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.
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.
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