Proposition needed to be proven

  • Thread starter Werg22
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In summary: I forgot to mention that you should look the lemma up, as it is a well-known one.In summary, The discussion involves a proposition conjecturing that for any n in the set of non-negative integers, there are no a, b, c in the set of non-negative integers such that 4n + 3 = 5^a 13^b 17^c. The conversation also includes suggestions for approaching the problem, such as trying small values of n and working mod 4. There is also a mention of a lemma from the book "Proofs from the BOOK" that may be helpful in this case.
  • #1
Werg22
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A relatively lengthy proof I am writing for an assignment leads me to a proposition (which I need to turn into a lemma for the proof to be complete) conjecturing that for any [tex]n \in \mathbb{Z^{*}} [/tex], there are no [tex]a, b, c \in \mathbb{Z^{*}}[/tex] such that [tex]4n + 3 = 5^{a}13^{b}17^{c}[/tex]. I haven't been able to find to tackle the problem, any suggestions?
 
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  • #2
I don't understand you question...do you mean that the sequence defined by [tex]a_n = 4n +3 , n \in \mathbb{Z}[/tex] must generate some primes for n other than 5, 13, 17 and 21?

The digits 4 and 3 add up to 7, a prime. So a value of n that would keep the digits the same already rules the number out for a heap of divisibility tests for small numbers. Since it only rules out small numbers, try small values of n. n=1, a_1 = 7, prime. n=10, a_10 = 43, prime.

I don't think that's what you are asking, because I'm sure you would have spotted n=1 straight away.
 
  • #3
What do you mean by "4n + 3 must have primes other than 5, 13, 17 and 21"?

21 isn't prime :confused:

5, 13, 17, 21 are all of the form 4n+1 not 4n+3 :confused:
 
  • #4
Sorry for stating that 21 is prime, it was 4 am here, easy to say gibberish at this time. That said, I have rectified the original question, so please read it.
 
  • #5
(4n+1)(4k+1) = 16nk+4(n+k)+1 = 4m+1. The form is preserved under multiplication.
 
  • #6
OK, I read it, but robert Ihnot got there first...

5 = 13 = 17 = 1 mod 4, so 5^a 13^b 17^c = 1 mod 4.
 
  • #7
Thanks allot, the proof is complete. :smile:
 
  • #8
work mod 4. the lhs is 3 mod 4. what aboutn the rhs? 5=1 mod 4, 13=1 mod 4, and also 17=1 mod 4, so the lhs =3 and the rhs =1.
 
  • #9
Sorry to sound ignorant but what does the "*" represent in [tex]\mathbb{Z}^{*}[/tex]
 
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  • #10
uart said:
Sorry to sound ignorant but what does the "*" represent in [tex]\mathbb{Z}^{*}[/tex]

Non-negative integers.
 
  • #11
"Non-negative". Ok thanks.
 
  • #12
I'm curious as to why you ask this question, Werg22. I vaguely remember using/reading this lemma, as proven by robert Ihnot here, in a maths book. It was titled "Proofs from the BOOK" or something along those lines. I believe the chapter I found it in was something on the representation of integers as sum of primes or something like that.
 

FAQ: Proposition needed to be proven

What is a proposition that needs to be proven?

A proposition that needs to be proven is a statement or claim that has not yet been confirmed or established as true. It requires evidence or logical reasoning to support it and convince others of its validity.

How do you prove a proposition?

To prove a proposition, you need to gather evidence or data that supports the statement and use logical reasoning to show that the evidence supports the claim. This can include conducting experiments, analyzing data, or providing logical arguments based on existing knowledge.

What makes a proposition valid?

A proposition is considered valid if it is supported by evidence and logical reasoning. This means that the evidence and reasoning used to support the proposition are sound and convincing, and can withstand scrutiny and criticism from others.

Can a proposition be proven wrong?

Yes, a proposition can be proven wrong if the evidence or reasoning used to support it is found to be flawed or incomplete. Science is an ever-evolving process, and new evidence or discoveries can sometimes contradict previously accepted propositions.

Why is it important to prove a proposition?

Proving a proposition is important because it allows us to gain a deeper understanding of the world around us. It helps us separate fact from fiction, and make informed decisions based on evidence and logic rather than assumptions or beliefs. Proving propositions also allows for progress and advancement in science, as it leads to the development of new theories and ideas.

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