How can you quickly determine if a number is prime or composite?

I can quickly determine that any even number larger than 2 is composite, and any odd number larger than 5 which ends with 5 is also composite.

That leaves numbers ending with 1,3,7,9. Is there a quick way to determine odd numbers ending with one of those especially, when you get past 100.
So it seems the short answer is: find a chart of prime numbers.

Look for a chart on the Internet. I found one chart up to 10,000 without breaking a sweat.

http://www.google.com/search?q=prime+number+chart&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

If there were an easy formula, people wouldn’t be spending centuries trying to find new primes.

This entry was posted on Sunday, May 30th, 2010 at 9:59 am and is filed under composite. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

One Response to “How can you quickly determine if a number is prime or composite?”

Eric on May 30th, 2010 at 3:23 pm

Look for a chart on the Internet. I found one chart up to 10,000 without breaking a sweat.

http://www.google.com/search?q=prime+number+chart&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

If there were an easy formula, people wouldn’t be spending centuries trying to find new primes.
References :

Composite