number theory into its modern subfieldsâ€”in particular, analytic and algebraic number theory.
Algebraic number theory may be said to start with the study of reciprocity and cyclotomy, but truly came into its own with the development of abstract algebra and early ideal theory and valuation theory; see below. A conventional starting point for analytic number theory is Dirichlet's theorem on arithmetic progressions (1837),{{sfn|Apostol|1976|p=7}} {{sfn|Davenport|Montgomery|2000|p=1}} whose proof introduced L-functions and involved some asymptotic analysis and a limiting process on a real variable.See the proof in {{harvnb|Davenport|Montgomery|2000|loc=section 1}} The first use of analytic ideas in number theory actuallygoes back to Euler (1730s),{{sfn|Iwaniec|Kowalski|2004|p=1}} {{sfn|Varadarajan|2006|loc=sections 2.5, 3.1 and 6.1}} who used formal power series and non-rigorous (or implicit) limiting arguments. The use of complex analysis in number theory comes later: the work of Bernhard Riemann (1859) on the zeta function is the canonical starting point;{{sfn|Granville|2008|pp=322â€“48}} Jacobi's four-square theorem (1839), which predates it, belongs to an initially different strand that has by now taken a leading role in analytic number theory (modular forms).See the comment on the importance of modularity in {{harvnb|Iwaniec|Kowalski|2004|p=1}}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 ErdÅ‘s and Selberg.{{sfn|Goldfeld|2003}} The term is somewhat ambiguous: for example, proofs based on complex Tauberian theorems (for example, Wienerâ€“Ikehara) 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.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.See, for example, the initial comment in {{harvnb|Iwaniec|Kowalski|2004|p=1}}.

### Analytic number theory

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- Complex zeta.jpg -
Riemann zeta function Î¶(s) in the complex plane. The color of a point s gives the value of Î¶(s): dark colors denote values close to zero and hue gives the value's argument.
File:ModularGroup-FundamentalDomain.svg|thumb|The action of the modular group on the upper half plane. The region in grey is the standard fundamental domainfundamental domainAnalytic 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;{{sfn|Apostol|1976|p=7}} or
• in terms of its concerns, as the study within number theory of estimates on size and density, as opposed to identities.{{harvnb|Granville|2008|loc=section 1}}: "The main difference is that in algebraic number theory [...] one typically considers questions with answers that are given by exact formulas, whereas in analytic number theory [...] one looks for good approximations."
Some subjects generally considered to be part of analytic number theory, for example, sieve theory,Sieve theory figures as one of the main subareas of analytic number theory in many standard treatments; see, for instance, {{harvnb|Iwaniec|Kowalski|2004}} or {{harvnb|Montgomery|Vaughan|2007}} are better covered by the second rather than the first definition: some of sieve theory, for instance, uses little analysis,This is the case for small sieves (in particular, some combinatorial sieves such as the Brun sieve) rather than for large sieves; the study of the latter now includes ideas from harmonic and functional analysis. yet it does belong to analytic number theory.The following are examples of problems in analytic number theory: the prime number theorem, the Goldbach conjecture (or the twin prime conjecture, or the Hardyâ€“Littlewood conjectures), 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 (or, rather, the study of their properties). The theory of modular forms (and, more generally, automorphic forms) also occupies an increasingly central place in the toolbox of analytic number theory.See the remarks in the introduction to {{harvnb|Iwaniec|Kowalski|2004|p=1}}: "However much stronger...".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 (generalizations of prime numbers in the field of algebraic numbers) 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.{{harvnb|Granville|2008|loc=section 3}}: "[Riemann] defined what we now call the Riemann zeta function [...] Riemann's deep work gave birth to our subject [...]" This is an example of a general procedure in analytic number theory: deriving information about the distribution of a sequence (here, prime ideals or prime numbers) from the analytic behavior of an appropriately constructed complex-valued function.See, for example, {{harvnb|Montgomery|Vaughan|2007}}, p. 1.

