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Is there a consensus in the mathematical community, or some accepted authority, to determine whether zero should be classified as a natural number?

It seems as though formerly $0$ was considered in the set of natural numbers, but now it seems more common to see definitions saying that the natural numbers are precisely the positive integers.

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It may be Italian education, but I've always been told, from 1st grade to 3rd year of my engineering degree course (present), that 0 ∈ ℕ, and never had any reason to believe the countrary. (We have ℕ₀=ℕ\{0} when the need does arise.) –  badp Jul 21 '10 at 12:06
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voted to close. The question is subjective, as is clearly indicated by the first sentence of the wikipedia article on Natural Numbers –  Tom Stephens Jul 23 '10 at 17:05
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While definitely subjective, it might be the case that the asker genuinely does not know about the controversy and is in need of an answer to say "There is no answer". Whatever the case, I still voted to close. –  Justin L. Jul 23 '10 at 20:49
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@Justin, I know that there are mixed views (as indicated in the second paragraph of my question). But for the case of 1 being classified as a prime number, it seems the consensus view of the Mathematical community is that it should not count as a prime number. My actual question is 'Is there a consensus on whether zero is a natural number?' (although the question's title is simpler), so a suitable answer would be 'No, there is no consensus' combined with a quick demonstration from a few Mathematical dictionaries or articles that there are conflicting definitions. –  bryn Jul 24 '10 at 2:37
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@Nick The responses to this question indicate that the definitions you propose are far from universally accepted. I agree that it would be great if everyone agreed on a standard, but I would argue strongly for the convention that 0 is a natural number. The convention $0\in\mathbb{N}$ doesn't mean you have to start counting at 0! –  Alex Kruckman Dec 2 '13 at 18:24

6 Answers 6

up vote 20 down vote accepted

Simple answer: sometimes yes, sometimes no, it's usually stated (or impled by notation). From the Wikipedia article:

In mathematics, there are two conventions for the set of natural numbers: it is either the set of positive integers $\{1, 2, 3, \dots\}$ according to the traditional definition; or the set of non-negative integers $\{0, 1, 2,\dots\}$ according to a definition first appearing in the nineteenth century.

Saying that, more often than not I've seen the natural numbers only representing the 'counting numbers' (i.e. excluding zero). This was the traditional historical definition, and makes more sense to me. Zero is in many ways the 'odd one out' - indeed, historically it was not discovered (described?) until some time after the natural numbers.

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I see plenty of both these days, but when I was at school and at university, I almost only saw them defined to be {0, 1, ..}. The elements of {1, 2, ..} were called the whole numbers in my school days. –  Charles Stewart Jul 21 '10 at 9:06
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Maybe it's because I'm a student of physics that we do things slightly differently, but we seem to call only the counting numbers 'natural numbers'. 'Whole numbers' is just an informal way of describing all integers. –  Noldorin Jul 21 '10 at 9:12
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Indeed, I can see why they were defined that way. There's a surprising lack of consistency in this area of naming. –  Noldorin Jul 21 '10 at 13:50
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I would say that in number theory you will probably see $\mathbb N=\{1,2,\ldots\}$, but in set-theoretical textbooks $0$ will be included as a natural number. (It is a natural approach in that contexts, since they're defined as finite ordinals. –  Martin Sleziak Sep 17 '11 at 12:09
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@Kaveh: Not including $0$ simplifies other things. For example, you can then define rational numbers as (equivalence classes of) pairs of an integer and a natural number; no explicit exception for $0$ needed. And also the equivalence relation can then be easily stated by $(a,b) \equiv (ac,bc)$ for any $c\in\mathbb N$ (again, no exception needed). –  celtschk Aug 1 '13 at 17:12

There is no "official rule", it depends from what you want to do with natural numbers. Originally they started from $1$ because $0$ was not given the status of number.

Nowadays if you see $\mathbb{N}^+$ you may be assured we are talking about numbers from $1$ above; $\mathbb{N}$ is usually for numbers from $0$ above.

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"$\mathbb{N}$ is usually for numbers from $0$ above." Can you point to evidence supporting this "usually"? –  Jonas Meyer Aug 1 '13 at 4:01
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For evidence that the issue exists, mathworld.wolfram.com/NaturalNumber.html is a source; as for the "usually", I should dig my old books, I think. –  mau Aug 1 '13 at 13:38

There are the two definitions, as you say. However the set of strictly positive numbers being the natural numbers is actually the older definition. Inclusion of $0$ in the natural numbers is a definition for them that first occurred in the 19th century.

