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Frequently asked questions

Q: Are you positive that 0.999... equals 1 exactly, not approximately?

A: In the set of real numbers, yes. This is covered in the article. If you still have doubts, you can discuss it at Talk:0.999.../Arguments. However, please note that original research should never be added to a Wikipedia article, and original arguments and research in the talk pages will not change the content of the article—only reputable secondary and tertiary sources can do so.

Q: Can't "1 - 0.999..." be expressed as "0.000...1"?

A: No. The string "0.000...1" is not a meaningful real decimal because, although a decimal representation of a real number has a potentially infinite number of decimal places, each of the decimal places is a finite distance from the decimal point; the meaning of digit d being k places past the decimal point is that the digit contributes d · 10^-k toward the value of the number represented. It may help to ask yourself how many places past the decimal point the "1" is. It cannot be an infinite number of real decimal places, because all real places must be finite. Also ask yourself what the value of

{\frac {0.000\dots 1}{10}}

would be. Those proposing this argument generally believe the answer to be 0.000...1, but, basic algebra shows that, if a real number divided by 10 is itself, then that number must be 0.

Q: The highest number in 0.999... is 0.999...9, with a last '9' after an infinite number of 9s, so isn't it smaller than 1?

A: If you have a number like 0.999...9, it is not the last number in the sequence (0.9, 0.99, ...); you can always create 0.999...99, which is a higher number. The limit

0.999\ldots =\lim _{n\to \infty }0.\underbrace {99\ldots 9} _{n}

is not defined as the highest number in the sequence, but as the smallest number that is higher than any number in the sequence. In the reals, that smallest number is the number 1.

Q: 0.9 < 1, 0.99 < 1, and so forth. Therefore it's obvious that 0.999... < 1.

A: No. By this logic, 0.9 < 0.999...; 0.99 < 0.999... and so forth. Therefore 0.999... < 0.999..., which is absurd.

Something that holds for various values need not hold for the limit of those values. For example, f (x)=x³/x is positive (>0) for all values in its implied domain (x ≠ 0). However, the limit as x goes to 0 is 0, which is not positive. This is an important consideration in proving inequalities based on limits. Moreover, although you may have been taught that

0.x_{1}x_{2}x_{3}...

must be less than

1.y_{1}y_{2}y_{3}...

for any values, this is not an axiom of decimal representation, but rather a property for terminating decimals that can be derived from the definition of decimals and the axioms of the real numbers. Systems of numbers have axioms; representations of numbers do not. To emphasize: Decimal representation, being only a representation, has no associated axioms or other special significance over any other numerical representation.

Q: 0.999... is written differently from 1, so it can't be equal.

A: 1 can be written many ways: 1/1, 2/2, cos 0, ln e, i⁴, 2 - 1, 1e0, 1₂, and so forth. Another way of writing it is 0.999...; contrary to the intuition of many people, decimal notation is not a bijection from decimal representations to real numbers.

Q: Is it possible to create a new number system other than the reals in which 0.999... < 1, the difference being an infinitesimal amount?

A: Yes, although such systems are neither as used nor as useful as the real numbers, lacking properties such as the ability to take limits (which defines the real numbers), to divide (which defines the rational numbers, and thus applies to real numbers), or to add and subtract (which defines the integers, and thus applies to real numbers). Furthermore, we must define what we mean by "an infinitesimal amount." There is no nonzero constant infinitesimal in the real numbers; quantities generally thought of informally as "infinitesimal" include ε, which is not a fixed constant; differentials, which are not numbers at all; differential forms, which are not real numbers and have anticommutativity; 0⁺, which is not a number, but rather part of the expression

\lim _{x\rightarrow 0^{+}}f(x)

, the right limit of x (which can also be expressed without the "+" as

\lim _{x\downarrow 0}f(x)

); and values in number systems such as dual numbers and hyperreals. In these systems, 0.999... = 1 still holds due to real numbers being a subfield. As detailed in the main article, there are systems for which 0.999... and 1 are distinct, systems that have both alternative means of notation and alternative properties, and systems for which subtraction no longer holds. These, however, are rarely used and possess little to no practical application.

