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数学 / 几何原本 / Proposition IX.20

第9卷命题 20 · 素数无穷定理

Prime numbers are more than any assigned multitude of prime numbers.

素数的个数多于任意给定的素数集合。

A A B B C C D E F G G
fig-1

素数无穷:取已知素数 A、B、C 的最小公倍数 DE,在 E 处加单位 EF 得 DF;DF 要么是新素数,要么有不同于 A、B、C 的素因子 G。

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分步证明Step-by-step proof
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  1. Let A, B, C be the assigned prime numbers; I say that there are more prime numbers than A, B, C. For let the least number measured by A, B, C be taken, and let it be DE; let the unit DF be added to DE. Then EF is either prime or not. First, let it be prime; then the prime numbers A, B, C, EF have been found which are more than A, B, C.

    设A、B、C为给定的素数,取被A、B、C整除的最小数DE。

  2. Next, let EF not be prime; therefore it is measured by some prime number. [VII. 31] Let it be measured by the prime number G. I say that G is not the same with any of the numbers A, B, C.

    在DE上加单位DF得EF。若EF是素数,则已找到A、B、C、EF四个素数,多于原集合。

  3. For, if possible, let it be so. Now A, B, C measure DE; therefore G also will measure DE. But it also measures EF.

    若EF不是素数,则它被某个素数G整除。

  4. Therefore G, being a number, will measure the remainder, the unit DF: which is absurd. Therefore G is not the same with any one of the numbers A, B, C. And by hypothesis it is prime.

    若G与A、B、C之一相同,则G整除DE,又整除EF,从而整除差DF(单位),矛盾。故G是不同于A、B、C的新素数。