# 半素数

## 例子与种类

4, 6, 9, 10, 14, 15, 21, 22, 25, 26, 33, 34, 35, 38, 39, 46, 49, 51, 55, 57, 58, 62, 65, 69, 74, 77, 82, 85, 86, 87, 91, 93, 94, 95 （OEIS數列A001358）.

6, 10, 14, 15, 21, 22, 26, 33, 34, 35, 38, 39, 46, 51, 55, 57, 58, 62, 65, 69, 74, 77, 82, 85, 86, 87, 91, 93, 94, 95, ... （OEIS數列A006881

1, 2, 3, 4, 5, 6, 7, 9, 10, 11, 13, 14, 15, 17, 19, 21, 22, 23, 25, 26, 29, 31, 33, 34, 35, 37, 38, 39, 41, 43, 46, 47, 49, ... （OEIS數列A037143

## 性质

${\displaystyle \varphi (n)=(p-1)(q-1)=n-(p+q)+1.}$

${\displaystyle \varphi (n)=p(p-1)=n-p.}$

## 应用

1974年，阿雷西博信息通过无线电信号被发向星团。其由1679个二进制数字组成，这些数字的用意是让接收方将信息解析成位图图像。选择数字${\displaystyle 1679=23\cdot 73}$是因为其是一个半素数，只存在一种构成矩形图像的可能（up to 图像平面的旋转和反射）。[6]

## 參考資料與附註

1. ^ Sloane, N.J.A. (编). Sequence A001358. The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
2. ^ Stewart, Ian. Professor Stewart's Cabinet of Mathematical Curiosities. Profile Books. 2010: 154 [2018-07-14]. ISBN 9781847651280. （原始内容存档于2021-04-28）.
3. ^ French, John Homer. Advanced Arithmetic for Secondary Schools. New York: Harper & Brothers. 1889: 53.
4. Cozzens, Margaret; Miller, Steven J., The Mathematics of Encryption: An Elementary Introduction, Mathematical World 29, American Mathematical Society: 237, 2013 [2018-07-14], ISBN 9780821883211, （原始内容存档于2019-07-22）
5. ^ The RSA Factoring Challenge. [2012-08-04]. （原始内容存档于2013-07-27）.
6. ^ du Sautoy, Marcus. The Number Mysteries: A Mathematical Odyssey through Everyday Life. St. Martin's Press. 2011: 19 [2018-07-14]. ISBN 9780230120280. （原始内容存档于2021-04-28）.