Register to reply 
Cantor's Diagonalization Proof of the uncountability of the real numbers 
Share this thread: 
#73
Feb712, 04:02 AM

Emeritus
Sci Advisor
PF Gold
P: 16,091

I don't have time to write more, so I'll make a series of brief comments.
Saying "the list X contains 0.1, 0.11, 0.111, 0.1111, and every other (terminating) decimal of that form" is very different from saying "the list X contains 1/9". In fact, the two statements have absolutely nothing to do with one another. A representation cannot become "more accurate". "0.9999..." is notation for a specific decimal: the decimal with a 9 in every position to the right of the decimal point (and that decimal, in turn, can be interpreted as notation for the real number 1). When I say every position, I really mean every. The decimal 0.999 has a zero in its millionth's place, for example. 0.999... has a 9 in that place. Pick positive integer x you like: 0.999... has a 9 in the position x places to the right of the decimal. 0.9999 is an approximation to 0.999.... 0.999999 is another (better) approximation to 0.999.... An algorithm that sequentially considers the numbers 0.9, 0.99, 0.999, 0.9999, and so forth does not produce a "representation that changes to become more accurate". Instead, it produces an infinite sequence of different representations of approximations to 1, each one more accurate than the last. An algorithm that behaves in the aforementioned way, incidentally, is in of itself an exact representation of 1. (where the notation is that we interpret a program that outputs a convergent sequence as notating the limit of that sequence) Unless you're designing the algorithm specifically for that purpose (or a similar one), it is rather difficult to devise an algorithm whose first time outputting a 'simple' number (like 1/9) happens after so many iterations that you cannot write it down  even if we insist that you have to write it in decimal form. The number of digits in the decimal representation of [itex]\sqrt{2}[/itex] doesn't "grow without bound"  it's just a number, it doesn't make sense to "grow". The number is [itex]\aleph_0[/itex]. Rational numbers (even integers) can be limits of sequences of terminating decimals whose number of nonzero digits grows without bound. (e.g. the sequence 0.1, 0.11, 0.111, ...) (e.g. the sequence 0.1, 0.011, 0.00111, 0.0001111, 0.000011111, ...) 


#74
Feb712, 07:45 AM

P: 497

Yes andrewr you can add √2 to the countably many rational numbers and you will still have countably many numbers. By extension you can add all the quadratic irrationals and you will still have countably many, in fact you can add all the roots of polynomial equations and you will have all the algebraic numbers in your list, which is countable.
Are you satisfied that you can create any number possible and put it on the list? So therefore there does not exist any number that is not on the list? Good, because that is exactly the hypothesis that starts Cantor's proof  that all real numbers can be written down in a list such that each real number can be mapped to an integer (its place on the list). Cantor's diagonal argument constructs a number that can plainly be seen not to be on the list: if you pick any number in the list in position n, the constructed number is different from the number on the list at least in it's nth decimal digit. The hypothesis was clearly wrong. Far from contradicting Cantor's proof, you have simply demonstrated that you can produce a list that is the starting point for that proof. 


#75
Feb712, 10:14 AM

P: 800

Cantor asks us to consider a list of ALL the real numbers. He then constructs a real number not on that list, showing that ANY such list is necessarily incomplete. Therefore there can be no list of all real numbers. Cantors proof has NOTHING to do with taking some countable list and trying to add numbers to it. That's a misunderstanding that's a source of much confusion. 


#76
Feb712, 01:07 PM

P: 497

Yes my last sentence was not correct. What I should have said was:
Far from contradicting Cantor's proof, you can use Cantor's diagonal argument to construct a counterexample to show that your (andrewr's) assertion that you can list all of the real numbers using some algorithm is incorrect. 


