Legendre symbol (a/p)
for various a (along top) and p (along left side).
a
p
0 1 2 3 4 5 6 7 8 9 10
3 0 1 −1
5 0 1 −1 −1 1
7 0 1 1 −1 1 −1 −1
11 0 1 −1 1 1 1 −1 −1 −1 1 −1

Only 0 ≤ a < p are shown, since due to the first property below any other a can be reduced modulo p. Quadratic residues are highlighted in yellow, and correspond precisely to the values 0 and 1.

In number theory, the Legendre symbol is a multiplicative function with values 1, −1, 0 that is a quadratic character modulo of an odd prime number p: its value at a (nonzero) quadratic residue mod p is 1 and at a non-quadratic residue (non-residue) is −1. Its value at zero is 0.

The Legendre symbol was introduced by Adrien-Marie Legendre in 1798[1] in the course of his attempts at proving the law of quadratic reciprocity. Generalizations of the symbol include the Jacobi symbol and Dirichlet characters of higher order. The notational convenience of the Legendre symbol inspired introduction of several other "symbols" used in algebraic number theory, such as the Hilbert symbol and the Artin symbol.

Definition

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Let   be an odd prime number. An integer   is a quadratic residue modulo   if it is congruent to a perfect square modulo   and is a quadratic nonresidue modulo   otherwise. The Legendre symbol is a function of   and   defined as

 

Legendre's original definition was by means of the explicit formula

 

By Euler's criterion, which had been discovered earlier and was known to Legendre, these two definitions are equivalent.[2] Thus Legendre's contribution lay in introducing a convenient notation that recorded quadratic residuosity of a mod p. For the sake of comparison, Gauss used the notation aRp, aNp according to whether a is a residue or a non-residue modulo p. For typographical convenience, the Legendre symbol is sometimes written as (a | p) or (a/p). For fixed p, the sequence   is periodic with period p and is sometimes called the Legendre sequence. Each row in the following table exhibits periodicity, just as described.

Table of values

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The following is a table of values of Legendre symbol   with p ≤ 127, a ≤ 30, p odd prime.

a
p
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
3 1 −1 0 1 −1 0 1 −1 0 1 −1 0 1 −1 0 1 −1 0 1 −1 0 1 −1 0 1 −1 0 1 −1 0
5 1 −1 −1 1 0 1 −1 −1 1 0 1 −1 −1 1 0 1 −1 −1 1 0 1 −1 −1 1 0 1 −1 −1 1 0
7 1 1 −1 1 −1 −1 0 1 1 −1 1 −1 −1 0 1 1 −1 1 −1 −1 0 1 1 −1 1 −1 −1 0 1 1
11 1 −1 1 1 1 −1 −1 −1 1 −1 0 1 −1 1 1 1 −1 −1 −1 1 −1 0 1 −1 1 1 1 −1 −1 −1
13 1 −1 1 1 −1 −1 −1 −1 1 1 −1 1 0 1 −1 1 1 −1 −1 −1 −1 1 1 −1 1 0 1 −1 1 1
17 1 1 −1 1 −1 −1 −1 1 1 −1 −1 −1 1 −1 1 1 0 1 1 −1 1 −1 −1 −1 1 1 −1 −1 −1 1
19 1 −1 −1 1 1 1 1 −1 1 −1 1 −1 −1 −1 −1 1 1 −1 0 1 −1 −1 1 1 1 1 −1 1 −1 1
23 1 1 1 1 −1 1 −1 1 1 −1 −1 1 1 −1 −1 1 −1 1 −1 −1 −1 −1 0 1 1 1 1 −1 1 −1
29 1 −1 −1 1 1 1 1 −1 1 −1 −1 −1 1 −1 −1 1 −1 −1 −1 1 −1 1 1 1 1 −1 −1 1 0 1
31 1 1 −1 1 1 −1 1 1 1 1 −1 −1 −1 1 −1 1 −1 1 1 1 −1 −1 −1 −1 1 −1 −1 1 −1 −1
37 1 −1 1 1 −1 −1 1 −1 1 1 1 1 −1 −1 −1 1 −1 −1 −1 −1 1 −1 −1 −1 1 1 1 1 −1 1
41 1 1 −1 1 1 −1 −1 1 1 1 −1 −1 −1 −1 −1 1 −1 1 −1 1 1 −1 1 −1 1 −1 −1 −1 −1 −1
43 1 −1 −1 1 −1 1 −1 −1 1 1 1 −1 1 1 1 1 1 −1 −1 −1 1 −1 1 1 1 −1 −1 −1 −1 −1
47 1 1 1 1 −1 1 1 1 1 −1 −1 1 −1 1 −1 1 1 1 −1 −1 1 −1 −1 1 1 −1 1 1 −1 −1
53 1 −1 −1 1 −1 1 1 −1 1 1 1 −1 1 −1 1 1 1 −1 −1 −1 −1 −1 −1 1 1 −1 −1 1 1 −1
59 1 −1 1 1 1 −1 1 −1 1 −1 −1 1 −1 −1 1 1 1 −1 1 1 1 1 −1 −1 1 1 1 1 1 −1
61 1 −1 1 1 1 −1 −1 −1 1 −1 −1 1 1 1 1 1 −1 −1 1 1 −1 1 −1 −1 1 −1 1 −1 −1 −1
67 1 −1 −1 1 −1 1 −1 −1 1 1 −1 −1 −1 1 1 1 1 −1 1 −1 1 1 1 1 1 1 −1 −1 1 −1
71 1 1 1 1 1 1 −1 1 1 1 −1 1 −1 −1 1 1 −1 1 1 1 −1 −1 −1 1 1 −1 1 −1 1 1
73 1 1 1 1 −1 1 −1 1 1 −1 −1 1 −1 −1 −1 1 −1 1 1 −1 −1 −1 1 1 1 −1 1 −1 −1 −1
79 1 1 −1 1 1 −1 −1 1 1 1 1 −1 1 −1 −1 1 −1 1 1 1 1 1 1 −1 1 1 −1 −1 −1 −1
83 1 −1 1 1 −1 −1 1 −1 1 1 1 1 −1 −1 −1 1 1 −1 −1 −1 1 −1 1 −1 1 1 1 1 1 1
89 1 1 −1 1 1 −1 −1 1 1 1 1 −1 −1 −1 −1 1 1 1 −1 1 1 1 −1 −1 1 −1 −1 −1 −1 −1
97 1 1 1 1 −1 1 −1 1 1 −1 1 1 −1 −1 −1 1 −1 1 −1 −1 −1 1 −1 1 1 −1 1 −1 −1 −1
101 1 −1 −1 1 1 1 −1 −1 1 −1 −1 −1 1 1 −1 1 1 −1 1 1 1 1 1 1 1 −1 −1 −1 −1 1
103 1 1 −1 1 −1 −1 1 1 1 −1 −1 −1 1 1 1 1 1 1 1 −1 −1 −1 1 −1 1 1 −1 1 1 1
107 1 −1 1 1 −1 −1 −1 −1 1 1 1 1 1 1 −1 1 −1 −1 1 −1 −1 −1 1 −1 1 −1 1 −1 1 1
109 1 −1 1 1 1 −1 1 −1 1 −1 −1 1 −1 −1 1 1 −1 −1 −1 1 1 1 −1 −1 1 1 1 1 1 −1
113 1 1 −1 1 −1 −1 1 1 1 −1 1 −1 1 1 1 1 −1 1 −1 −1 −1 1 −1 −1 1 1 −1 1 −1 1
127 1 1 −1 1 −1 −1 −1 1 1 −1 1 −1 1 −1 1 1 1 1 1 −1 1 1 −1 −1 1 1 −1 −1 −1 1

