251 (number)
From Infogalactic: the planetary knowledge core



Cardinal  two hundred fiftyone  
Ordinal  251st (two hundred and fiftyfirst) 

Factorization  251  
Prime  54th  
Roman numeral  CCLI  
Binary  11111011_{2}  
Ternary  100022_{3}  
Quaternary  3323_{4}  
Quinary  2001_{5}  
Senary  1055_{6}  
Octal  373_{8}  
Duodecimal  18B_{12}  
Hexadecimal  FB_{16}  
Vigesimal  CB_{20}  
Base 36  6Z_{36} 
251 (two hundred [and] fiftyone) is the natural number between 250 and 252. It is also a prime number.
In mathematics
251 is a Sophie Germain prime.^{[1]} Every 5 × 5 matrix has exactly 251 square submatrices.^{[2]} 251 is a de Polignac number, meaning that it is odd and cannot be formed by adding a power of two to a prime number.^{[3]}^{[4]} It is the smallest number that can be formed in more than one way by summing three positive cubes:^{[5]}^{[6]}
References
 ↑ "Sloane's A005384 : Sophie Germain primes p: 2p+1 is also prime", The OnLine Encyclopedia of Integer Sequences. OEIS Foundation.
 ↑ "Sloane's A030662 : Number of combinations of n things from 1 to n at a time, with repeats allowed", The OnLine Encyclopedia of Integer Sequences. OEIS Foundation.
 ↑ "Sloane's A006285 : Odd numbers not of form p + 2^x (de Polignac numbers)", The OnLine Encyclopedia of Integer Sequences. OEIS Foundation.
 ↑ Kozek, Mark Robert (2007), Applications of Covering Systems of Integers and Goldbach's Conjecture for Monic Polynomials, PhD dissertation, University of South Carolina, p. 14, ISBN 9780549210207.
 ↑ "Sloane's A008917 : Numbers that are the sum of 3 positive cubes in more than one way", The OnLine Encyclopedia of Integer Sequences. OEIS Foundation.
 ↑ De Koninck, JeanMarie (2009), Those fascinating numbers, Providence, RI: American Mathematical Society, p. 64, ISBN 9780821848074, MR 2532459.