In number theory, the law of quadratic reciprocity is a theorem about modular arithmetic that gives conditions for the solvability of quadratic equations modulo prime numbers. There are a number of equivalent statements of the theorem.
Although the law can be used to tell whether any quadratic equation modulo a prime number has a solution, it does not provide any help at all for actually finding the solution. (The article on quadratic residues discusses algorithms for this.)
- The fundamental theorem must certainly be regarded as one of the most elegant of its type. (Art. 151)
The first section of this article gives a special case of quadratic reciprocity that is representative of the general case. The second section gives the formulations of quadratic reciprocity found by Legendre and Gauss.
Consider the polynomial f(n) = n2 − 5 and its values for n = 1, 2, 3, 4, ... The prime factorizations of these values are given as follows:
A striking feature of the data is that with the exceptions of 2 and 5, the prime numbers that appear as factors are precisely those with final digit 1 or 9.
Another way of phrasing this is that the primes p for which there exists an n such that n2 ≡ 5 (mod p) are precisely 2, 5, and those primes p that are ≡ 1 or 4 (mod 5).
The law of quadratic reciprocity gives a similar characterization of prime divisors of f(n) = n2 − c for any integer c.
Terminology, data, and two statements of the theorem
A quadratic residue (mod n) is any number congruent to a square (mod n). A quadratic nonresidue (mod n) is any number that is not congruent to a square (mod n). The adjective "quadratic" can be dropped if the context makes it clear that it is implied. When working modulo primes (as in this article), it is usual to treat zero as a special case. By doing so, the following statements become true:
- Modulo a prime, there are an equal number of quadratic residues and nonresidues.
- Modulo a prime, the product of two quadratic residues is a residue, the product of a residue and a nonresidue is a nonresidue, and the product of two nonresidues is a residue.
Table of quadratic residues
This table is complete for odd primes less than 50. To check whether a number n is a quadratic residue mod one of these primes p, find a ≡ n (mod p) and 0 ≤ a < p. If a is in row p it is a residue (mod p); if it is not in row p of the table, it is a nonresidue (mod p).
The quadratic reciprocity law is the statement that certain patterns found in the table are true in general.
In this article, p and q always refer to distinct positive odd prime numbers.
−1 and the first supplement
First of all, for which prime numbers is −1 a quadratic residue? Examining the table, we find −1 in rows 5, 13, 17, 29, 37, and 41 but not in rows 3, 7, 11, 19, 23, 31, 43 or 47.
(−1 ≡ 2 (mod 3), −1 ≡ 4 (mod 5), −1 ≡ 10 (mod 11), etc.)
The former primes are all ≡ 1 (mod 4), and the latter are all ≡ 3 (mod 4). This leads to
The first supplement to quadratic reciprocity:
±2 and the second supplement
For which prime numbers is 2 a quadratic residue? Examining the table, we find 2 in rows 7, 17, 23, 31, 41, and 47, but not in rows 3, 5, 11, 13, 19, 29, 37, or 43.
The former primes are all ≡ ±1 (mod 8), and the latter are all ≡ ±3 (mod 8). This leads to
The second supplement to quadratic reciprocity:
−2 is in rows 3, 11, 17, 19, 41, 43, but not in rows 5, 7, 13, 23, 29, 31, 37, or 47. The former are ≡ 1 or ≡ 3 (mod 8), and the latter are ≡ 5 or ≡ 7 (mod 8).
3 is in rows 11, 13, 23, 37, and 47, but not in rows 5, 7, 17, 19, 29, 31, 41, or 43.
The former are ≡ ±1 (mod 12) and the latter are all ≡ ±5 (mod 12).
−3 is in rows 7, 13, 19, 31, 37, and 43 but not in rows 5, 11, 17, 23, 29, 41, or 47. The former are ≡ 1 (mod 3) and the latter ≡ 2 (mod 3).
Since the only residue (mod 3) is 1, we see that −3 is a quadratic residue modulo every prime which is a residue (mod 3).
5 is in rows 11, 19, 29, 31, and 41 but not in rows 3, 7, 13, 17, 23, 37, 43, or 47.
The former are ≡ ±1 (mod 5) and the latter are ≡ ±2 (mod 5).
Since the only residues (mod 5) are ±1, we see that 5 is a quadratic residue modulo every prime which is a residue (mod 5).
−5 is in rows 3, 7, 23, 29, 41, 43, and 47 but not in rows 11, 13, 17, 19, 31, or 37. The former are ≡ 1, 3, 7, 9 (mod 20) and the latter are ≡ 11, 13, 17, 19 (mod 20).
