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 November 24th, 2017, 10:06 AM #1 Newbie   Joined: Apr 2014 From: Edmonton Posts: 4 Thanks: 0 A question about primes Hi, I have a question about primes here. I was just wondering if any whole number plus one half could be used to find primes. for example, 1.5,2.5,3.5... The way I see it, you shouldn't round up or down from these numbers and I was wondering if anybody could establish for me the logic behind rounding up, say, from 1.5 to 2.
 November 24th, 2017, 10:16 AM #2 Global Moderator     Joined: Oct 2008 From: London, Ontario, Canada - The Forest City Posts: 7,661 Thanks: 965 Math Focus: Elementary mathematics and beyond Once you have a candidate how would you test it for primality?
November 24th, 2017, 06:59 PM   #3
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Quote:
 Originally Posted by Pegleggedninja Hi, I have a question about primes here. I was just wondering if any whole number plus one half could be used to find primes. for example, 1.5,2.5,3.5... The way I see it, you shouldn't round up or down from these numbers and I was wondering if anybody could establish for me the logic behind rounding up, say, from 1.5 to 2.
What in the world does this have to do with prime numbers? When thinking about prime numbers, we are only dealing with whole numbers.

If you round up 3.5 you get 4, which is NOT prime.

Last edited by skipjack; November 24th, 2017 at 09:44 PM.

 November 24th, 2017, 10:33 PM #4 Global Moderator   Joined: Dec 2006 Posts: 18,248 Thanks: 1439 That wouldn't stop one from asking whether, for some prime $p$ (such as 131), $q = \sqrt{4p-163}$ is an integer, and $p + q + 1$ another prime.

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