# Refuting the Anti-Cantor Cranks

by lugita15
Tags: anticantor, cranks, refuting
P: 1,781
 Quote by lugita15 No, Cantor is saying "suppose there is a complete list of real numbers". Then he is using the diagonal construction to show that there is a real number not on the list. Thus, he is concluding that the assumption that there is a complete list of real numbers is wrong.
I'm not trying to split the hair that finely. Nothing I'm saying changes if you say "suppose there is a complete list of real numbers". I think we're ok here as long as everyone agrees that the phrase in quotes is the obvious contradiction and that the diagonal construction of a new real is unnecessary.
P: 1,583
 Quote by Antiphon I'm not trying to split the hair that finely. Nothing I'm saying changes if you say "suppose there is a complete list of real numbers". I think we're ok here as long as everyone agrees that the phrase in quotes is the obvious contradiction and that the diagonal construction of a new real is unnecessary.
No, it is not such an obvious fact that you get a contradiction if you assume that there exists a complete list of real numbers. It takes a nontrivial proof like Cantor's.
P: 606
 Quote by Antiphon I was trying to lead the horse to water. The infinite limit I presented is an algorithmic construction that in principle is no different from the way one might sum an infinite series. The point of it was to show that one simply cannot arrive at Cantor's result without starting off with unbounded sets as you pointed out. That you cannot approach the result the way you might show that .999... is the same as 1.0. But let's move on because this doesn't invalidate the Cantor result. A couple of posts back I did exactly give you as requested "any list of real numbers". It was in fact the first three entries of an infinite countable list exactly as you specified, and between 0 and 1 to keep things from getting messy. But that's not good enough apparently. So it looks like you need to tighten up your specs before we can go to the next step. Is your objection to my list the fact that it excluded 0.05? Because perhaps that was the very Cantor number that would be added by diagonolization. Next you're going to say that I'm not getting it, that the first entry corresponding to natural number 1 is all zeros (say) and the second entry which corresponds to natural number 2 is "the next real number after 0.000..." so that I haven't skipped any. Which brings us full circle to my very first post. It seems the point of the constructed proof is to arrive at the contradiction that by adding one more real, I wasn't able to enumerate them all after all. But one doesn't need to add the diagonalized real to arrive at this conclusion. You arrived at it YOURSELF when you kept saying that my finite, truncated, first few entries of the countably infinite list I provided aren't good enough because why? Because they didn't contain all the real numbers! That's like saying sin(x) does not equal x+x^3/6+... because I didn't start the series with all the terms already there.

Check this: as already noted, by "list" one has to understand "elements of an infinite countable set".

Next: nobody adds anything to anything. Upon getting any list, one can construct a number which is not in that specific list.

Next: perhaps you think you've arrived to a great insight when you use exclamation marks, but I honestly can't see it. Your example

of a finite list with three elements shows nothing as I can construct easily a number not in it without even using the diagonal trick.

Next: I don't care, nor any other mathematician would, what's the first element in such a list. It can be 0, 1, 0.5 or whatever. Anyway, after

you're done with your list, I can always construct a number not in it.

DonAntonio
P: 606
 Quote by Antiphon Let me further add (because I don't want to fly off on a tangent here) that I understand the result but disagree so far with the soundness of the diagonalization proof as its been presented. I get it if you say that the terms in a series like sin(x) are given by x^n/n! and clearly you can't "shoehorn in" a new term between the nth and n+1th. Cantor does exactly this, shoehorns in another term thus showing there is not the one to one association as in the series for sin(x). I get it. The gist of what I'm trying to say (as a non-mathematician) is this: why is it that I can enumerate the first 3 natural numbers in an ordered set of them beginning with 0, but the same cannot be done with the reals beyond the first entry?

This is what Cantor's theorem states: the set of real numbers is infinite yet uncountable, i.e. it cannot be ennumerated. That's why.

 If Cantor or anyone wants to start there and work toward the idea these sets are fundamentally different in nature, there'd be no Cantor Cranks. Cantor forms this very list that cant be formed and then a step *later* brings us to a contradiction?

Either you're addressing something else or you're completely lost in this matter: neither Cantor nor anyone else trying to prove this theorem

"forms" any list at all. We assume such a list exists AND THEN we prove that there's always a real number not belonging to the list.

 Sorry but that's a little too much like Morpheous jumping from building to building. Telling me to "free my mind" isn't going to cut it.

I would never tell you to free your mind, but I' definitely tell you to learn some mathematics. If you want to understand this stuff, that is.

