Check whether a number is prime, and see the nearest primes. Enter a whole number.
Result
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Prime or composite.Usage Tip
To test a small number, you only need to check division by primes up to its square root.
THE MATH
Prime: a whole number above 1 with exactly two divisors (1 and itself)
Tested by trial division up to the square root
Tested by trial division up to the square root
Prime numbers are the building blocks of the integers; every whole number factors uniquely into primes.
Composite numbers have at least one divisor besides 1 and themselves.
Composite numbers have at least one divisor besides 1 and themselves.
2 is the only even prime; all other evens are composite.
1 is neither prime nor composite by definition.
The next and previous primes are shown for context.
1 is neither prime nor composite by definition.
The next and previous primes are shown for context.