I'm working on finding prime numbers and wanted someone to let me know if this method has already been used.

Using Eratosthenes sieve method, we know that multiples of prime numbers cannot be prime, since they can be divided by the prime number itself.

For example: 3*2=6 cannot be prime since it's divisible by 3.

Since 2 and 5 are prime, this means all even numbers >2 and all multiples of 5 that are >5 are not prime.

So just by looking at these numbers, you know they aren't prime because they are either even or a multiply of 5:

192838717485
818273718298
etc.

But I noticed that if you put the multiples of a number into the same number, that number is also not prime.

For example, here are the multiples of 3:

3
6
9
12
15

That means if I put 12 and 15 together like this: 1215, that is not a prime number. Since it is composed of numbers that are multiples of 3.

I haven't found anything to suggest that people are aware of this, but perhaps it is a way to make the sieve more efficient?

Basically, we should be able to feed a computer a bunch of multiples of primes and then have it spit out all the primes from 0 to n through deduction.