Английская Википедия:281 (number)
Материал из Онлайн справочника
281 is the natural number following 280 and preceding 282. It is also a prime number.
In mathematics
281 is a twin prime with 283,[1] Sophie Germain prime,[2] sum of the first fourteen primes, sum of seven consecutive primes (29 + 31 + 37 + 41 + 43 + 47 + 53), Chen prime, Eisenstein prime with no imaginary part, and a centered decagonal number.
281 is the smallest prime p such that the decimal period length of the reciprocal of p is (p−1)/10, i.e. the period length of 1/281 is 28. However, in binary, it has period length 70.
The generalized repunit number <math>\frac{281^p-1}{280}</math> is composite for all prime p < 60000.
References
Шаблон:Reflist Шаблон:Integers