#77
Feb712, 01:10 PM

P: 39

This seems to be the point that everyone is not fully understanding. I'm not the slightest bit worried about the list that you care "creating" as you are crossing numbers out. That list is totally meaningless. Why? Well it should be obvious. Stop your diagonalization process at any point. You claim to now have a number that is not on the list above where you are working. So? Big deal. What so you have at that point? What number have you just created thus far? Well, clearly since you've stopped you have a truncated decimal number. I can absolutely guarantee that you are standing there holding a valid Rational number. In other words, you haven't created a "new number" that isn't already on the list Rational numbers. All you've succeeded in doing it proving that the number you've created thus far is not on the list above where you are working. But you haven't demonstrated that it is a "new number" that can't already be on the list of Rational numbers. In fact, it can easily be shown that this will always be the case. Let say that you have gone out ten places and you have created the following "new number": 0.3854736394 That number cannot possibly be on your list above where you are working. But clearly as a truncated number it must necessarily already be on the list of rational number. Moreover, no matter which numeral your process chooses for the next supposedly "New Number" I can guarantee that the number you create will be on the list of Rational Numbers. This should be extremely ease to see. How many digits do we have to chose from to use as our replacement digists? Well we have ten of them 0 thru 9. So let's try that and see if we can create a "New Number" that isn't already a valid Rational Number using this process: Here's the number we're claiming is already a "New Number" that can't already be on the list of Rational Numbers: 0.3854736394 Granted this is cheating because I've stopped the process. But bear with me,.... What can we possible add to this number next? Well we can tack on anything between or including 0 thru 9. So lets do that,... 0.38547363940 0.38547363941 0.38547363942 0.38547363943 0.38547363944 0.38547363945 0.38547363946 0.38547363947 0.38547363948 0.38547363949 I've just created ten supposedly "New Numbers" that potentially could not be on the list above where I'm working. So big deal? All of these numbers (ever single last one of them) must necessarily be on a list of Rational numbers. In fact, you can take that even further and proclaim that there must necessarily be infinitely many Rational numbers that begin with any one of the numbers listed above and have a different sequence of numerals after that. Take the original number we've created and just add any arbitrary digits to it that you care to chose on a whim and you've got yet another Rational number: 0.3854736394abcdefghijklmnopqrstuvwzyz etc. (potentially to infinity, what's going to stop you ever) Chose any arbitrary digit from 0 to 9 for the letter variables above and you'll have a perfectly legitimate Rational number. So at not point in this process can you claim to have created a "New Number" that can't be on the list of Rational Numbers. In fact, you can be absolutely guaranteed that you will always have a valid Rational Number at ALL TIMES in your process. So what's Cantor's point? That if you continue this process out to "infinity" you will have somehow miraculously created a sequence of numerals that can't be Rationalized. But wait? When would this occur? When could Cantor ever actually COMPLETE this infinite process in order to claim to have created a NEW NUMBER that isn't already on the list of Rational Numbers? He hasn't proven anything at all. All he's done is rely on the "the definition" that we claim that real decimal expansion never quist going whereas a supposedly "Rational Number" must truncate. But what sense is there truly to even claiming that a Rational Number "must truncate"? What sense does that truly make? What would ever prevent you from adding yet another numeral digit to a string of numerals representing a Rational Number? At the very BEST, all Cantor has done is claim that Real numbers must be expressed using infinite decimal expansions and all Rational Numbers must truncate. But that's not a proof of anything. It's just an assumption of an arbitrary manmade definition. Unless Cantor can actually show that he can successfully "complete" this infinite process and actually have obtained a "new sequence of numerals" that can't already be on a list of Rational numbers, then he hasn't proven anything. This is why I agree with Henri Poincare on this point. Pretending that an infinite process could be "completed" is an absurd idea to begin with and will always result in utterly meaningless or absurd results. Cantor hasn't proven anything with this proof other than he's still clinging to the idea of a "completed infinity". He's trying to treat infinity as though it can be thought of as being finite. It makes not sense for Cantor to even proclaim to have created a "new number" unless he can "complete" his process and show us this "new number" that he has created. But he can NEVER do that because his process requires that it must never STOP, if it ever is said to STOP what does he have? He's standing there holding perfectly valid Rational Number. So his socalled "proof" is nothing short of utter absurdity. In fact, I take precisely the opposite view of Micromass. If you can show me graphically, or physically something the totally defines mathematical formalism, then as far as I'm concerned it's the mathematical formalism that must then necessarily be wrong. Certainly not physical reality. And currently we have no physical proof (or even reason to believe) that anything can be infinite much less that multiple cardinal sizes of infinities should exist. 


#78
Feb712, 01:20 PM

Mentor
P: 18,209

Sigh...
I'm not even reading the rest since it is so comically wrong. Instead of giving all these arguments, why not find an error in the formal proof?? 


