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Prime Numbers to 400

Ok, if you've read the first blog on prime numbers, you know some rules that help you reduce the number of numbers you have to check to see if a number is prime.

One more rule that may help is the rule that:

**All prime numbers are either +1 or -1 from a multiple of 6.** Not all +1's and -1's are primes, but all primes are +1 or -1.

What this rule does is move you one away from all the numbers that can be divided by 2 or 3.

Here's another chart of what numbers you have to check as divisors for all primes up to 400.

This chart doesn't prove it, but it gives you hint of a basic rule of all numbers:

**Every number is either prime or can be factored (divided) into primes.**

This is true just from the way we define prime numbers

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