# Euler's criterion

In number theory, Euler's criterion is a formula for determining whether an integer is a quadratic residue modulo a prime. Precisely,

Let p be an odd prime and a an integer coprime to p. Then

$a^{\tfrac {p-1}{2}}\equiv {\begin{cases}\;\;\,1{\pmod {p}}&{\text{ if there is an integer }}x{\text{ such that }}a\equiv x^{2}{\pmod {p}}\\-1{\pmod {p}}&{\text{ if there is no such integer.}}\end{cases}}$ Euler's criterion can be concisely reformulated using the Legendre symbol:

$\left({\frac {a}{p}}\right)\equiv a^{\tfrac {p-1}{2}}{\pmod {p}}.$ The criterion first appeared in a 1748 paper by Leonhard Euler.

## Proof

The proof uses the fact that the residue classes modulo a prime number are a field. See the article prime field for more details.

Because the modulus is prime, Lagrange's theorem applies: a polynomial of degree k can only have at most k roots. In particular, x2 a (mod p) has at most 2 solutions for each a. This immediately implies that besides 0, there are at least p 1/2 distinct quadratic residues modulo p: each of the p 1 possible values of x can only be accompanied by one other to give the same residue.

In fact, $(p-x)^{2}\equiv x^{2}{\pmod {p}}.$ This is because $(p-x)^{2}\equiv p^{2}-{2}{x}{p}+x^{2}\equiv x^{2}{\pmod {p}}.$ So, the ${\tfrac {p-1}{2}}$ distinct quadratic residues are: $1^{2},2^{2},...,({\tfrac {p-1}{2}})^{2}({\pmod {p}})$ As a is coprime to p, Fermat's little theorem says that

$a^{p-1}\equiv 1{\pmod {p}},$ which can be written as

$\left(a^{\tfrac {p-1}{2}}-1\right)\left(a^{\tfrac {p-1}{2}}+1\right)\equiv 0{\pmod {p}}.$ Since the integers mod p form a field, for each a, one or the other of these factors must be zero.

Now if a is a quadratic residue, ax2,

$a^{\tfrac {p-1}{2}}\equiv {(x^{2})}^{\tfrac {p-1}{2}}\equiv x^{p-1}\equiv 1{\pmod {p}}.$ So every quadratic residue (mod p) makes the first factor zero.

Applying Lagrange's theorem again, we note that there can be no more than p 1/2 values of a that make the first factor zero. But as we noted at the beginning, there are at least p 1/2 distinct quadratic residues (mod p) (besides 0). Therefore they are precisely the residue classes that make the first factor zero. The other p 1/2 residue classes, the nonresidues, must make the second factor zero, or they would not satisfy Fermat's little theorem. This is Euler's criterion.

## Examples

Example 1: Finding primes for which a is a residue

Let a = 17. For which primes p is 17 a quadratic residue?

We can test prime p's manually given the formula above.

In one case, testing p = 3, we have 17(3 − 1)/2 = 171 ≡ 2 ≡ −1 (mod 3), therefore 17 is not a quadratic residue modulo 3.

In another case, testing p = 13, we have 17(13 − 1)/2 = 176 ≡ 1 (mod 13), therefore 17 is a quadratic residue modulo 13. As confirmation, note that 17 ≡ 4 (mod 13), and 22 = 4.

We can do these calculations faster by using various modular arithmetic and Legendre symbol properties.

If we keep calculating the values, we find:

(17/p) = +1 for p = {13, 19, ...} (17 is a quadratic residue modulo these values)
(17/p) = −1 for p = {3, 5, 7, 11, 23, ...} (17 is not a quadratic residue modulo these values).

Example 2: Finding residues given a prime modulus p

Which numbers are squares modulo 17 (quadratic residues modulo 17)?

We can manually calculate it as:

12 = 1
22 = 4
32 = 9
42 = 16
52 = 25 ≡ 8 (mod 17)
62 = 36 ≡ 2 (mod 17)
72 = 49 ≡ 15 (mod 17)
82 = 64 ≡ 13 (mod 17).

So the set of the quadratic residues modulo 17 is {1,2,4,8,9,13,15,16}. Note that we did not need to calculate squares for the values 9 through 16, as they are all negatives of the previously squared values (e.g. 9 ≡ −8 (mod 17), so 92 ≡ (−8)2 = 64 ≡ 13 (mod 17)).

We can find quadratic residues or verify them using the above formula. To test if 2 is a quadratic residue modulo 17, we calculate 2(17 − 1)/2 = 28 ≡ 1 (mod 17), so it is a quadratic residue. To test if 3 is a quadratic residue modulo 17, we calculate 3(17 − 1)/2 = 38 ≡ 16 ≡ −1 (mod 17), so it is not a quadratic residue.

Euler's criterion is related to the Law of quadratic reciprocity.

## Applications

In practice, it is more efficient to use an extended variant of Euclid's algorithm to calculate the Jacobi symbol $\left({\frac {a}{n}}\right)$ . If $n$ is an odd prime, this is equal to the Legendre symbol, and decides whether $a$ is a quadratic residue modulo $n$ .

On the other hand, since the equivalence of $a^{\frac {n-1}{2}}$ to the Jacobi symbol holds for all odd primes, but not necessarily for composite numbers, calculating both and comparing them can be used as a primality test, specifically the Solovay-Strassen primality test. Composite numbers for which the congruence holds for a given $a$ are called Euler–Jacobi pseudoprimes to base $a$ .