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A number is prime if it has exactly two factors ... 1 and itself. For example, 17 is prime because its only factors are 1 and 17. However, 35 is not prime, since it has factors 1, 35, as well as 5 and 7 The number 2 is prime, since its factors are 1 and 2 only. The number 2 is the only even prime number. No other even number can be prime, because it will always have 2 as a factor. For example, 18 isn't prime ... besides 1 and 18, it must also have factor 2 because it's even. Here are the first ten prime numbers: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, ... The 'prime' definition only applies to natural numbers, not to zero, negative numbers or non-whole numbers. It can sometimes be difficult to tell if a very large odd number is prime. For example, is 493 prime? The answer is no ... it has factors 1, 493, and also 17 and 29. Below is a handy prime number checker. Enter any number and press the 'calculate' button; it will tell you whether the number is prime or not, and if not, it will tell you one additional factor. Give it a try ... enter 31031 (don't use commas). |