Prime factors of odd composites

In summary, if n is odd composite, then there is a prime decomposition such that the smallest prime is 5.
  • #1
GlassBones
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1

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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  • #2
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?
 
  • #3
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?
 
  • #4
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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  • #5
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
 
  • #6
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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  • #7
Oh for some reason I thought the only odd composite numbers was divisible by 5. Not sure why I was thinking that. Thanks
 

What are prime factors?

Prime factors are the numbers that, when multiplied together, result in a given number. They are the building blocks of composite numbers.

What are odd composites?

Odd composites are numbers that are greater than 1 and have more than two factors. They are not prime numbers, and their prime factors are all odd.

How do you find the prime factors of an odd composite number?

To find the prime factors of an odd composite number, you can use a process called prime factorization. This involves dividing the number by its smallest prime factor, then dividing the result by its smallest prime factor, and so on until all factors are prime.

Why is it important to know the prime factors of odd composites?

Knowing the prime factors of odd composites can be useful in many mathematical applications, such as finding the greatest common factor or simplifying fractions. It can also help in cryptography and number theory problems.

What is the largest prime factor of an odd composite number?

The largest prime factor of an odd composite number is always smaller than the square root of the number. This is because if a number has a prime factor larger than its square root, then it would also have a smaller prime factor, making it a composite number.

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