# 半素数

（重定向自半質數

## 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中的数列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中的数列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中的数列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. ISBN 9781847651280.
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, ISBN 9780821883211
5. ^ The RSA Factoring Challenge
6. ^ du Sautoy, Marcus. The Number Mysteries: A Mathematical Odyssey through Everyday Life. St. Martin's Press. 2011: 19. ISBN 9780230120280.