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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  
Senior Member Joined: May 2016 From: USA Posts: 857 Thanks: 348  Quote:
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), $q = \sqrt{4p163}$ is an integer, and $p + q + 1$ another prime.


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