Question about primes and divisibility abstract algebra/number theory

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Discussion Overview

The discussion revolves around determining the greatest integer that divides the expression p^4 - 1 for every prime number p greater than 5. The scope includes number theory and abstract algebra concepts, particularly focusing on divisibility and properties of prime numbers.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant expresses difficulty in approaching the problem due to a lack of strong background in algebra and number theory.
  • Another suggests starting by computing the greatest integer that divides p^4 - 1 for primes in a specific range, proposing to begin with primes less than 10.
  • A participant questions whether p^4 - 1 itself could be the answer, indicating a possible misunderstanding of the problem's requirements.
  • Clarification is provided that the question seeks a single integer that divides p^4 - 1 for all primes p > 5, emphasizing that it cannot depend on p.
  • One participant factors p^4 - 1 into (p+1)(p-1)(p^2+1) and discusses the contributions of these factors to divisibility by 16 and 3, suggesting that the answer lies between 48 and 240.
  • Another participant analyzes the expression modulo 5, concluding that p^4 - 1 is congruent to 0 mod 5 for primes greater than 5.

Areas of Agreement / Disagreement

Participants express differing views on the approach to the problem and the interpretation of the question. There is no consensus on the greatest integer that divides p^4 - 1 for all primes greater than 5, and multiple competing ideas are presented.

Contextual Notes

Participants have not fully resolved the mathematical steps involved in determining the greatest integer, and assumptions about the properties of primes and their residues modulo various bases are still being explored.

AxiomOfChoice
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Can someone please tell me how to go about answering a question like this? I've been racking my brain for a long time and still don't have a clue...I guess because my background in algebra/number theory really isn't that strong.

"What is the greatest integer that divides [itex]p^4 - 1[/itex] for every prime number [itex]p[/itex] greater than 5?"

Thanks!
 
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There seems an obvious first thing to try:
Compute the greatest integer that divides [itex]p^4 - 1[/itex] for every prime number p in the range 5 < p < N​

where N is whatever number you like. I'd probably start with 10 and then increase it a few times until I had an idea what was going on.
 
AxiomOfChoice said:
What is the greatest integer that divides [itex]p^4 - 1[/itex] for every prime number [itex]p[/itex] greater than 5?"

Wouldn't it be [tex]p^4-1[/tex]? Maybe I'm not understanding the question.
 
Sorry; this is a multiple choice question off of an old Math Subject GRE exam. There are five answer choices:

(A) 12
(B) 30
(C) 48
(D) 120
(E) 240
 
This is what I have so far.

[tex]p^4-1= (p+1)(p-1)(p^2+1)[/tex]

p is odd so [tex]p = 1 \text{ or } 3[/tex] (mod 4) so there are three 2's in (p+1) and (p-1) plus another in [tex](p^2+1)[/tex] so [tex]16|p^4-1[/tex]. Furthermore, 3 does not divide p (since p>5) so (p-1) or (p+1) does and so [tex]3|p^4-1[/tex]. Now it's between 240 and 48.
 
qntty said:
Wouldn't it be [tex]p^4-1[/tex]? Maybe I'm not understanding the question.

Yes, you are. The question is about a single number that divides [itex]p^4- 1[/itex] for all primes p> 5. It cannot depend on p.
 
Alright I found the the last factor.

1^2 = 1 mod 5
2^2 = 4
3^2 = 4
4^2 = 1


So [tex]p^2[/tex] = 1 or 4 mod 5

[tex](p^2)^2 = 1[/tex] mod 5

[tex]p^4-1 = 0[/tex] mod 5
 

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