How do you determine whether a number is prime or composite? - Answers

Negative numbers, zero, and one are neither prime nor composite. See the related questions as to an explanation why. See whether the number is divisible by 2, and by all odd numbers up to the square root of the number. For numbers up to 100, it is enough to check the factors 2, 3, 5, and 7. For higher numbers you need to check more factors. For very large numbers (for example, hundreds of digits), more efficient methods are known, but those methods are also more complicated. Check the related link for a number factoring calculator.