Properties of the Legendre symbol

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There are a number of useful properties of the Legendre symbol which, together with the law of quadratic reciprocity, can be used to compute it efficiently.

  • Given a generator  , if  , then   is a quadratic residue if and only if   is even. This shows that half of the elements in   are quadratic residues.
  • If   then the fact that
      gives us that   is a square root of the quadratic residue  .
  • The Legendre symbol is periodic in its first (or top) argument: if ab (mod p), then
     
  • The Legendre symbol is a completely multiplicative function of its top argument:
     
  • In particular, the product of two numbers that are both quadratic residues or quadratic non-residues modulo p is a residue, whereas the product of a residue with a non-residue is a non-residue. A special case is the Legendre symbol of a square:
     
  • When viewed as a function of a, the Legendre symbol   is the unique quadratic (or order 2) Dirichlet character modulo p.
  • The first supplement to the law of quadratic reciprocity:
     
  • The second supplement to the law of quadratic reciprocity:
     
  • Special formulas for the Legendre symbol   for small values of a:
    • For an odd prime p ≠ 3,
       
    • For an odd prime p ≠ 5,
       
  • The Fibonacci numbers 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, ... are defined by the recurrence F1 = F2 = 1, Fn+1 = Fn + Fn−1. If p is a prime number then
     
For example,
 

Legendre symbol and quadratic reciprocity

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Let p and q be distinct odd primes. Using the Legendre symbol, the quadratic reciprocity law can be stated concisely:

 

Many proofs of quadratic reciprocity are based on Euler's criterion

 

In addition, several alternative expressions for the Legendre symbol were devised in order to produce various proofs of the quadratic reciprocity law.

 
in his fourth[4] and sixth[5] proofs of quadratic reciprocity.
 
Reversing the roles of p and q, he obtains the relation between (p/q) and (q/p).
 
Using certain elliptic functions instead of the sine function, Eisenstein was able to prove cubic and quartic reciprocity as well.
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  • The Jacobi symbol (a/n) is a generalization of the Legendre symbol that allows for a composite second (bottom) argument n, although n must still be odd and positive. This generalization provides an efficient way to compute all Legendre symbols without performing factorization along the way.
  • A further extension is the Kronecker symbol, in which the bottom argument may be any integer.
  • The power residue symbol (a/n)n generalizes the Legendre symbol to higher power n. The Legendre symbol represents the power residue symbol for n = 2.

Computational example

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The above properties, including the law of quadratic reciprocity, can be used to evaluate any Legendre symbol. For example:

 

Or using a more efficient computation:

 

The article Jacobi symbol has more examples of Legendre symbol manipulation.

Since no efficient factorization algorithm is known, but efficient modular exponentiation algorithms are, in general it is more efficient to use Legendre's original definition, e.g.

 

using repeated squaring modulo 331, reducing every value using the modulus after every operation to avoid computation with large integers.

Notes

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  1. ^ Legendre, A. M. (1798). Essai sur la théorie des nombres. Paris. p. 186.
  2. ^ Hardy & Wright, Thm. 83.
  3. ^ Ribenboim, p. 64; Lemmermeyer, ex. 2.25–2.28, pp. 73–74.
  4. ^ Gauss, "Summierung gewisser Reihen von besonderer Art" (1811), reprinted in Untersuchungen ... pp. 463–495
  5. ^ Gauss, "Neue Beweise und Erweiterungen des Fundamentalsatzes in der Lehre von den quadratischen Resten" (1818) reprinted in Untersuchungen ... pp. 501–505
  6. ^ Lemmermeyer, ex. p. 31, 1.34
  7. ^ Lemmermeyer, pp. 236 ff.

References

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