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August 7th, 2009, 07:48 PM  #1 
Senior Member Joined: Apr 2007 Posts: 2,140 Thanks: 0  International Mathematical Olympiad, year 1971
1971/3. Prove that the set of integers of the form contains an infinite subset in which every two members are relatively prime.

August 10th, 2009, 12:45 PM  #2 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  Re: International Mathematical Olympiad, year 1971
Hmm. If a  2^n  3, then a divides 2^(n + a  1)  3 because the period of 2^k  3 mod a divides a  1. So unless a < 5 (and even then it should work out, only a few cases must be tested) you have a does not divide 2^(n + a)  3. Not quite enough, but it feels like it's a step toward an answer. 
August 10th, 2009, 03:22 PM  #3 
Senior Member Joined: Apr 2007 Posts: 2,140 Thanks: 0 
Good solution to start out with, CRGreathouse. Honestly, I am a total beginner to the field of number theory. It would be a ways off until I can master number theory and tackle such IMO number theory problems...

August 10th, 2009, 08:07 PM  #4  
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  Re: Quote:
 
August 10th, 2009, 09:22 PM  #5 
Senior Member Joined: Apr 2007 Posts: 2,140 Thanks: 0 
IMO problems are supposed to be hard problems. But, when you take a look at the solution of a IMO problem without solving it, you will be amazed how simple the solution is, so like you have said, there could be a simple trick that goes along with the above problem.

August 10th, 2009, 11:41 PM  #6  
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  Re: Quote:
Hint on the problem: the approach of choosing congruence classes recursively by primes doesn't work (if the prime divides one less than the current modulus); I need a less restrictive approach.  
August 31st, 2009, 04:47 AM  #7 
Senior Member Joined: Dec 2008 Posts: 160 Thanks: 0  Re: International Mathematical Olympiad, year 1971
Honestly, I do not see how tip from CG lead to the solution. Here is a solution I propose: Let we already have a sequence of that  are relatively prime to each other. I show how to find that sequence is a sequence where all are relatively prime. Let Put  Euler function, then and is relatively prime with all 
August 31st, 2009, 04:40 PM  #8 
Senior Member Joined: Apr 2007 Posts: 2,140 Thanks: 0  zolden, are you like mathematically gifted or something. . .?

September 1st, 2009, 10:51 AM  #9 
Senior Member Joined: Dec 2008 Posts: 160 Thanks: 0  Re: International Mathematical Olympiad, year 1971
I am just hanging around. Sometimes, interesting problems get posted.

September 1st, 2009, 03:44 PM  #10 
Senior Member Joined: Apr 2007 Posts: 2,140 Thanks: 0 
Well, it's good to have good people on the forum! Stick around if you want, and we will discuss things, indeed! 

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