How can you tell if a number is prime? And what is the fastest way to tell if a number is a prime number?
A prime number is a number that has just two factors: the number 1 and itself. Therefore, when finding all the factors of a number if you end up with just two numbers then you have a prime number.
Using the same process as you would normally when finding factors for a given number, the same is applied when determining whether a number is prime. Once you’ve completed finding all the factors simply add up all the factors and if you see just the number 1 and your original number, then you likely have a prime number.
So if 2 is an even number and is a prime number, does that mean the next even number, 4, is a prime too?
To find out if 4 is or is not a prime number apply the simple step by step technique below.
Step By Step Prime Number Example
Here is the process if you want to find if a number is a prime number. I’ll use for this example the number 4.
To see if 4 is prime start at the first factor 2 (as all numbers have 1 as a factor).
Does 2 divide into 4 without leaving a remainder?
Yes, 4 is an even number, therefore, it has 2 as a factor.
Therefore, 4 is not a prime number.
As you can see with this same simple step by step guide that’s exactly the same process when finding factors of a number, you can see that if you find a factor you can stop as you know this number is not a prime number.
Therefore, whenever you’re finding out if a number is prime you can short-circuit the factor finding process and stop whenever you find a factor that isn’t 1 or the original number itself.
Summary
A prime number is a number that has only 2 factors: the number 1 and itself.
The number 4 is not a prime number as it has 1, 2 and 4 as it’s factors.