Can Arithmetic Progressions Form Infinite Relatively Prime Subsequences?

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Discussion Overview

The discussion revolves around the question of whether an arithmetic progression of the form ax+b can contain an infinite subsequence where every two elements are relatively prime. The conversation explores theoretical aspects, assumptions about the parameters a and b, and potential mathematical tools like the Chinese Remainder Theorem (CRT) and Dirichlet's theorem.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that the Chinese Remainder Theorem may be relevant but is uncertain about its application.
  • Another participant questions the validity of the proposition by providing a counterexample when a=2 and b=0, later clarifying that a and b should be non-zero.
  • A subsequent post indicates that a=2 and b=2 serves as a counterexample, proposing that a and b need to be coprime for the original claim to hold.
  • It is noted that the problem specifies that (a,b) = 1, prompting a participant to reconsider the applicability of the CRT in this context.
  • There is a mention of deriving a solution without using Dirichlet's theorem, implying a desire for a more nuanced approach to the problem.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which the arithmetic progression can yield an infinite relatively prime subsequence. While there is some agreement on the necessity for a and b to be coprime, the applicability of the CRT and the implications of Dirichlet's theorem remain points of contention.

Contextual Notes

Participants have not fully resolved the implications of the Chinese Remainder Theorem in this context, and there are unresolved assumptions regarding the nature of a and b, particularly their coprimality.

robin_vanp
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a question came up

"show that the arithmetic progression ax+b contains an infinite subsequence (not necessarily a progression), every two of whose elements are relatively prime."

i have a hunch that the chinese remainder theorem has something to do with this, but I'm not sure how. any thoughts?
 
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Is that true? What if a=2, b=o?
 
sorry, assuming a, b are non zero
 
Then a=2, b=2 is a counterexample. I think you really need that a and b are coprime, in which case the sequence actually contains infinitely many primes.
 
right again. its actually a two part question so it says on the top that (a,b) = 1, i forget to mention; if so (now that we finally got the problem) how is the CRT applicable here?
 
and deriving some sort of solution that does not employ dirichlet's theorem, i think, because then that would be obvious; i really do not know how the CRT can be used here.
 

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