DonAntonio
 P: 800 Fascinating to see the thread degeneration here. Any discussion of anti-Cantor cranks draws anti-Cantor cranks. Purely from a behavioral point of view, the Cantor deniers and the Cantor denier refutors seem equally obsessive. The fact that one group is mathematically correct and the other not, is irrelevant. Because after all, most people manage to avoid these discussions altogether. And now you can see why. They always, always, always end up in exactly the same place. What I used to like about sci.math on Usenet was that at least there, you could toss in gratuitous personal insults. Here you can't do that. So it's much less fun to tease and torment the deniers.
P: 31
 Quote by Antiphon Let me further add (because I don't want to fly off on a tangent here) that I understand the result but disagree so far with the soundness of the diagonalization proof as its been presented. I get it if you say that the terms in a series like sin(x) are given by x^n/n! and clearly you can't "shoehorn in" a new term between the nth and n+1th. Cantor does exactly this, shoehorns in another term thus showing there is not the one to one association as in the series for sin(x). I get it.
He doesn't do that at all. Plus, you can replace any term in that series and it will still converge to some real number. It just will no longer be sin(x).

Similarly any decimal expansion 0.a1a2a3... where a1, a2, a3,... are digits 0-9 is short for the series a1*10^-1 + a2*10^-2 + ... which converges to a real number between zero and one no matter what the digits are chosen to be.

 The gist of what I'm trying to say (as a non-mathematician) is this: why is it that I can enumerate the first 3 natural numbers in an ordered set of them beginning with 0, but the same cannot be done with the reals beyond the first entry? If Cantor or anyone wants to start there and work toward the idea these sets are fundamentally different in nature, there'd be no Cantor Cranks.
Because well-ordered-ness is a completely separate issue from cardinality. The rational numbers are also not well ordered, yet the rational numbers can be put into 1-1 correspondence with the natural numbers and thus the Cantor diagonal argument fails. The existence of irrational numbers with non-terminating decimal expansions is necessary for the Cantor diagonal argument to work.

 Cantor forms this very list that cant be formed and then a step *later* brings us to a contradiction? Sorry but that's a little too much like Morpheous jumping from building to building. Telling me to "free my mind" isn't going to cut it.
The list is hypothetical. That's the essence of proof by contradiction. Do you have a problem with other proofs by contradiction? What about the proof that the square root of two is irrational? It's normal to assume there are some natural numbers n and m such that n^2 / m^2 = 2 and then show that the existence of such numbers leads to an absurdity. Do you also question the proof that the square root of 2 is irrational? If not, then what is fundamentally different about Cantor's diagonal argument?
P: 606
 Quote by SteveL27 Fascinating to see the thread degeneration here. Any discussion of anti-Cantor cranks draws anti-Cantor cranks. Purely from a behavioral point of view, the Cantor deniers and the Cantor denier refutors seem equally obsessive. The fact that one group is mathematically correct and the other not, is irrelevant. Because after all, most people manage to avoid these discussions altogether. And now you can see why. They always, always, always end up in exactly the same place. What I used to like about sci.math on Usenet was that at least there, you could toss in gratuitous personal insults. Here you can't do that. So it's much less fun to tease and torment the deniers.

Indeed. For the time being the thread hasn't degenerated that much, imo. I won't get into any crank-bashing or

crank-educating rant here. For that we have sci.math. So far, though, Antiphon is not a crank but someone with

some doubts and some rather deserted areas in his/her mathematical education...for

now. If and when he, or anyone else, slip into crankhood I, for one, shall bail out of the thread.

DonAntonio
P: 31
 Quote by DonAntonio Indeed. For the time being the thread hasn't degenerated that much, imo. I won't get into any crank-bashing or crank-educating rant here. For that we have sci.math. So far, though, Antiphon is not a crank but someone with some doubts and some rather deserted areas in his/her mathematical education...for now. If and when he, or anyone else, slip into crankhood I, for one, shall bail out of the thread. DonAntonio
I agree Antiphon does not appear to be a "crank". I certainly hope he's not scared off by the implications and condescension being tossed around in this thread. Also if someone is honestly questioning at least give them credit for having the curiosity to want to learn rather than knocking them on their lack of current education.
P: 800
 Quote by mbs I agree Antiphon does not appear to be a "crank". I certainly hope he's not scared off by the implications and condescension being tossed around in this thread. Also if someone is honestly questioning at least give them credit for having the curiosity to want to learn rather than knocking them on their lack of current education.
Sorry. I officially apologize for implying that doubters are the same as cranks. Antiphon, come back.
P: 4,572
That is the best way to prove something IMO because you in doing so, you are accepting the premise that what you are trying to disprove is in fact true and from this a contradiction (if it's not true under some axiomatic system) is found and thus you have shown it's false.

