# Difference between revisions of "Smallest quadratic nonresidue is less than square root plus one"

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==Statement== | ==Statement== | ||

− | Suppose <math>p</math> is an odd prime. Suppose <math>a</math> is the smallest positive integer less than <math>p</math> that is a [[quadratic nonresidue]] modulo <math>p</math>. Then: | + | Suppose <math>p</math> is an odd prime. Suppose <math>a</math> is the [[smallest quadratic nonresidue]] modulo <math>p</math>: <math>a</math> is the smallest positive integer less than <math>p</math> that is a [[quadratic nonresidue]] modulo <math>p</math>. Then: |

<math>a < \sqrt{p} + 1</math>. | <math>a < \sqrt{p} + 1</math>. | ||

Note that we need the <math>+1</math> additive correction term. For instance, in the cases <math>p = 3</math> and <math>p = 7</math>, the smallest quadratic nonresidues (<math>2</math> and <math>3</math> respectively) are greater than the squareroot. | Note that we need the <math>+1</math> additive correction term. For instance, in the cases <math>p = 3</math> and <math>p = 7</math>, the smallest quadratic nonresidues (<math>2</math> and <math>3</math> respectively) are greater than the squareroot. | ||

+ | |||

+ | In fact, since the smallest quadratic nonresidue modulo <math>p</math> must be prime, we conclude that there is a prime less than <math>\sqrt{p} + 1</math> that is a quadratic nonresidue modulo <math>p</math>. | ||

==Related facts== | ==Related facts== |

## Revision as of 18:27, 2 May 2009

## Statement

Suppose is an odd prime. Suppose is the smallest quadratic nonresidue modulo : is the smallest positive integer less than that is a quadratic nonresidue modulo . Then:

.

Note that we need the additive correction term. For instance, in the cases and , the smallest quadratic nonresidues ( and respectively) are greater than the squareroot.

In fact, since the smallest quadratic nonresidue modulo must be prime, we conclude that there is a prime less than that is a quadratic nonresidue modulo .

## Related facts

- Extended Riemann hypothesis: A stronger form of the Riemann hypothesis that is equivalent to the statement that the smallest quadratic nonresidue modulo , for any odd prime , is less than , where here is the natural logarithm.
- Artin's conjecture on primitive roots: This is a closely related conjecture that states that every integer that is not or a perfect square occurs as a primitive root for infinitely many primes.

## Proof

**Given**: An odd prime , is the smallest positive integer that is a quadratic nonresidue modulo .

**To prove**: .

**Proof**: Let be the smallest positive integer such that , and . Since is prime, , so . Further, by the multiplicativity of Legendre symbols:

.

Since is a quadratic nonresidue mod , the first term is , so at least one of and is a nonresidue. Since and is the least nonresidue by assumption, must be a quadratic residue, forcing to be a quadratic nonresidue, and hence, . Since , we get . In particular, since , from which we can deduce that .