[the frog]

Hello you guys. I am studying for the GRE... and I had found that I forgot most high school math. I remember that once sometime told me that there was a way to now if a more than two digit number was a prime, but I don´t remember it. I have already researched about it in your post but can´t find an easy, fast, and quick way. Thanks!!

[ROLCAM]

PLEASE SEE:-

List of prime numbers Summary.

Enjoy!

[ebaines]

If you want to test whether a given number is prime, the basic technique is to try dividing the number in turn by each of the prime numbers start with 2, then 3, then 5, etc. If you get up to testing a prime which is greater than the square root of the number without having any that successfully divides into the number, then the number is prime.

For example: consider the number 97. You can test it as follows:

a. Divisible by 2? No, since 97 is not an even number.
b. Divisible by 3? No, since 9 + 7 =16 is not divisible by 3.
c. Divisible by 5? No, since the last digit is neither a 5 nor 0.
d. Divisible by 7? No - 7 divides into 91, so if you tried to divide into 97 you would have a remainder of 6.

Note that at this point you have tested all primes up through 7. Since 7^2 = 49, which is less than 97, we need to keep going.

e. Divisible by 11? You can check easily enough, but you really don't have to - since 11^2 = 121, which is greater than 97, there is really no need as we have checked all we need to. We can conclude that 97 is a prime number.

Of course, this method requires that you know all the primes in order up through the square root of the number. If you're not sure, it's best to err by trying to divide the number by a non-prime - it doesn't hurt. So in his example if you forgot that 9 was not prime, and tried to see if 97 is divisible by 9, you would find it is not and just keep going.