This is important not just mathematically but also psychologically because when most people start off disproving something, in the back of their mind they assume that what they are proving is undoubtedly true which ends up screwing up their analysis, proof and way of thinking whereas the above method psychologically says "OK this is what you said, let's go along with this and see what happens" which is a much better approach because mentally you are saying "I'm going to disregard my own prejudices for the moment and I'm going to assume that you are right". It's very subtle, but it's so important as a logical tool and I'm afraid it's not used as much as it could (and should) be.

You definitely have the right approach and mindset for analyzing things not only mathematically, but in general situations overall.
P: 1,622
 Quote by Antiphon But I'd bet it's not legitimate logic to start with the absurd result and reason your way backward to the expression above.
Whenever you do a proof by contradiction you are starting with an absurd result (by definition), so I don't really understand your objection here.

 What am I missing?
Assuming that the real numbers are countable is exactly akin to assuming that $\sqrt{2} = \frac{p}{q}$ where $p,q \in \mathbb{Z}$ are relatively prime. Constructing a real number not on the list is exactly like showing that 2 divides both $p$ and $q$. The type of reasoning is identical.
 P: 250 Is this a closed subject in the mathematical world? I ask because I have found this http://en.wikipedia.org/wiki/Controv...tor%27s_theory And sure everyone is entitled to have an opinion but I'd like to know if experts logicians have reached an agreement on this.
P: 606
 Quote by viraltux Is this a closed subject in the mathematical world? I ask because I have found this http://en.wikipedia.org/wiki/Controv...tor%27s_theory And sure everyone is entitled to have an opinion but I'd like to know if experts logicians have reached an agreement on this.

This is one of the instances where Wiki, a very good source of immediate though generally not deep and sometimes

"In mathematical logic, the theory of infinite sets was first developed by Georg Cantor. Although this work has found some acceptance

in the mathematics community, it has been criticized in several areas by mathematicians and philosophers."

The words "some acceptance" are unduly and unjustly misleading: infinite sets, in this or that acception, are widel accepted by an

overwhelming majority of mathematicians. Period.

Now, the controversy exists within very narrow and, if may I add, unimportant frames and individuals, and it surely isn't something

that, as far as we know right now, would affect in some dramatic way neither the development of most of mathematics nor

most of its applications to other sciences, technology and/or the "real" world, whatever that is.

Back again with cranks: these persons are characterised by an inner and utterly unjustified certainty that they are right and ALL

the others are wrong, even when they are NOT mathematicians (99% of the cases) and the others are.

DonAntonio
 P: 250 OK, I just thought up this counter-argument Let's imaging it does exist a list of all real numbers such that they have only zeros in the decimal part. The list will look something like: S 1 xxxxxxxxxxxxxxxxxxx.00000000000000000 2 yyyyyyyyyyyyyyyyyyy.00000000000000000 3 xyxyxyxyxyxyxyxyxyx.00000000000000000 4 yxyxyxyxyxyxyxyxyxy.00000000000000000 Then we try to construct S0 using a different digit from the Diagonal yxyy... .00000000000000000 Then S0 cannot possibly be in that list, therefore that list cannot exist... but since the list have only zeros in the decimal places that list is equivalent to the Natural numbers which we know we can count, therefore the diagonal argument makes no sense. What is it that I did wrong?
 P: 95 As an aside, you don't need to phrase the diagonal argument as a contradiction, you can just use it to show that any function from the naturals to the reals must fail to be onto.
P: 95
 Quote by viraltux OK, I just thought up this counter-argument Let's imaging it does exist a list of all real numbers such that they have only zeros in the decimal part. The list will look something like: S 1 xxxxxxxxxxxxxxxxxxx.00000000000000000 2 yyyyyyyyyyyyyyyyyyy.00000000000000000 3 xyxyxyxyxyxyxyxyxyx.00000000000000000 4 yxyxyxyxyxyxyxyxyxy.00000000000000000 Then we try to construct S0 using a different digit from the Diagonal yxyy... .00000000000000000 Then S0 cannot possibly be in that list, therefore that list cannot exist... but since the list have only zeros in the decimal places that list is equivalent to the Natural numbers which we know we can count, therefore the diagonal argument makes no sense. What is it that I did wrong?
S0, even if you can define it at each step, which you may not be able to do, will have an infinite number of digits before the decimal point, and so will not be a real number at all.
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