Proof: x^n + 1 is Valid for Odd n".

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Discussion Overview

The discussion centers around the validity of the expression \(x^n + 1\) for odd integers \(n\) and the implications of this expression for even integers. Participants explore the conditions under which the factorization holds and the nature of roots associated with the expression.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant asserts that \(x^n + 1\) can be factored as \((x+1)(x^n - x^{n-2} + \ldots + (x^2 - x + 1)\) only when \(n\) is an odd integer.
  • Another participant notes that the notation implies \(n\) is an integer and claims that even values of \(n\) lead to an incorrect sign for the leading order term on the right-hand side.
  • A different participant challenges the factorization by stating that if \((x+1)\) is a factor, then \(x = -1\) should be a root, which they argue it is not.
  • One participant attempts to clarify the intended meaning of the original statement, suggesting that it was meant to prove the expression false for even \(n\) rather than true for odd \(n\).
  • Another participant agrees with this interpretation but argues that the statement is incorrect for odd \(n\) as well, providing a counterexample with \(X^3 + 1\).
  • One participant introduces a more complex argument involving \(n = (2^r)q\) and discusses the implications for primality of \((2^n) + 1\), suggesting that it cannot be prime unless certain conditions are met.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the factorization for odd and even \(n\). There is no consensus on the correctness of the statements made, and multiple competing interpretations of the original claim exist.

Contextual Notes

Some participants' arguments depend on specific interpretations of the notation and definitions used, and there are unresolved questions regarding the implications of the proposed factorization for different values of \(n\).

ascheras
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Prove the following is valid only when n is an odd integer.

x^n + 1= (x+1)(x^n - X^n-2 + ... + (x^2 - x + 1).

It's an easy 3 line proof.
 
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The notation given already implies that n is an integer. From there, it's easy to check that even values of n result in the wrong sign for the leading order term on RHS.
 
(x+1) a factor implies x = -1 is a root, but it ain't. (one liner)
 
The way that is meant is (X^(2n+1)+1) =(X+1)(X^(2n)-X^(2n-1)+X^(2n-2)-+-..+1).
 
robert, how can you tell he meant the converse of what he said? what he said was to prove his statement false for even n, not to prove it true for odd n. that at least is how i translate the word "only".
 
mathwonk said:
robert, how can you tell he meant the converse of what he said? what he said was to prove his statement false for even n, not to prove it true for odd n. that at least is how i translate the word "only".

Yes, that is true. Even so statement is not correct for odd n since

(X^3+1) not equal to (X+1)(X^3-X+1) for all X.
 
Last edited:
Let n= (2^r)*q, where q has no prime divisors. Then you can write (2^n) +1= (y^q) +1, where y=(2^r). Then (2^n) +1= (y+1)(y^(q-1)- y^(q-2) +...+ (y^2) - y+1). Here, y+1= (2^r)+1 >1 and there are two factors. This (2^n)+1 cannot be prime if the other factor is also > 1. That happens unless y^(q-1) +...+ (y^2)- y+1 reduces to 1, i.e. q=1. Therefore (2^n)+1 is prime, which implies that q=1 and n=(2^r) for some r as claimed.
 

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