Q: Are you sure 0.999... equals 1 in hyperreals?

A: If notation '0.999...' means anything useful in hyperreals, it still means number 1. There are several ways to define hyperreal numbers, but if we use the construction given here, the problem is that almost same sequences give different hyperreal numbers,

0.(9)<0.9(9)<0.99(9)<0.(99)<0.9(99)<0.(999)<1\;

, and even the '()' notation doesn't represent all hyperreals. The correct notation is (0.9; 0.99; 0,999; ...).

Q: If it is possible to construct number systems in which 0.999... is less than 1, shouldn't we be talking about those instead of focusing so much on the real numbers? Aren't people justified in believing that 0.999... is less than one when other number systems can show this explicitly?

A: At the expense of abandoning many familiar features of mathematics, it is possible to construct a system of notation in which the string of symbols "0.999..." is different than the number 1. This object would represent a different number than the topic of this article, and this notation has no use in applied mathematics. Moreover, it does not change the fact that 0.999... = 1 in the real number system. The fact that 0.999... = 1 is not a "problem" with the real number system and is not something that other number systems "fix". Absent a WP:POV desire to cling to intuitive misconceptions about real numbers, there is little incentive to use a different system.

Q: The initial proofs don't seem formal and the later proofs don't seem understandable. Are you sure you proved this? I'm an intelligent person, but this doesn't seem right.

A: Yes. The initial proofs are necessarily somewhat informal so as to be understandable by novices. The later proofs are formal, but more difficult to understand. If you haven't completed a course on real analysis, it shouldn't be surprising that you find difficulty understanding some of the proofs, and, indeed, might have some skepticism that 0.999... = 1; this isn't a sign of inferior intelligence. Hopefully the informal arguments can give you a flavor of why 0.999... = 1. If you want to formally understand 0.999..., however, you'd be best to study real analysis. If you're getting a college degree in engineering, mathematics, statistics, computer science, or a natural science, it would probably help you in the future anyway.

Q: But I still think I'm right! Shouldn't both sides of the debate be discussed in the article?

A: The criteria for inclusion in Wikipedia is for information to be attributable to a reliable published source, not an editor's opinion. Regardless of how confident you may be, at least one published, reliable source is needed to warrant space in the article. Until such a document is provided, including such material would violate Wikipedia policy. Arguments posted on the Talk:0.999.../Arguments page are disqualified, as their inclusion would violate Wikipedia policy on original research.

0.999... is a featured article; it (or a previous version of it) has been identified as one of the best articles produced by the Wikipedia community. Even so, if you can update or improve it, please do so.

This article appeared on Wikipedia's Main Page as Today's featured article on October 25, 2006.

Article milestones
Date	Process	Result
May 5, 2006	Articles for deletion	Kept
October 10, 2006	Featured article candidate	Promoted
August 31, 2010	Featured article review	Kept

Current status: Featured article

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Arguments Archives: Index, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12

Yet another anon

Moved to Arguments subpage

Terminating decimals

"Terminating decimal" is a technical term that must be linked in such an elementary article. Previously, it was linked to Repeating decimal. I agree that this is not a convenient target. I have created a redirect, and linked it to an anchor in the lead of Decimal. If this link is not correct, this is not a problem of 0.999... but a problem of the redirect page or of the target page. In any case, the link Terminating decimal must be kept. D.Lazard (talk) 15:09, 27 October 2023 (UTC)[reply]

Agree. Since this term may be unfamiliar to some of our readers a link is necessary. If the article that it the target of the link needs improvement that should be discussed/implemented there, not here. Mr. Swordfish (talk) 16:18, 27 October 2023 (UTC)[reply]

I am a native speaker of English, which is fundamentally the target of WP:en. "Terminating decimal" is not a technical term, at all, it is simply the participle adjective "terminating", which means "it stops", qualifying "decimal". If there really were an article "terminating decimal", a link would be unnecessary, IMO, but not confusing. The "repeating decimal" article is not very good, since the first paragraph tells us that a terminating decimal is not a "repeating decimal", then the second paragraph backtracks, and says that a "terminating decimal" is one where the repeating sequence is just zeros. It cannot help to link a self-explanatory term to this. Imaginatorium (talk) 19:09, 27 October 2023 (UTC)[reply]