#79
Feb712, 01:27 PM

P: 39

But how can he claim to have ever created a number that is not already on the list of ALL rational numbers? The way he can possibly claim to have done this if he does indeed take his process out finitely (without ever terminating it). But if he never terminates it, then how can he claim to have "created" a new number? It seems to me that all he's doing is trying to depend on an idea that "real" numbers must be infinite whilst rational numbers are thought of as somehow needing to terminate. But in truth, why should a Rational number need to ever terminate? What would cause a Rational number need to terminate? Aren't the Rational numbers themselves supposed to be infinite in size? (i.e. unbounded, unending) All a Rational number is, is a fraction of Natural Numbers. So we can write: 123/321 That's a rational number. How about 1234/4321? That's a rational number too. How about 12345/54321 Again a rational number. In fact, is there any reason why we would need to stop this process? Clearly there is no end to the Rational numbers we can create. That's why they are considered to be an infinite set of numbers. So what sense does it even make to claim that a Rational Number should necessarily "terminate"? We have no reason to even place this restriction on the numerators and denominators of the fractions that define them. In short, we actual have no logical reason to claim that a Rational number must terminate. That's just an arbitrary idea made up by mankind to try to make a distinction between a "Rational Number" and a "Real Number". But do we truly even need a concept of "Real Numbers" at all? I say no, we don't. The very idea of "Real Numbers" is an erroneous idea that is not even required to describe physical reality. There is nothing in physical reality that even requires this concept. 


#80
Feb712, 01:31 PM

P: 497



#81
Feb712, 01:34 PM

P: 497




#82
Feb712, 01:36 PM

Mentor
P: 18,209

0.1 0.11 0.111 0.1111 0.11111 0.111111 ... then Cantor diagonalization yields 0.1111111111... which is not on this list. This is a simple example. Cantor diagonalization ALWAYS yields a number not on the list. Cantor wants you to look at a list of ALL real numbers and it will construct a number not on the list. 


#83
Feb712, 01:37 PM

P: 39

I'm telling you that Henri Poincare is right. The very idea that you can pretend to have completed an infinite process without stopping it is an utterly absurd and meaningless idea. It's simply nonsense. It makes no sense. Why should I apply a mathematical formalism to reality if that formalism has nothing to do with reality? But as a physicist why should I be interested in playing such games? I may as well play dominoes if I'm going to do that. I'm more interesting in finding (or creating) a "mathematical formalism" that can actually be used to properly describe the true nature of reality. 


#84
Feb712, 01:43 PM

P: 497

If I want to put a tyre on a wheel I will need ∏ which is not rational. If I want to calculate how much ^{235}U I have left in a rector after 1 month I need e which is not rational. 


#85
Feb712, 01:45 PM

Mentor
P: 18,209

This thread is going offtopic. Therefore:
Moderator note: This thread will only deal with Cantor's diagonalization proof. The invalidity of infinity and real numbers will be considered offtopic and the post will be deleted. If you want to discuss this topic, create a new thread. Furthermore, since informal arguments are clearly not convincing. Moderator note: This thread will only deal with the rigorous version of Cantor's proof which is found in post 24 or in any serious set theory book. 


#86
Feb712, 02:04 PM

P: 39

You've answered my question and solved the whole problem. I was indeed viewing the proof incorrectly. Unfortunately it doesn't help. Why does it not help? Because it would be a totally futile and meaningless "proof". In fact, he would be supposedly 'proving' something that necessarily falls out of how he is defining a "Real Number". He's simply demanding that a "Real Number" is necessarily an "infinite" decimal expansion by definition and assuming that a "Rational Number" cannot be described by an "infinite" decimal expansion. Another problem is that his "proof" would obviously work for any numbers. Not just reals. Think of it this way. Just take the decimal point in his proof in his proof away. What do you have left? A list of natural numbers. You can go down that list in precisely the same way and proof that even the Natural Numbers can't be put into a correspondence with themselves. How so? Well, let's try it. Start our list with the arbitrary natural number of 1 1 We cross that out and replace it with any arbitrary digit (say 9) So we have a "new number" 9. Lets continue, we'll need a two digit number to work on next so lets choose 23 our list so far looks as follows: 1 23 Replace the 3 with 5. Our newly constructed number is 95. Move down another row pick another arbitrary natural number, we'll need one that is at least 3 digits wide or more, lets pick 3476 just for fun. Here's our new list 1 23 3476 Now we need to replace the 7 with something other than 7, let's choose 8. Our new number is now 958, and so on. So we currently have a list of natural numbers 1 23 3476 And thus far we've created a "new number" 958 that isn't on this list. And we can continue down the list like this just like Cantor and end up with precisely the same situation he has. (i.e. we can create a supposedly "new" natural number that isn't already on the list of numbers that we have created in this way) Does that mean that we can create a new Natural Number that isn't already on the list of Natural Numbers? And that the Natural Numbers must then be "uncountably infinite" just like the Reals? This socalled "proof" doesn't prove anything. In fact, if it proves anything at all it proves that the Natural Numbers must have the same weird cardinality as the reals because we can use the very same process as Cantor used to prove it for the Natural Numbers too. So how are we any further ahead toward proving anything unique? 


