Prime factors of odd composites

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Homework Help Overview

The discussion revolves around proving that for any odd composite number ##n##, all of its prime factors are at most ##\frac{n}{3}##. Participants are exploring the implications of prime factorization and the properties of composite numbers.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants express uncertainty about how to begin the proof and whether they can use prime decomposition. There is discussion about the fundamental theorem of arithmetic and its relevance to the problem. Some question the assumption that the smallest prime factor must be 5 or 3.

Discussion Status

There is an ongoing exploration of different approaches to the problem, with some participants suggesting connections between the properties of primes and composite numbers. While some hints and ideas have been shared, there is no explicit consensus on a method or solution yet.

Contextual Notes

Participants are navigating assumptions about the nature of odd composite numbers and the use of established theorems in their reasoning. There is a mention of potential constraints regarding the professor's instructions on what can be assumed or used in the proof.

GlassBones
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Homework Statement


Let ##n## be odd and a composite number, prove that all of its prime is at most ##\frac{n}{3} ##

Homework Equations


Some theorems might help?
Any ##n>1## must have a prime factor
if n is composite then there is a prime ##p<√n## such that ##p|n##

The Attempt at a Solution


I'm at a standstill, not sure how to start off this question? Can anyone prove hints to get me going
 
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GlassBones said:

Homework Statement


Let ##n## be odd and a composite number, prove that all of its prime is at most ##\frac{n}{3} ##

Homework Equations


Some theorems might help?
Any ##n>1## must have a prime factor
if n is composite then there is a prime ##p<√n## such that ##p|n##

The Attempt at a Solution


I'm at a standstill, not sure how to start off this question? Can anyone prove hints to get me going
Are you allowed to use the prime decomposition of integers?
 
I'm pretty sure I can (prof didn't talk about it but I'm assuming we should know about it).
With prime decomposition, I'm not sure how to generate it. Can we just assume there is a prime decomposition because n is composite?
 
GlassBones said:
I'm pretty sure I can (prof didn't talk about it but I'm assuming we should know about it).
With prime decomposition, I'm not sure how to generate it. Can we just assume there is a prime decomposition because n is composite?
The fundamental theorem of arithmetic says, that every integer can be written as a product of primes. If you are allowed to use this, and proofs can easily be found, e.g. on Wikipedia, then you only have to write this in a formula and think about what the smallest prime in there is.
 
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Since n is odd composite, the smallest prime must be 5. So n is divisible by 5.
I see where this is going, but ill probably go over the proof again to make sure I get it
 
Start with the replacement of ##5## by ##3## in your argument. You don't need the fundamental theorem of arithmetic, but you have somehow to connect ##n## with primes. A prime number ##p## has the property, that if it divides a composite number, then it divides a factor. This can also be used. It is called Euclid's lemma, but it is actually the correct definition of a prime.
 
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Oh for some reason I thought the only odd composite numbers was divisible by 5. Not sure why I was thinking that. Thanks
 

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