Does this small odd and even proof works?

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    even Proof Works
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Discussion Overview

The discussion revolves around a proof regarding the oddness of the number 101, initiated by a participant who seeks to explore an alternative approach to the proof provided in a mathematical text. The conversation includes various perspectives on the validity and completeness of the proof, as well as related definitions and implications regarding odd and even numbers.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes a proof by contradiction, assuming 101 is even and leading to a contradiction with the divisibility of 102.
  • Another participant suggests that the proof should explicitly demonstrate that 102 is divisible by 2, noting an alternative representation of 101 as 50*2 + 1.
  • Concerns are raised about the assumption that if a number is not even, it must be odd, with a participant indicating that this may not have been proven yet.
  • A later reply suggests that it would be beneficial to prove that a number of the form 2b + 1 is odd, emphasizing the need to show it is not even.
  • A participant presents a proof for the statement that 2b + 1 is odd, leading to a conclusion about the impossibility of having an integer that is both larger and smaller than its consecutive integer.
  • Responses express appreciation for the proof presented, highlighting a sense of beauty in the mathematical reasoning.

Areas of Agreement / Disagreement

Participants express differing views on the completeness and validity of the initial proof regarding 101's oddness. There is no consensus on the necessity of proving certain definitions or implications related to odd and even numbers.

Contextual Notes

Some participants note that certain foundational definitions and proofs may not have been established or agreed upon within the context of the discussion, which could affect the validity of the arguments presented.

Seydlitz
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This is taken from Peter J. Eccles, Introduction to Mathematical Reasoning, page 17. This is not a homework because it is an example in the text. Prove that 101 is an odd number. The text has given a way of proving it and I just want to do it with my own approach.

Prove that 101 is an odd number.

Assume 101 is even. There is a number ##b## such that ##101=2b##. Adds 1 to both side. ##102=2b+1## The right side shouldn't be divisible by 2 but the left side can be divided by 2. A contradiction? Hence 101 is an odd number.
 
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Given the level of the question it seems like you should prove that 102 can be divided by 2 (by stating what the multiplication is). At that point it's just as easy to say 101 = 50*2+1 so is odd but them's the shakes
 
Office_Shredder said:
Given the level of the question it seems like you should prove that 102 can be divided by 2 (by stating what the multiplication is). At that point it's just as easy to say 101 = 50*2+1 so is odd but them's the shakes

Well ok then, I just want to know if that contradiction works.
 
Your proof also uses the fact that if a number isn't even, then it's odd. Again considering the level of the question, this fact may not have been proven yet.
 
Tobias Funke said:
Your proof also uses the fact that if a number isn't even, then it's odd. Again considering the level of the question, this fact may not have been proven yet.

It is not proven but it is used in the definition that if a number is not even then it is odd.
 
Seydlitz said:
It is not proven but it is used in the definition that if a number is not even then it is odd.

Then you should probably think about proving that if a number is of the form 2b+1 then it's odd (in particular you have to prove it's not even)
 
Prove that ##2b+1## is odd.

Suppose ##2b+1## is even, then it exists an integer ##c##, where ##2b+1=2c##

##2c+1=2b+2## and ##2c-1 = 2b##, hence ##2b<2c<2b+2##. Further ##b<c<b+1##

But there is no integer which is larger and smaller than the next consecutive integer. So ##2b+1## must be odd.

I think this opens a new bag of cats, but at least I found this myself from scratch!

(The book standard proof implicitly assumes this I believe)
 
Last edited:
That's a very nice proof
 
Office_Shredder said:
That's a very nice proof

I think I can at least feel this small 'beauty' feeling after sketching it. Thanks!
 

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