#87
Feb712, 02:08 PM

Mentor
P: 18,209

The difference with Cantor diagonalization on the reals is that you end up with a number which extends indefinitely after the decimal point. This IS a real number. 


#88
Feb712, 02:20 PM

P: 497

If there are as many rows (real numbers) as there are columns then you can construct the diagonal number which is not in any row, hence the contradiction that shows that this cannot be the case. If there are more rows (real numbers) than there are columns (natural numbers) then... we don't need to construct anything  this is exactly what we are trying to prove! 


#89
Feb712, 02:24 PM

P: 159




#90
Feb712, 03:08 PM

P: 39

You just asked me yourself, "What process?? Do you mean taking the limit?? The limit of rational numbers doesn't need to be rational." A question like that demands addressing the very concept of LIMIT. You, as a mathematician, should be fully aware that the LIMIT process of calculus does not prove the existence of what the limit equals. On the contrary all that is required to prove a limit is that certain trends and conditions have been proven. The actual result (i.e. the number that the limit is equal to) cannot be said to necessarily "exist". Surely you're aware of this. You can prove that a "limit exists" for functions where the value of the limit is undefined and therefore in terms of the "function" itself that point does not exist. So a taking any process to a limit does not imply that the result has any actual "existence". That may seem to have nothing to do with Cantor's diagonalization proof, but it's very much a part of it. Cantor is claiming that because he can take something to a limit that necessarily proves that the thing the limit is pointing too exists. That's actually a false use of Limits anyway. The epsilondelta definition of limits won't even support any such conclusions. So is that "offtopic" or is it required information concerning the proof in question? The proof basically demands that something has been taken to a limit. So how could that topic not be part of the proof? If an informal intuitive or graphical argument can be shown to trump a mathematical axiom, then which should be accepted as being more reasonable? Actually if you demand that we stick solely to the axioms of set theory then how could we ever show that they are flawed? They would necessarily contain those flaws. Finally, and very sincerely,... If you don't feel that this forum is the proper venue for fleshing out these kinds of ideas, then may I ask if you can point to an appropriate forum or web site where an intuitive approach to reason is deemed acceptable to discuss. It not my intent to step on anyone's toes. But I seriously would like to flesh out these ideas with people who are at least intelligent and professional enough to be capable of comprehending the points I'm attempting to address. Clearly since these are concerns associated with mathematical formalism, it only makes sense to discuss them with people who at least have some understanding of mathematics. But it seems to be rather fruitless if they are just going to demand that the status quo cannot be challenged without first accepting the very axioms that are in question. Where can I propose ideas like these without having them immediately dismissed simply because they are indeed challenging sacred axioms? Or has mathematics become a religion where any suggestion that its axioms might contain flaws is considered totally unacceptable blaspheme? You can't very well use a formal axiomatic system to disprove itself. If there are "flaws" in the formal system they must necessarily be address from an external intuitive approach. That's my position on that. So can you suggest a web site math forum where intuitive reasoning has not yet been cast asunder as being totally worthless? 


Register to reply 
Related Discussions  
Use Cantor's Diagonalization on the set of Natural Numbers?  Calculus & Beyond Homework  8  
Cantor diagonalization argument  Set Theory, Logic, Probability, Statistics  2  
Cantor's diagonalization  Calculus & Beyond Homework  5  
A new point of view on Cantor's diagonalization arguments  General Physics  177  
A question on Cantor's second diagonalization argument  General Math  36 