# Difference between revisions of "Bertrand's postulate"

(New page: Bertrand's postulate asserts that for every positive integer N, there is a prime between N and 2N. Despite its name, it is actually a theorem rather than a postulate. For large N, the ...) |
(Added segue to proof, and statement of Chebyshev bound) |
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− | Bertrand's postulate | + | Despite it's name, Bertrand's postulate is actually a theorem rather than a postulate: |

− | For | + | '''Theorem (Bertrand's Postulate):''' For every integer <math>n \geq 2</math>, there is a prime <math>p</math> satisfying <math>n < p < 2n</math>. |

− | The relevance of the postulate for the [[finding primes]] problem is that it guarantees the existence of a k-digit prime for any k. Brute force search thus yields a k-digit prime after about <math>O(10^k)</math> steps; this can be considered the "trivial bound" for the problem. | + | The relevance of the postulate for the [[finding primes]] problem is that it guarantees the existence of a <math>k</math>-digit prime for any <math>k</math>. Brute force search thus yields a <math>k</math>-digit prime after about <math>O(10^k)</math> steps; this can be considered the "trivial bound" for the problem. |

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+ | This result was apparently first proved by Chebyshev. For large <math>n</math>, the claim follows as a consequence of the prime number theorem. We will give an elementary proof due to Erdos. | ||

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+ | Our proof of Bertrand's postulate starts with a result independent interest, due to Chebyshev. | ||

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+ | '''Lemma (Chebyshev bound):''' For integers <math>n \geq 2</math>, | ||

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+ | <math> \prod_{p \leq n} p \leq 4^n, </math> | ||

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+ | where the product is over all primes <math>p</math> less than or equal to <math>n</math>. | ||

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+ | [TBC] |

## Revision as of 15:57, 11 August 2009

Despite it's name, Bertrand's postulate is actually a theorem rather than a postulate:

**Theorem (Bertrand's Postulate):** For every integer [math]n \geq 2[/math], there is a prime [math]p[/math] satisfying [math]n \lt p \lt 2n[/math].

The relevance of the postulate for the finding primes problem is that it guarantees the existence of a [math]k[/math]-digit prime for any [math]k[/math]. Brute force search thus yields a [math]k[/math]-digit prime after about [math]O(10^k)[/math] steps; this can be considered the "trivial bound" for the problem.

This result was apparently first proved by Chebyshev. For large [math]n[/math], the claim follows as a consequence of the prime number theorem. We will give an elementary proof due to Erdos.

Our proof of Bertrand's postulate starts with a result independent interest, due to Chebyshev.

**Lemma (Chebyshev bound):** For integers [math]n \geq 2[/math],

[math] \prod_{p \leq n} p \leq 4^n, [/math]

where the product is over all primes [math]p[/math] less than or equal to [math]n[/math].

[TBC]