The observations about −3 and +5 continue to hold: −7 is a residue (mod p) if and only if p is a residue (mod 7), −11 is a residue (mod p) if and only if p is a residue (mod 11), +13 is a residue (mod p) if and only if p is a residue (mod 13), ...
The more complicated-looking rules for the quadratic characters of +3 and −5, which depend upon congruences (mod 12) and (mod 20) respectively, are simply the ones for −3 and +5 working with the first supplement.
For example, for −5 to be a residue (mod p), either both 5 and −1 have to be residues (mod p) or they both have to be nonresidues:
i.e., p has to be ≡ ±1 (mod 5) and ≡ 1 (mod 4), which is the same thing as p ≡ 1 or 9 (mod 20), or p has to be ≡ ±2 mod 5 and ≡ 3 mod 4, which is the same as p ≡ 3 or 7 (mod 20). See Chinese remainder theorem.
The generalization of the rules for −3 and +5 is Gauss's statement of quadratic reciprocity:
These statements may be combined:
- Let q* = (−1)(q−1)/2q. Then the congruence x2 ≡ p (mod q) is solvable if and only if x2 ≡ q* (mod p) is.
Table of quadratic character of primes
|R||q is a residue (mod p)||q ≡ 1 (mod 4) or p ≡ 1 (mod 4) (or both)|
|N||q is a nonresidue (mod p)|
|R||q is a residue (mod p)||both q ≡ 3 (mod 4) and p ≡ 3 (mod 4)|
|N||q is a nonresidue (mod p)|
Another way to organize the data is to see which primes are residues mod which other primes, as illustrated in the above table. The entry in row p column q is R if q is a quadratic residue (mod p); if it is a nonresidue the entry is N.
If the row, or the column, or both, are ≡ 1 (mod 4) the entry is blue or green; if both row and column are ≡ 3 (mod 4), it is yellow or orange.
The blue and green entries are symmetric around the diagonal: The entry for row p, column q is R (resp N) if and only if the entry at row q, column p, is R (resp N).
The yellow and orange ones, on the other hand, are antisymmetric: The entry for row p, column q is R (resp N) if and only if the entry at row q, column p, is N (resp R).
This observation is Legendre's statement of quadratic reciprocity:
It is a simple exercise to prove that Legendre's and Gauss's statements are equivalent – it requires no more than the first supplement and the facts about multiplying residues and nonresidues.
Connection with cyclotomy
The early proofs of quadratic reciprocity are relatively unilluminating. The situation changed when Gauss used Gauss sums to show that quadratic fields are subfields of cyclotomic fields, and implicitly deduced quadratic reciprocity from a reciprocity theorem for cyclotomic fields. His proof was cast in modern form by later algebraic number theorists. This proof served as a template for class field theory, which can be viewed as a vast generalization of quadratic reciprocity
- I confess that, as a student unaware of the history of the subject and unaware of the connection with cyclotomy, I did not find the law or its so-called elementary proofs appealing. I suppose, although I would not have (and could not have) expressed myself in this way that I saw it as little more than a mathematical curiosity, fit more for amateurs than for the attention of the serious mathematician that I then hoped to become. It was only in Hermann Weyl's book on the algebraic theory of numbers that I appreciated it as anything more.
History and alternative statements
There are a number of ways to state the theorem. Keep in mind that Euler and Legendre did not have Gauss's congruence notation, nor did Gauss have the Legendre symbol.
In this article p and q always refer to distinct positive odd primes.
He did not state the law of quadratic reciprocity, although the cases −1, ±2, and ±3 are easy deductions from these and other of his theorems.
He also claimed to have a proof that if the prime number p ends with 7, (in base 10) and the prime number q ends in 3, and p ≡ q ≡ 3 (mod 4), then
Euler conjectured, and Lagrange proved, that
Proving these and other statements of Fermat was one of the things that led mathematicians to the reciprocity theorem.
Translated into modern notation, Euler stated:
- If q ≡ 1 (mod 4) then q is a quadratic residue (mod p) if and only if p ≡ r (mod q), where r is a quadratic residue of q.
- If q ≡ 3 (mod 4) then q is a quadratic residue (mod p) if and only if p ≡ ±b2 (mod 4q), where b is odd and not divisible by q.
This is equivalent to quadratic reciprocity.
He could not prove it, but he did prove the second supplement.
