“Euclid's lemma”的意思、由来-开放百科全书

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Euclid's lemma

释义

Formulations
History
Proof
Proof using Bézout's lemma Proof of Elements
See also
Footnotes
Notes Citations
References
External links

In number theory, Euclid's lemma is a lemma that captures a fundamental property of prime numbers, namely:{{refn|group=note|It is also called Euclid's first theorem^[1]^[2] although that name more properly belongs to the side-angle-side condition for showing that triangles are congruent.^[3]}} {{math_theorem|name=Euclid's lemma|math_statement=If a prime {{math|p}} divides the product {{math|ab}} of two integers {{math|a}} and {{math|b}}, then {{math|p}} must divide at least one of those integers {{math|a}} and {{math|b}}.}} For example, if {{math|p {{=}} 19}}, {{math|a {{=}} 133}}, {{math|b {{=}} 143}}, then {{math|ab {{=}} 133 × 143 {{=}} 19019}}, and since this is divisible by 19, the lemma implies that one or both of 133 or 143 must be as well. In fact, {{math| 133 {{=}} 19 × 7}}.

Inherently, if the premise of the lemma does not hold, i.e., {{math|p}} is a composite number, its consequent may be either true or false. For example, in the case of {{math|p {{=}} 10}}, {{math|a {{=}} 4}}, {{math|b {{=}} 15}}, composite number 10 divides {{math|ab {{=}} 4 × 15 {{=}} 60}}, but 10 divides neither 4 nor 15.

This property is the key in the proof of the fundamental theorem of arithmetic.{{refn|group=note|In general, to show that a domain is a unique factorization domain, it suffices to prove Euclid's lemma and the ascending chain condition on principal ideals (ACCP)}} It is used to define prime elements, a generalization of prime numbers to arbitrary commutative rings. Euclid's Lemma shows that in the integers irreducible elements are also prime elements. The proof uses induction so it does not apply to all integral domains.

Formulations

Let {{math|p}} be a prime number, and assume {{math|p}} divides the product of two integers {{math|a}} and {{math|b}}. (In symbols this is written {{math|p|ab}}. Its negation, {{math|p}} does not divide {{math|ab}} is written {{math|p∤ab}}.) Then {{math|p|a}} or {{math|p|b}} (or both). Equivalent statements are:

If {{math|p∤a}} and {{math|p∤b}}, then {{math|p∤ab}}.
If {{math|p∤a}} and {{math|p|ab}}, then {{math|p|b}}.

Euclid's lemma can be generalized from prime numbers to any integers:

{{math_theorem|If n|ab, and n is relatively prime to a, then n|b.}}

This is a generalization because if {{math|n}} is prime, either

{{math|n|a}} or
{{math|n}} is relatively prime to {{math|a}}. In this second possibility, {{math|n∤a}} so {{math|n|b}}..

History

The lemma first appears as proposition 30 in Book VII of Euclid's Elements. It is included in practically every book that covers elementary number theory.^[4]^[5]^[6]^[7]^[8]

The generalization of the lemma to integers appeared in Jean Prestet's textbook Nouveaux Elémens de Mathématiques in 1681.^[9]

In Carl Friedrich Gauss's treatise Disquisitiones Arithmeticae, the statement of the lemma is Euclid's Proposition 14 (Section 2), which he uses to prove the uniqueness of the decomposition product of prime factors of an integer (Theorem 16), admitting the existence as "obvious." From this existence and uniqueness he then deduces the generalization of prime numbers to integers.^[10] For this reason, the generalization of Euclid's lemma is sometimes referred to as Gauss's lemma, but some believe this usage is incorrect^[11] due to confusion with Gauss's lemma on quadratic residues.

Proof

Proof using Bézout's lemma

The usual proof involves another lemma called Bézout's identity. ^[12] This states that if {{math|x}} and {{math|y}} are relatively prime integers (i.e. they share no common divisors other than 1 and -1) there exist integers {{math|r}} and {{math|s}} such that

Let {{math|a}} and {{math|n}} be relatively prime, and assume that {{math|n|ab}}. By Bézout's identity, there are {{math|r}} and {{math|s}} making

Multiply both sides by {{math|b}}:

The first term on the left is divisible by {{math|n}}, and the second term is divisible by {{math|ab}}, which by hypothesis is divisible by {{math|n}}. Therefore their sum, {{math|b}}, is also divisible by {{math|n}}. This is the generalization of Euclid's lemma mentioned above.

