281 (number)

← 280 281 282 →
← 200 210 220 230 240 250 260 270 280 290 → List of numbers Integers ← 0 100 200 300 400 500 600 700 800 900 →
Cardinal	two hundred eighty-one
Ordinal	281st (two hundred eighty-first)
Factorization	prime
Prime	60th
Greek numeral	ΣΠΑ´
Roman numeral	CCLXXXI, cclxxxi
Binary	1000110012
Ternary	1011023
Senary	11456
Octal	4318
Duodecimal	1B512
Hexadecimal	11916

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, ^[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 ${\displaystyle {\frac {281^{p}-1}{280}}}$ is composite for all prime p < 60000.

References

1. Sloane, N. J. A. (ed.). "SequenceA001359(Lesser of twin primes)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
2. Sloane, N. J. A. (ed.). "SequenceA005384(Sophie Germain primes p: 2p+1 is also prime)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
3. Sloane, N. J. A. (ed.). "SequenceA109611(chen prime)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.
4. Sloane, N. J. A. (ed.). "SequenceA062786(centered 10-gonal numbers)". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation.