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  • I strongly agree with you, and while the people replying aren't wrong, they're arguing for something that I don't think you said.

    1/3 ≈ 0.333... in the same way that approximating a circle with polygons of increasing side number has a limit of a circle, but will never yeild a circle with just geometry.

    0.999... ≈ 1 in the same way that shuffling infinite people around an infinite hotel leaves infinite free rooms, but if you try to do the paperwork, no one will ever get anywhere.

    Decimals require you to check the end of the number to see if you can round up, but there never will be an end. Thus we need higher mathematics to avoid the halting problem. People get taught how decimals work, find this bug, and then instead of being told how decimals are broken, get told how they're wrong for using the tools they've been taught.

    If we just accept that decimals fail with infinite steps, the transition to new tools would be so much easier, and reflect the same transition into new tools in other sciences. Like Bohr's Atom, Newton's Gravity, Linnaean Taxonomy, or Comte's Positivism.

    • Decimals require you to check the end of the number to see if you can round up, but there never will be an end.

      The character sequence "0.999..." is finite and you know you can round up because you've got those three dots at the end. I agree that decimals are a shit representation to formalise rational numbers in but it's not like using them causes infinite loops. Unless you insist on writing them, that is. You can compute with infinities just fine as long as you keep them symbolic.

      That only breaks down with the reals where equality is fundamentally incomputable. Equality of the rationals and approximate equality of reals is perfectly computable though, the latter meaning that you can get equality to arbitrary, but not actually infinite, precision. You can specify a number of digits you want, you can say "don't take longer than ten seconds to compute", any kind of bound. Once the precision goes down to plank lengths I think any reasonable engineer would build a bridge with it.

      ...sometimes I do think that all those formalists with all those fancy rules about fancy limits are actually way more confused about infinity than freshman CS students.

      • Eh, if you need special rules for 0.999... because the special rules for all other repeating decimals failed, I think we should just accept that the system doesn't work here. We can keep using the workaround, but stop telling people they're wrong for using the system correctly.

        The deeper understanding of numbers where 0.999... = 1 is obvious needs a foundation of much more advanced math than just decimals, at which point decimals stop being a system and are just a quirky representation.

        Saying decimals are a perfect system is the issue I have here, and I don't think this will go away any time soon. Mathematicians like to speak in absolutely terms where everything is either perfect or discarded, yet decimals seem to be too simple and basal to get that treatment. No one seems to be willing to admit the limitations of the system.

        • Noone in the right state of mind uses decimals as a formalisation of numbers, or as a representation when doing arithmetic.

          But the way I learned decimal division and multiplication in primary school actually supported periods. Spotting whether the thing will repeat forever can be done in finite time. Constant time, actually.

          The deeper understanding of numbers where 0.999… = 1 is obvious needs a foundation of much more advanced math than just decimals

          No. If you can accept that 1/3 is 0.333... then you can multiply both sides by three and accept that 1 is 0.99999.... Primary school kids understand that. It's a bit odd but a necessary consequence if you restrict your notation from supporting an arbitrary division to only divisions by ten. And that doesn't make decimal notation worse than rational notation, or better, it makes it different, rational notation has its own issues like also not having unique forms (2/6 = 1/3) and comparisons (larger/smaller) not being obvious. Various arithmetic on them is also more complicated.

          The real take-away is that depending on what you do, one is more convenient than the other. And that's literally all that notation is judged by in maths: Is it convenient, or not.

          • I never commented on the convenience or usefulness of any method, just tried to explain why so many people get stuck on 0.999... = 1 and are so recalcitrant about it.

            If you can accept that 1/3 is 0.333... then you can multiply both sides by three and accept that 1 is 0.99999....

            This is a workaround of the decimal flaw using algebraic logic. Trying to hold both systems as fully correct leads to a conflic, and reiterating the algebraic logic (or any other proof) is just restating the problem.

            The problem goes away easily once we understand the limits of the decimal system, but we need to state that the system is limited! Otherwise we get conflicting answers and nothing makes sense.

            • The problem goes away easily once we understand the limits of the decimal system, but we need to state that the system is limited!

              But the system is not limited: It has a representation for any rational number. Subjectively you may consider it inelegant, you may consider its use in some area inconvenient, but it is formally correct and complete.

              I bet there's systems where rational numbers have unique representations (never looked into it), and I also bet that they're awkward AF to use in practice.

              This is a workaround of the decimal flaw using algebraic logic.

              The representation has to reflect algebraic logic, otherwise it would indeed be flawed. It's the algebraic relationships that are primary to numbers, not the way in which you happen to put numbers onto paper.

              And, honestly, if you can accept that 1/3 == 2/6, what's so surprising about decimal notation having more than one valid representation for one and the same number? If we want our results to look "clean" with rational notation we have to normalise the fraction from 2/6 to 1/3, and if we want them to look "clean" with decimal notation we, well, have to normalise the notation, from 0.999... to 1. Exact same issue in a different system, and noone complains about.

              • Decimals work fine to represent numbers, it's the decimal system of computing numbers that is flawed. The "carry the 1" system if you prefer. It's how we're taught to add/subtract/multiply/divide numbers first, before we learn algebra and limits.

                This is the flawed system, there is no method by which 0.999... can become 1 in here. All the logic for that is algebraic or better.

                My issue isn't with 0.999... = 1, nor is it with the inelegance of having multiple represetations of some numbers. My issue lies entirely with people who use algebraic or better logic to fight an elementary arithmetic issue.

                People are using the systems they were taught, and those systems are giving an incorrect answer. Instead of telling those people they're wrong, focus on the flaws of the tools they're using.

                • This is the flawed system, there is no method by which 0.999… can become 1 in here.

                  Of course there is a method. You might not have been taught in school but you should blame your teachers for that, and noone else. The rule is simple: If you have a nine as repeating decimal, replace it with a zero and increment the digit before that.

                  That's it. That's literally all there is to it.

                  My issue lies entirely with people who use algebraic or better logic to fight an elementary arithmetic issue.

                  It's not any more of an arithmetic issue than 2/6 == 1/3: As I already said, you need an additional normalisation step. The fundamental issue is that rational numbers do not have unique representations in the systems we're using.

                  And, in fact, normalisation in decimal representation is way easier, as the only case to worry about is indeed the repeating nine. All other representations are unique while in the fractional system, all numbers have infinitely many representations.

                  Instead of telling those people they’re wrong, focus on the flaws of the tools they’re using.

                  Maths teachers are constantly wrong about everything. Especially in the US which single-handedly gave us the abomination that is PEMDAS.

                  Instead of blaming mathematicians for talking axiomatically, you should blame teachers for not teaching axiomatic thinking, of teaching procedure instead of laws and why particular sets of laws make sense.

                  That method I described to get rid of the nines is not mathematical insight. It teaches you nothing. You're not an ALU, you're capable of so much more than that, capable of deeper understanding that rote rule application. Don't sell yourself short.

                  EDIT: Bijective base-10 might be something you want to look at. Also, I was wrong, there's way more non-unique representations: 0002 is the same as 2. Damn obvious, that's why it's so easy to overlook. Dunno whether it easily extends to fractions can't be bothered to think right now.

                  • I don't really care how many representations a number has, so long as those representations make sense. 2 = 02 = 2.0 = 1+1 = -1+3 = 8/4 = 2x/x. That's all fine, we can use the basic rules of decimal notation to understand the first three, basic arithmetic to understand the next three, and basic algebra for the last one.

                    0.999... = 1 requires more advanced algebra in a pointed argument, or limits and infinite series to resolve, as well as disagreeing with the result of basic decimal notation. It's steeped in misdirection and illusion like a magic trick or a phishing email.

                    I'm not blaming mathematicians for this, I am blaming teachers (and popular culture) for teaching that tools are inflexible, instead of the limits of those systems.

                    In this whole thread, I have never disagreed with the math, only it's systematic perception, yet I have several people auguing about the math with me. It's as if all math must be regarded as infinitely perfect, and any unbelievers must be cast out to the pyre of harsh correction. It's the dogmatic rejection I take issue with.

                    • 0.999… = 1 requires more advanced algebra in a pointed argument,

                      You're used to one but not the other. You convinced yourself that because one is new or unacquainted it is hard, while the rest is not. The rule I mentioned Is certainly easier that 2x/x that's actual algebra right there.

                      It’s as if all math must be regarded as infinitely perfect, and any unbelievers must be cast out to the pyre of harsh correction

                      Why, yes. I totally can see your point about decimal notation being awkward in places though I doubt there's a notation that isn't, in some area or the other, awkward, and decimal is good enough. We're also used to it, that plays a big role in whether something is judged convenient.

