Prime numbers explained

by Ben Alford

A prime number can only be divided by itself and by 1, without a remainder . For example, 11 can be divided only by 11 and by 1. 2 is the only even prime number. All other even numbers can be divided by 2. See below for a table of all the prime numbers up to 1000.

Some facts:

  • If the sum of a number’s digits is a multiple of 3, that number can be divided by 3.
  • No prime number greater than 5 ends in a 5. Any number greater than 5 that ends in a 5 can be divided by 5.
  • Zero and 1 are not considered prime numbers.
  • Except for 0 and 1, a number is either a prime number or a composite number. A composite number is defined as any number, greater than 1, that is not prime.

How to prove whether a number is prime

first try dividing it by 2, and see if you get a whole number. If you do, it can’t be a prime number. If you don’t get a whole number, next try dividing it by prime numbers: 3, 5, 7, 11 (9 is divisible by 3) and so on, always dividing by a prime number.

 First 1000 prime numbersScreen Shot 2013-03-05 at 14.52.41

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