Legendre and his symbol
Fermat proved that if p is a prime number and a is an integer,
Thus, if p does not divide a,
Legendre lets a and A represent positive primes ≡ 1 (mod 4) and b and B positive primes ≡ 3 (mod 4), and sets out a table of eight theorems that together are equivalent to quadratic reciprocity:
|Theorem||When||it follows that|
He says that since expressions of the form
Legendre's version of quadratic reciprocity
He notes that these can be combined:
The supplementary laws using Legendre symbols
Legendre's attempt to prove reciprocity is based on a theorem of his:
This technique doesn't work for Theorem VIII. Let b ≡ B ≡ 3 (mod 4), and assume Then if there is another prime p ≡ 1 (mod 4) such that the solvability of leads to a contradiction (mod 4). But Legendre was unable to prove there has to be such a prime p; he was later able to show that all that is required is "Legendre's lemma":
but he couldn't prove that either. Hilbert symbol (below) discusses how techniques based on the existence of solutions to can be made to work.
Gauss first proves the supplementary laws. He sets the basis for induction by proving the theorem for ±3 and ±5. Noting that it is easier to state for −3 and +5 than it is for +3 or −5, he states the general theorem in the form:
- If p is a prime of the form 4n + 1 then p, but if p is of the form 4n+3 then −p, is a quadratic residue (resp. nonresidue) of every prime, which, with a positive sign, is a residue (resp. nonresidue) of p.
In the next sentence, he christens it the "fundamental theorem" (Gauss never used the word "reciprocity").
Introducing the notation a R b (resp. a N b) to mean a is a quadratic residue (resp. nonresidue) (mod b), and letting a, a′, etc. represent positive primes ≡ 1 (mod 4) and b, b′, etc. positive primes ≡ 3 (mod 4), he breaks it out into the same 8 cases as Legendre:
|1)||±a R a′||±a′ R a|
|2)||±a N a′||±a′ N a|
|3)||+a R b
−a N b
|±b R a|
|4)||+a N b
−a R b
|±b N a|
|5)||±b R a||+a R b|
−a N b
|6)||±b N a||+a N b|
−a R b
|7)||+b R b′
−b N b′
|−b′ N b|
+b′ R b
|8)||−b N b′
+b R b′
|+b′ R b|
−b′ N b
In the next Article he generalizes this to what are basically the rules for the Jacobi symbol (below). Letting A, A′, etc. represent any (prime or composite) positive numbers ≡ 1 (mod 4) and B, B′, etc. positive numbers ≡ 3 (mod 4):
|9)||±a R A||±A R a|
|10)||±b R A||+A R b|
−A N b
|11)||+a R B||±B R a|
|12)||−a R B||±B N a|
|13)||+b R B||−B N b|
+N R b
|14)||−b R B||+B R b|
−B N b
All of these cases take the form "if a prime is a residue (mod a composite), then the composite is a residue or nonresidue (mod the prime), depending on the congruences (mod 4)". He proves that these follow from cases 1) - 8).
Gauss needed, and was able to prove, a lemma similar to the one Legendre needed:
The proof of quadratic reciprocity is by complete induction (i.e. assuming it is true for all numbers less than n allows the deduction it is true for n) for each of the cases 1) to 8).
Gauss's version in Legendre symbols
These can be combined:
Note that the statements in this section are equivalent to quadratic reciprocity: if, for example, Euler's version is assumed, the Legendre-Gauss version can be deduced from it, and vice-versa.
This form of quadratic reciprocity is derived from Euler's work:
Euler's statement can be proved by using Gauss's lemma.
Gauss's fourth proof consists of proving this theorem (by comparing two formulas for the value of Gauss sums) and then restricting it to two primes:
Let a, b, c, ... be unequal positive odd primes, whose product is n, and let m be the number of them that are ≡ 3 (mod 4); check whether n/a is a residue of a, whether n/b is a residue of b, .... The number of nonresidues found will be even when m ≡ 0, 1 (mod 4), and it will be odd if m ≡ 2, 3 (mod 4).
He gives the example. Let a = 3, b = 5, c = 7, and d = 11. Three of these, 3, 7, and 11 ≡ 3 (mod 4), so m ≡ 3 (mod 4).
5×7×11 R 3; 3×7×11 R 5; 3×5×11 R 7; and 3×5×7 N 11, so there are an odd number of nonresidues.
