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Natural number
281 is the natural number following 280 and preceding 282. It is also a prime number.
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, [3] Eisenstein prime with no imaginary part, and a centered decagonal number. [4]
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 281 p − 1 280 {\displaystyle {\frac {281^{p}-1}{280}}} is composite for all prime p < 60000.
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