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Prime Pairs (Posted on 2002-09-01) Difficulty: 4 of 5
Pairs of primes separated by a single number are called prime pairs. Examples are 17 and 19. Prove that the number between a prime pair is always divisible by 6 (assuming both numbers in the pair are greater than 6).

(From http://www.ocf.berkeley.edu/~wwu/riddles/intro.shtml )

  Submitted by levik    
Rating: 3.3636 (11 votes)
Solution: (Hide)
Let's say the prime numbers in a given prime pair are a-1 and a+1. Since both are greater than six, neither can be divisible by either 2 or 3.

But if a-1 is not divisible by 2, a must be divisible by 2.

Also, if you take any three consecutive numbers, one of them must be divisible by 3, and since neither a-1 nor a+1 are, then a must be.

Since a has to be divisible by both 2 and 3, it must be divisible by 6 as well.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
SolutionThis is D1, not D4!Math Man2013-01-24 20:37:24
SolutionSolutionK Sengupta2008-11-06 05:47:18
SolutionSolutionPraneeth2007-08-14 03:01:13
re: Stronger resultRichard2006-06-08 19:48:38
Stronger resultSteve Herman2005-04-30 15:17:20
SolutionSimple SolutionGamer2003-10-11 17:11:29
SolutionSixesTomM2002-09-02 12:00:23
Solutionfriedlinguini2002-09-02 04:34:39
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