@Imaginatorium: Adjectives need precise definitions in math texts and this is one instance. A decimal expansion is not an event in time or place in space so the English definition does not apply, and is in any case too imprecise. It is very unlikely you are going to get consensus in favor of your view.--Jasper Deng (talk) 19:27, 27 October 2023 (UTC)[reply]

It seems also that Imaginatorium did not notice that "terminating decimal" does not link anymore to Repeating decimal; the target is an anchor in Decimal . D.Lazard (talk) 19:38, 27 October 2023 (UTC)[reply]

One more proof

According to the formation rule, the reciprocal part of the number is 9 and the non-revolving part is zero. Accordingly (9-0)/9=1. Please add this.

Bera678 (talk) 19:14, 15 December 2023 (UTC)[reply]

This is not a proof. Moreover, for being added here, a proof requires to be published in a textbook, and you do not provide any source. D.Lazard (talk) 09:36, 16 December 2023 (UTC)[reply]

Maybe I didn't fully express what I meant. But I'm sure it's proof. Although this is based on personal research, we can find a reference. Bera678 (talk) 09:44, 16 December 2023 (UTC)[reply]

It certainly is not a proof. It is merely quoting a rule of thumb for obtaining the value, but the rule of thumb is valid only because there is a proof of it. JBW (talk) 18:30, 16 December 2023 (UTC)[reply]

Did you look at the formation rule in the 'in compressed form' section of the Repeating decimal article? If you looked you can see that our number is equal to 9/9 to 1. Moreover, this evidence may be more understandable to readers. Bera678 (talk) 12:26, 17 December 2023 (UTC)[reply]

Yes, that may be the most convincing "evidence" for many readers, but it is not proof, since it hinges on arithmetic algorithms that first should be proven to be valid for infinite decimals. Nø (talk) 13:27, 17 December 2023 (UTC)[reply]

OK Bera678 (talk) 13:51, 17 December 2023 (UTC)[reply]

FAR notice

This is an FA from 2006 that underwent FAR in 2010 and was kept. This article does not currently meet the featured article criteria:

It uses a mixture of parenthetical referencing, which is deprecated, and inline references, failing 2.c.
The "Elementary proof" section is entirely unreferenced, and many other sections have unreferenced paragraphs, some of which appears to contain OR (see, e.g., "Impossibility of unique representation"), failing 1.c.
There are weasel words and editorializing throughout and the writing style is at times casual, failing 2.

Pinging @JBL; I saw your recent FA and hoped you might be able to take a look. voorts (talk/contributions) 02:23, 18 January 2024 (UTC)[reply]

@Voorts I'll deal the citation style. I'm changing to sfnp for all short citations. Dedhert.Jr (talk) 02:31, 18 January 2024 (UTC)[reply]

The rest of the cites need work too; many of them don't use any citation formats and some of them are ref tags with {{harv}}s inside them. Since there are variations in citation style, I think they can all be changed to {{sfnp}} for conformity. voorts (talk/contributions) 02:36, 18 January 2024 (UTC)[reply]

One additional thing: I don't see any kind of thorough source checking in either the FA or FAR discussions. voorts (talk/contributions) 02:40, 18 January 2024 (UTC)[reply]

@Voorts An additional thing but optional likely. I do think that this article uses many types of math templates, math in TeX, and by simply just using HTML code. So I prefer to use Tex instead, right after completing the citations format problems. Dedhert.Jr (talk) 05:45, 19 January 2024 (UTC)[reply]

I'm not really well-versed in math templates on Wikipedia, so I can't really opine on what to use, but I agree that using plain html code is not the best. voorts (talk/contributions) 05:56, 19 January 2024 (UTC)[reply]

Thanks for the ping, voorts. Unfortunately I've discovered about myself that I'm good at starting something more or less from scratch, and good at local spot-checking, but not very good at the kind of work needed here. I'll try to take a look, though. --JBL (talk) 20:56, 21 January 2024 (UTC)[reply]