### Algebraic number theory

An algebraic number is any complex number that is a solution to some polynomial equation f(x)=0 with rational coefficients; for example, every solution x of x^5 + (11/2) x^3 - 7 x^2 + 9 = 0 (say) 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.{{harvid|Milne|2014}}, p. 2. 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 (viz., quadratic fields) were already studied by Gauss, as the discussion of quadratic forms in Disquisitiones arithmeticae can be restated in terms of ideals andnorms in quadratic fields. (A quadratic field consists of allnumbers of the form a + b sqrt{d}, wherea and b are rational numbers and dis a fixed rational number whose square root is not rational.)For that matter, the 11th-century chakravala method amountsâ€”in modern termsâ€”to an algorithm for finding the units of a real quadratic number field. However, neither BhÄskara 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. (For example, in the field generated by the rationalsand sqrt{-5}, the number 6 can be factorised both as 6 = 2 cdot 3 and
6 = (1 + sqrt{-5}) ( 1 - sqrt{-5}); all of 2, 3, 1 + sqrt{-5} and
1 - sqrt{-5}
are irreducible, and thus, in a naÃ¯ve sense, analogous to primes among the integers.) The initial impetus for the development of ideal numbers (by Kummer) seems to have come from the study of higher reciprocity laws,{{sfn|Edwards|2000|p=79}} 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.(For example, the complex numbers C are an extension of the reals R, and the reals R are an extension of the rationals Q.)Classifying the possible extensions of a given number field is a difficult and partially open problem. Abelian extensionsâ€”that is, extensions L of K such that the Galois groupThe Galois group of an extension K/L consists of the operations (isomorphisms) that send elements of L to other elements of L while leaving all elements of K fixed.Thus, for instance, Gal(C/R) consists of two elements: the identity element(taking every element x + iy of C to itself) and complex conjugation(the map taking each element x + iy to x âˆ’ iy).The Galois group of an extension tells us many of its crucial properties. The study of Galois groups started with Ã‰variste Galois; in modern language, the main outcome of his work is that an equation f(x) = 0 can be solved by radicals(that is, x can be expressed in terms of the four basic operations togetherwith square roots, cubic roots, etc.) if and only if the extension of the rationals by the roots of the equation f(x) = 0 has a Galois group that is solvablein the sense of group theory. ("Solvable", in the sense of group theory, is a simple property that can be checked easily for finite groups.) Gal(L/K) of L over K is an abelian groupâ€”are relatively well understood.Their classification was the object of the programme of class field theory, which was initiated in the late 19th century (partly by Kronecker and Eisenstein) and carried out largely in 1900â€“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 (points all of whose coordinates are rationals) orintegral points (points all of whose coordinates are integers) 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: are there finitelyor infinitely many rational points on a given curve (or surface)? What about integer points?An example here may be helpful. Consider the Pythagorean equation x^2+y^2 = 1;we would like to study its rational solutions, that is, its solutions(x,y) such thatx and y are both rational. This is the same as asking for all integer solutionsto a^2 + b^2 = c^2; any solution to the latter equation givesus a solution x = a/c, y = b/c to the former. It is also thesame as asking for all points with rational coordinates on the curvedescribed by x^2 + y^2 = 1. (This curve happens to be a circle of radius 1 around the origin.)Image:ECClines-3.svg|right|thumb|300px|Two examples of an elliptic curve, that is, a curveof genus 1 having at least one rational point. (Either graph can be seen as a slice of a torustorusThe 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 curveâ€”that is, rational or integer solutions to an equation f(x,y)=0, where f is a polynomial in two variablesâ€”turns out to depend crucially on the genus of the curve. The genus can be defined as follows:It may be useful to look at an example here. Say we want to study the curve y^2 = x^3 + 7. We allow x and y to be complex numbers: (a + b i)^2 = (c + d i)^3 + 7. This is, in effect, a set of two equations on four variables, since both the realand the imaginary part on each side must match. As a result, we get a surface (two-dimensional) in four-dimensional space. After we choose a convenient hyperplane on which to project the surface (meaning that, say, we choose to ignore the coordinate a), we canplot the resulting projection, which is a surface in ordinary three-dimensional space. Itthen becomes clear that the result is a torus, loosely speaking, the surface of a doughnut (somewhatstretched). A doughnut has one hole; hence the genus is 1. allow the variables in f(x,y)=0 to be complex numbers; then f(x,y)=0 defines a 2-dimensional surface in (projective) 4-dimensional space (since two complex variables can be decomposed into four real variables, that is, four dimensions). Countthe number of (doughnut) holes in the surface; call this number the genus of f(x,y)=0. Other geometrical notions turn out to be just as crucial.There is also the closely linked area of Diophantine approximations: given a number x, how well can it be approximated by rationals? (We are looking for approximations that are good relative to the amount of space that it takes to write the rational: call a/q (with gcd(a,q)=1) a good approximation to x if |x-a/q|

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