                      On the other hand 0.9999... must be equal to 1. Because otherwise the system would be wrong: For the system to be acceptable, for it to be infinitely perfect in its consistency with everything else, it must work like that.

                      And that's what everyone's saying when they're throwing "1/3 = 0.333.... now multiply both by three" at you: That 1 = 0.9999... is necessary. That it must be that way. And because it must be like that, it is like that. Because the integrity of the system trumps your own understanding of what the rules of decimal notation are, it trumps your maths teacher, it trumps all the Fields medallists. That integrity is primal, it's always semantics first, then figure out some syntax to support it (unless you're into substructural logics, different topic). It's why you see mathematicians use the term "abuse of notation" but never "abuse of semantics".

                      • Again, I don't disagree with the math. This has never been about the math. I get that ever model is wrong, but some are useful. Math isn't taught like that though, and that's why people get hung up things like this.

                        Basic decimal notation doesn't work well with some things, and insinuates incorrect answers. People use the tools they were taught to use. People get told they're doing it wrong. People give up on math, stop trying to learn, and just go with what they can understand.

                        If instead we focus on the limitations of some tools and stop hammering people's faces in with bigger equations and dogma, the world might have more capable people willing to learn.

                  • Maths teachers are constantly wrong about everything

                    Very rarely wrong actually.

                    the abomination that is PEMDAS

                    The only people who think there's something wrong with PEMDAS are people who have forgotten one or more rules of Maths.

                • those systems are giving an incorrect answer

                  When there's an incorrect answer it's because the user has made a mistake.

                  Instead of telling those people they’re wrong

                  They were wrong, and I told them where they went wrong (did something to one side of the equation and not the other).

                  • The system I'm talking about is elementary decimal notation and basic arithmetic. Carry the 1 and all that. Equations and algebra are more advanced and not taught yet.

                    There is no method by which basic arithmetic and decimal notation can turn 0.999... into 1. All of the carry methods require starting at the smallest digit, and repeating decimals have no smallest digit.

                    If someone uses these systems as they were taught, they will get told they're wrong for doing so. If we focus on that person being wrong, then they're more likely to give up on math entirely, because they're wrong for doing as they were taught. If we focus on the limitstions of that system, then they have the explanation for the error, and an understanding of why the more complicated system is preferable.

                    All models are wrong, but some are useful.

                    • not taught yet

                      What do you mean not taught yet? There's nothing in the meme to indicate this is a primary school problem. In fact it explicitly has a picture of an adult, so high school Maths is absolutely on the table.

                      There is no method by which basic arithmetic and decimal notation can turn 0.999… into 1.

                      In high school we teach that they are the same thing. i.e. limits of accuracy, 1 isn't the same thing as 1.000..., but rather 1+/- some limit of accuracy (usually 1/2). Of course in programming it matters if you're talking about an integer 1 or a floating point 1.

                      If someone uses these systems as they were taught, they will get told they’re wrong for doing so

                      The only people I've seen get things wrong is people not using the systems correctly (such as the alleged "proof" in this thread, which broke several rules of Maths and as such didn't prove anything), and it's a teacher's job to point out how to use them correctly.

                      • What do you mean not taught yet?

                        I mean those more advanced methods are taught after basic arithmetic. There are plenty of adults that operate primarily with 5th grade math, and a scary number of them do finances...

                        limits of accuracy

                        This isn't about limits of accuracy, we're working with abstract values and ideal systems. Any inaccuracies must be introduced by those systems.

                        If you think the system isn't at fault here, please show me how basic arithmetic can make 0.999... into 1. Show me how the carry method deals with Infinity correctly. If every error is just using the system incorrectly, then a correct use of the system must be applicable to everything, right? You shouldn't need a new system like algebra to be correct, right?

        • The system works perfectly, it just looks wonky in base 10. In base 3 0.333... looks like 0.1, exactly 0.1

          • Oh the fundamental math works fine, it's the imperfect representation that is infinite decimals that is flawed. Every base has at least one.

    • That does very accurately sum up my understanding of the matter, thanks. I haven't been adding on to any of the other conversation in order to avoid putting my foot in my mouth further, but you've pretty much hit the nail on the head here. And the higher mathematics required to solve this halting problem are beyond me.

231 comments