One way is to test all the prime numbers equal or less than Sqrt[251]. These are: {2,3,5,7,11,13}.
We can eliminate 2, 3, and 5 because the number isn't odd, it's digits don't sum to a multiple of 3, and the last digit isn't a 5 or a 0.
For 7, note that 252 = 7*36, so 7 cannot divide 251 because that would imply 7 divides 1.
For 11, note that 253 = 11*23, so 11 cannot divide 251 because that would imply 11 divides 2.
For 13, note that 13*19 = 247, so 13 cannot divide 251 because that would imply 13 divides 4.
Hence, 251 is prime
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It is a prime number, and therefore not a composite number.251 is prime.
None. 251 is a prime number.
Just 1 and 251 because it is a prime number.
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