Eisenstein formulates this:
Mordell proved the following to be equivalent to quadratic reciprocity:
The Jacobi symbol is a generalization of the Legendre symbol; the main difference is that the bottom number has to be positive and odd, but does not have to be prime. If it is prime, the two symbols agree. It obeys the same rules of manipulation as the Legendre symbol. In particular
and if both numbers are positive and odd (this is sometimes called "Jacobi's reciprocity law"):
However, if the Jacobi symbol is +1 and the bottom number is composite, it does not necessarily mean that the top number is a quadratic residue of the bottom one. Gauss's cases 9) - 14) above can be expressed in terms of Jacobi symbols:
and since p is prime the left hand side is a Legendre symbol, and we know whether M is a residue (mod p) or not.
The formulas listed in the preceding section are true for Jacobi symbols as long as the symbols are defined. Euler's formula may be written
and 2 is a residue mod the primes 7, 23 and 31: 32 ≡ 2 (mod 7), 52 ≡ 2 (mod 23), and 82 ≡ 2 (mod 31), but 2 is not a quadratic residue (mod 5), so it can't be one (mod 15). This is related to the problem Legendre had: if we know that , we know that a is a nonresidue modulo every prime in the arithmetic series m + 4a, m + 8a, ..., if there are any primes in this series, but that wasn't proved until decades after Legendre.
Eisenstein's formula requires relative primality conditions (which are true if the numbers are prime)
The quadratic reciprocity law can be formulated in terms of the Hilbert symbol where a and b are any two nonzero rational numbers and v runs over all the non-trivial absolute values of the rationals (the archimedean one and the p-adic absolute values for primes p). The Hilbert symbol is 1 or −1. It is defined to be 1 if and only if the equation has a solution in the completion of the rationals at v other than . The Hilbert reciprocity law states that , for fixed a and b and varying v, is 1 for all but finitely many v and the product of over all v is 1. (This formally resembles the residue theorem from complex analysis.)
The proof of Hilbert reciprocity reduces to checking a few special cases, and the non-trivial cases turn out to be equivalent to the main law and the two supplementary laws of quadratic reciprocity for the Legendre symbol. There is no kind of reciprocity in the Hilbert reciprocity law; its name simply indicates the historical source of the result in quadratic reciprocity. Unlike quadratic reciprocity, which requires sign conditions (namely positivity of the primes involved) and a special treatment of the prime 2, the Hilbert reciprocity law treats all absolute values of the rationals on an equal footing. Therefore it is a more natural way of expressing quadratic reciprocity with a view towards generalization: the Hilbert reciprocity law extends with very few changes to all global fields and this extension can rightly be considered a generalization of quadratic reciprocity to all global fields.
There are also quadratic reciprocity laws in rings other than the integers.
In his second monograph on quartic reciprocity Gauss stated quadratic reciprocity for the ring Z[i] of Gaussian integers, saying that it is a corollary of the biquadratic law in Z[i], but did not provide a proof of either theorem. Peter Gustav Lejeune Dirichlet showed that the law in Z[i] can be deduced from the law for Z without using biquadratic reciprocity.
For an odd Gaussian prime π and a Gaussian integer α, gcd(α, π) = 1, define the quadratic character for Z[i] by the formula
Let λ = a + b i and μ = c + d i be distinct Gaussian primes where a and c are odd and b and d are even. Then
For an Eisenstein prime π, Nπ ≠ 3 and an Eisenstein integer α, gcd(α, π) = 1, define the quadratic character for Z[ω] by the formula
Let λ = a + b ω and μ = c + d ω be distinct Eisenstein primes where a and c are not divisible by 3 and b and d are divisible by 3. Eisenstein proved
Imaginary quadratic fields
Let k be an imaginary quadratic number field with ring of integers For a prime ideal with odd norm and define the quadratic character for by the formula
Let be an integral basis of
and define a function χ(ν) where ν has odd norm by
If m = Nμ and n = Nν are both odd, Herglotz proved
Also, if 
Polynomials over a finite field
Let F be a finite field with q = pn elements, where p is an odd prime number and n is positive, and let F[x] be the ring of polynomials in one variable with coefficients in F. If and f is irreducible, monic, and has positive degree, define the quadratic character for F[x] in the usual manner:
Dedekind proved that if are monic and have positive degrees,
The attempt to generalize quadratic reciprocity for powers higher than the second was one of the main goals that led 19th century mathematicians, including Carl Friedrich Gauss, Peter Gustav Lejeune Dirichlet, Carl Gustav Jakob Jacobi, Gotthold Eisenstein, Richard Dedekind, Ernst Kummer, and David Hilbert to the study of general algebraic number fields and their rings of integers; specifically Kummer invented ideals in order to state and prove higher reciprocity laws.