I am willing to help with this one. Ping me if want help with anything. I will conduct a source check. For the record: I do not see any problem with the casual writing style, given the readership of this article. Hawkeye7 (discuss) 21:20, 21 January 2024 (UTC)[reply]

"we should convert this into the book being used as a reference (but that would require access to it to see how to use it)" Fortunately, I do. Which is why I said I would look at the sources. Hawkeye7 (discuss) 23:36, 21 January 2024 (UTC)[reply]

Can you explain what "weasel words" means in this context? An example or two would help... Imaginatorium (talk) 09:38, 22 January 2024 (UTC)[reply]

Sure, here's a couple:

"While most authors choose to define"
"Many algebraic arguments have been provided"

voorts (talk/contributions) 21:19, 22 January 2024 (UTC)[reply]

See also WP:WEASEL. voorts (talk/contributions) 21:19, 22 January 2024 (UTC)[reply]

I don't understand the assertion that either of these is weasel-y. These assertions might or might not be adequately sourced (to be clear: I haven't checked), but if they reflect the sources I don't see what's objectionable about them. --JBL (talk) 23:10, 22 January 2024 (UTC)[reply]

"While most authors choose to define" is not in the source, so I have removed it. I'm not seeing support for the assertion "Division by zero occurs in some popular discussions of 0.999..." either. Unless someone can find one, I suggest we remove the entire bullet point. Apart from that sentence though, it is correctly sourced. Hawkeye7 (discuss) 02:10, 23 January 2024 (UTC)[reply]

@Hawkeye7 @JayBeeEll @Dedhert.Jr: Where are we on this? Has enough been done to fix this, or should this proceed to FAR? voorts (talk/contributions) 22:45, 21 February 2024 (UTC)[reply]

@Voorts I'm replying. Will trying to convert again as soon as possible, and copyedit; trying my best. Dedhert.Jr (talk) 13:37, 22 February 2024 (UTC)[reply]

I have converted the format footnotes into sfnp and harvtxt, and all math format in Tex. Dedhert.Jr (talk) 13:36, 23 February 2024 (UTC)[reply]

I have moved unused references to the Further reading section. Hawkeye7 (discuss) 19:16, 23 February 2024 (UTC)[reply]

The "Division by zero occurs in some popular discussions..." reads like WP:SYNTH to me (that is, WP:SYNTH dressed up with citations to the background topics being synthesized). There's maybe something to be said about how understanding limits can give a precise meaning to the intuitive idea of "division by zero" (or "division by infinity"), and limits are also important here, but without a source explicitly drawing that connection, we shouldn't include it. XOR'easter (talk) 01:42, 27 February 2024 (UTC)[reply]

Unless there are any objections, I plan on bringing this to FAR one week from now. voorts (talk/contributions) 03:06, 26 March 2024 (UTC)[reply]

Elementary proof supported by Stillwell?

It strikes me that the Stillwell reference for the section on the Elementary proof is not ideal. Can anyone find a better reference? Tito Omburo (talk) 22:20, 11 April 2024 (UTC)[reply]

Root of student confusion?

Is there any elementary education literature on confusion caused by teaching real numbers in terms of decimal expansions instead of axiomatically or geometrically? I believe that if such an RS exits then the article should discuss the issue. -- Shmuel (Seymour J.) Metz Username:Chatul (talk) 12:55, 14 April 2024 (UTC)[reply]

@@ Line 97: / Line 97: @@
 It strikes me that the Stillwell reference for the section on the Elementary proof is not ideal.  Can anyone find a better reference?  [[User:Tito Omburo|Tito Omburo]] ([[User talk:Tito Omburo|talk]]) 22:20, 11 April 2024 (UTC)
+== Root of student confusion? ==
+Is there any elementary education literature on confusion caused by teaching real numbers in terms of decimal expansions instead of axiomatically or geometrically? I believe that if such an [[WP:RS|RS]] exits then the article should discuss the issue. -- [[User:Chatul|Shmuel (Seymour J.) Metz Username:Chatul]] ([[User talk:Chatul|talk]]) 12:55, 14 April 2024 (UTC)