Proof of Elements

Euclid's lemma is proved at the Proposition 30 in Book VII of Elements. The original proof is difficult to understand as is, so we quote the commentary from {{Harvtxt|Euclid|1956|pp=319-332}}.

{{Anchor|Proposition 19|}}Proposition 19

If four numbers be proportional, the number produced from the first and fourth is equal to the number produced from the second and third; and, if the number produced from the first and fourth be equal to that produced from the second and third, the four numbers are proportional. {{refn|group=note|If a ： b＝c ： d, then ad＝bc; and conversely. ^[13]}}

{{Anchor|Proposition 20|}}Proposition 20

The least numbers of those that have the same ratio with them measures those that have the same ratio the same number of times—the greater the greater and the less the less. {{refn|group=note|If a ： b＝c ： d, and a, b are the least numbers among those that have the same ratio, then c＝na, d＝nb, where n is some integer. ^[14]}}

{{Anchor|Proposition 21|}}Proposition 21

Numbers prime to one another are the least of those that have the same ratio with them. {{refn|group=note|If a ： b＝c ： d, and a, b are prime to one another, then a, b are the least numbers among those that have the same ratio. ^[15]}}

{{Anchor|Proposition 29|}}Proposition 29

Any prime number is prime to any number it does not measure. {{refn|group=note|If a is prime and does not measure b, then a, b are prime to one another. ^[16]}}

{{Anchor|Proposition 30|}}Proposition 30

If two numbers, by multiplying one another, make the same number, and any prime number measures the product, it also measures one of the original numbers. {{refn|group=note|If c, a prime number, measure ab, c measures either a or b. ^[17]}}

Proof

If c, a prime number, measure ab, c measures either a or b.
Suppose c does not measure a.
Therefore c, a are prime to one another. ［VII. 29］
Suppose ab＝mc.
Therefore c ： a＝b ： m. ［VII. 19］
Hence ［VII. 20, 21］ b＝nc, where n is some integer.
Therefore c measures b.
Similarly, if c does not measure b, c measures a.
Therefore c measures one or other of the two numbers a, b.
Q.E.D. ^[18]

Footnotes

Notes

1. ^{{Harvnb|Bajnok|2013|loc=Theorem 14.5}}
2. ^{{Harvnb|Joyner|Kreminski|Turisco|2004|loc=Proposition 1.5.8, p. 25}}
3. ^{{Harvnb|Martin|2012|p=125}}
4. ^{{Harvnb|Gauss|2001|p=14}}
5. ^{{Harvnb|Hardy|Wright|Wiles|2008|loc=Theorem 3}}
6. ^{{Harvnb|Ireland|Rosen|2010|loc=Proposition 1.1.1}}
7. ^{{Harvnb|Landau|Goodman|1999|loc=Theorem 15}}
8. ^{{Harvnb|Riesel|1994|loc=Theorem A2.1}}
9. ^{{Harvnb|Euclid|1994|pp=338–339}}
10. ^{{Harvnb|Gauss|2001|loc=Article 19}}
11. ^{{mathworld |title = Euclid's Lemma |id = EuclidsLemma}}
12. ^{{Harvnb|Hardy|Wright|Wiles|2008|loc=§2.10}}
13. ^{{Harvnb|Euclid|1956|p=319}}
14. ^{{Harvnb|Euclid|1956|p=321}}
15. ^{{Harvnb|Euclid|1956|p=323}}
16. ^{{Harvnb|Euclid|1956|p=331}}
17. ^{{Harvnb|Euclid|1956|p=332}}
18. ^{{Harvnb|Euclid|1956|pp=331−332}}

Citations

References

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External links

{{MathWorld|title=Euclid's Lemma|id=EuclidsLemma}}

3 : Articles containing proofs|Lemmas|Theorems about prime numbers

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