The ninth in the list of 23 unsolved problems which David Hilbert proposed to the Congress of Mathematicians in 1900 asked for the "Proof of the most general reciprocity law [f]or an arbitrary number field". In 1923 Artin, building upon work by Furtwängler, Takagi, Hasse and others, discovered a general theorem for which all known reciprocity laws are special cases; he proved it in 1927.
The links below provide more detailed discussions of these theorems.
- Euler's criterion
- Zolotarev's lemma
- Proofs of quadratic reciprocity
- Cubic reciprocity
- Quartic reciprocity
- Eisenstein reciprocity
- Artin reciprocity
- Gauss, DA § 4, arts 107–150
- E.g. in his mathematical diary entry for April 8, 1796 (the date he first proved quadratic reciprocity). See facsimile page from Felix Klein's Development of Mathematics in the 19th century
- See F. Lemmermeyer's chronology and bibliography of proofs in the external references
- Lemmermeyer, pp. 2–3
- Gauss, DA, art. 182
- Lemmermeyer, p. 3
- Lemmermeyer, p. 5, Ireland & Rosen, pp. 54, 61
- Ireland & Rosen, pp. 69–70. His proof is based on what are now called Gauss sums.
- This section is based on Lemmermeyer, pp. 6–8
- The equivalence is Euler's criterion
- The analogue of Legendre's original definition is used for higher-power residue symbols
- E.g. Kronecker's proof (Lemmermeyer, ex. p. 31, 1.34) is to use Gauss's lemma to establish that
- Gauss, DA, arts 108–116
- Gauss, DA, arts 117–123
- Gauss, DA, arts 130
- Gauss, DA, Art 131
- Gauss, DA, arts. 125–129
- Gauss, DA, arts 135–144
- Because the basic Gauss sum equals
- Because the quadratic field is a subfield of the cyclotomic field
- Ireland & Rosen, pp 60–61.
- Gauss, "Summierung gewisser Reihen von besonderer Art", reprinted in Untersuchumgen uber hohere Arithmetik, pp.463–495
- Lemmermeyer, Th. 2.28, pp 63–65
- Lemmermeyer, ex. 1.9, p. 28
- By Peter Gustav Lejeune Dirichlet in 1837
- Gauss, BQ § 60
- Dirichlet's proof is in Lemmermeyer, Prop. 5.1 p.154, and Ireland & Rosen, ex. 26 p. 64
- Lemmermeyer, Prop. 5.1, p. 154
- Lemmermeyer, Thm. 7.10, p. 217
- Lemmermeyer, Thm 8.15, p.256 ff
- Lemmermeyer Thm. 8.18, p. 260
- Bach & Shallit, Thm. 6.7.1
- Lemmermeyer, p. 15, and Edwards, pp.79–80 both make strong cases that the study of higher reciprocity was much more important as a motivation than Fermat's Last Theorem was
- Lemmermeyer, p. viii
- Lemmermeyer, p. ix ff
The Disquisitiones Arithmeticae has been translated (from Latin) into English and German. The German edition includes all of Gauss's papers on number theory: all the proofs of quadratic reciprocity, the determination of the sign of the Gauss sum, the investigations into biquadratic reciprocity, and unpublished notes. Footnotes referencing the Disquisitiones Arithmeticae are of the form "Gauss, DA, Art. n".
The two monographs Gauss published on biquadratic reciprocity have consecutively numbered sections: the first contains §§ 1–23 and the second §§ 24–76. Footnotes referencing these are of the form "Gauss, BQ, § n".
These are in Gauss's Werke, Vol II, pp. 65–92 and 93–148. German translations are in pp. 511–533 and 534–586 of Untersuchungen über höhere Arithmetik.
Franz Lemmermeyer's Reciprocity Laws: From Euler to Eisenstein has many proofs (some in exercises) of both quadratic and higher-power reciprocity laws and a discussion of their history. Its immense bibliography includes literature citations for 196 different published proofs for the quadratic reciprocity law.
Kenneth Ireland and Michael Rosen's A Classical Introduction to Modern Number Theory also has many proofs of quadratic reciprocity (and many exercises), and covers the cubic and biquadratic cases as well. Exercise 13.26 (p 202) says it all
Count the number of proofs to the law of quadratic reciprocity given thus far in this book and devise another one.