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# Induction, what is and is not valid? watch

1. Anyway, a question came up http://www.dpmms.cam.ac.uk/site2002/...-2009/N+S1.pdf 8.(a).

I quess the base case was the problem. But, lol.

I was wondering, how do you if induction is valid? Maybe, the problem was with the base case i.e. that x=x is trivial and so shouldn't apply to most inductions. In the above question is the base case actually proven? maybe thats the problem. Although, x=x for all x's, so lol.

But still, n=1 could be true and n=k implies n=k+1, but the induction could be invalid. Thats weird. I don't know but I get the scary thought that say at n=3 the induction could fail but it could be true that n=1 and n=k implies n=k+1. Will this every happen?

P.S. You think you know something then bam right in the kisser.
P.P.S. To please Totally Tom I have edited out the question. Can someone anwser the question about induction i.e. can n=1 be true and n=k imply n=k+1 but the induction false?
2. (Original post by Simplicity)
Anyway, a question came up http://www.dpmms.cam.ac.uk/site2002/...-2009/N+S1.pdf 8.(a).

I quess the base case was the problem. But, lol.

I was also wondering is this a valid induction.

If there exist an injection , then

So this is the induction hypothesis. I won't prove the base case.

So the book constructs a function
g(i)={( i for ) and (i+1 for )

Now the book goes this. If then
is cleary an injection so
therefore . Is this valid? as surely, induction hypothesis is for m not m-1. I guess it is valid because , I think. But if m=1 then so does this mean the argument in the book is invalid?

Then, the other case i.e. is prove similarly.

I was wondering, how do you if induction is valid? Maybe, the problem was with the base case i.e. that x=x is trivial and so shouldn't apply to most inductions. In the above question is the base case actually proven? maybe thats the problem. Although, x=x for all x's, so lol.

But still, n=1 could be true and n=k implies n=k+1, but the induction could be invalid. Thats weird. I don't know but I get the scary thought that say at n=3 the induction could fail but it could be true that n=1 and n=k implies n=k+1. Will this every happen?

P.S. You think you know something then bam right in the kisser.

/rant.
3. (Original post by Totally Tom)

/rant.

This thread isn't even about the question. Okay, if it will make you happy I have edited it out.
4. (Original post by Simplicity)

This thread isn't even about the question. Okay, if it will make you happy I have edited it out.
ty.
5. the induction fails on the first one because nobody cares about the base case with 1 element, it doesn't work for the base case with 2 elements.
what's the 'definition' of an interesting number? why aren't they all interesting? i don't really see it proving anything unless we define what an interesting number is.
6. Thanks Totally Tom. I didn't know about interesting number paradox but yeah that explains it.

I was going to ask what is the well ordering principle but the wikipedia article about interesting number explains what it is. I guess that explain why the book calls it the least criminal and then you have to prove that there is no least criminal.
7. (Original post by Simplicity)
Thanks Totally Tom. I didn't know about interesting number paradox but yeah that explains it.

I was going to ask what is the well ordering principle but the wikipedia article about interesting number explains what it is. I guess that explain why the book calls it the least criminal and then you have to prove that there is no least criminal.
every non empty subset of the natural numbers has a least element
8. (Original post by Simplicity)
I quess the base case was the problem. But, lol.
I spent a very long time explaining to you why the base case wasn't the problem. In the given problem, n=k only implies n=k+1 for k >= 2; when k = 1, we can't deduce that n=1 implies n=2, so the inductive step fails.
9. (Original post by generalebriety)
I spent a very long time explaining to you why the base case wasn't the problem. In the given problem, n=k only implies n=k+1 for k >= 2; when k = 1, we can't deduce that n=1 implies n=2, so the inductive step fails.
lol, troll'd.
10. (Original post by Totally Tom)
lol, troll'd.
So I see.

Perhaps I'll steer clear of Simplicity's threads from now on.
11. (Original post by generalebriety)
I spent a very long time explaining to you why the base case wasn't the problem. In the given problem, n=k only implies n=k+1 for k >= 2; when k = 1, we can't deduce that n=1 implies n=2, so the inductive step fails.
Sorry, but that was bad wording. I think wikipedia has the best explanation, is that the argument relies on overlapping sets and this doesn't apply to a set of one element so the base case is really n=2.

(Original post by Totally Tom)
lol, troll'd.
Totally Tom

I didn't troll gb.
12. (Original post by Simplicity)
Sorry, but that was bad wording. I think wikipedia has the best explanation, is that the argument relies on overlapping sets and this doesn't apply to a set of one element so the base case is really n=2.
No, the base case is n = 1. (And I don't think Wikipedia says what you say it does - at least if you're referring to http://en.wikipedia.org/wiki/Mathema...erent_color.22)
13. Okay, that was bad wording. I don't know actually how to say it, so yeah it fails for n=2/

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