The Peano Axioms for natural numbers take $0$ to be one though, so if you are working with these axioms (and a lot of natural number theory does) then you take $0$ to be a natural number.

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Bourbaki included zero in 1935. That's not all that recent... –  user126 Jul 21 '10 at 10:08
    
More recently, rather than actually recent... but point taken, I'll revise the wording :) –  workmad3 Jul 21 '10 at 11:57
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"take $0$ to be one" is confusing phrasing. –  Jonas Meyer Aug 1 '13 at 4:00
    
@JonasMeyer: Actually it makes sense in a non-trivial way: From the axioms, the natural numbers are basically a set with a first number, and then a successor for each number. At that point, you cannot actually distinguish between the natural numbers starting at $0$ and the natural numbers starting at $1$, except for the (completely arbitrary) name for the initial number. Basically, it is the different definition for addition and multiplication which distinguishes the two choices. –  celtschk Aug 1 '13 at 16:57
    
Now if you say "one" is the name of the initial natural number, then "take $0$ to be one" would be interpreted as "take $0$ to be the initial number", that is, "start the natural numbers with $0$". –  celtschk Aug 1 '13 at 17:00

I remember all of my courses at University using only positive integers (not including $0$) for the Natural Numbers. It's possible that they had come to an agreement amongst the Maths Faculty, but during at least two courses we generated the set of natural numbers in ways that wouldn't make sense if $0$ was included.

One involved the cardinality of Sets of Sets, the other defined the natural numbers in terms of the number $1$ and addition only ($0$ and Negative Integers come into the picture later when you define an inverse to addition).

As a result when teaching the difference between Integers and Natural Numbers I always define $0$ as an integer that isn't a Natural Number.

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Obviously, defining ℕ from 0 and addition also works perfectly. I don't know what difference would 0 make to calculating the cardinality of P(ℕ) either. –  badp Jul 21 '10 at 12:08
    
The cardinality of sets of sets can certainly be $0$: All members of the empty set are sets. Indeed, in ZF all sets are sets of sets. –  celtschk Aug 1 '13 at 17:17

In consideration of symbols representing the counting (Natural) numbers, If $0$ is not included in the set $\mathbb{N}$, then there is no definition for the symbols $10, 100,\dots,100000$ etc. Since $0$ is undefined within the set $\mathbb{N}$.

For the system based on the number $10$ (Or any other base for that matter), the series $10$ to the $n$ is undefined for $n=0$ and we might just as well use the Roman symbol X or maybe even an Egyptian Hieroglyphic.

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It seems very unclear what you are trying to say and how it relates to the question. Moreover, the question has been properly answered already. –  Rasmus Jun 22 '12 at 14:04
    
@Rasmus: I think he is trying to say that digits are natural numbers so if zero was not a natural number we could not write $10$ as a natural number. I guess that in the naive way of thinking the representation of a number is closely related to the number. –  Asaf Karagila Jun 22 '12 at 15:09
    
Not so naive, as the lack of zero was an impediment to the introduction of positional notation. –  Jean-Claude Arbaut Mar 15 '13 at 19:14
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Whether or not $0$ is in the set $\mathbb N$ has no bearing on whether or not the symbol '$0$' may be used in representations of the elements of $\mathbb N$. This answer is incorrect. –  Jonas Meyer Aug 1 '13 at 4:13
    
While the answer isn't entirely clear, I think it makes a very good point. Excluding $0$ from the definition of $\mathbb{N}$ makes life difficult when you want to define base $b$ representation. For, you want to say that $104 = 1 \cdot 100 + 0 \cdot 10 + 4 \cdot 1$, but you can say no such thing if you don't have $0$ within your set of numbers. –  Goos Mar 12 at 17:58

Unambiguous names for the set of nonnegative integers $(0, 1, 2, 3, \dots)$ are whole numbers Unambiguous names for the set of positive integers $(1, 2, 3, \dots)$ are counting numbers and $\mathbb{N}^+$.
As other answers (and your own question) indicate, natural numbers, or $\mathbb{N}$, can have different meanings, depending on context. If which usage you mean is not clear from context, you are better off using one of the other terms.

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ℕ₀ is not an unambiguous name, we use it as ℕ₀ = ℕ \ {0}. –  badp Jul 23 '10 at 9:55
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@badp: I've never seen it used that way, but if there is someone else out there who does, I suppose that makes it ambiguous. Who's the we? –  Larry Wang Jul 23 '10 at 14:53
    
Italian education at least. –  badp Jul 24 '10 at 14:13
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"whole" and "counting" are ambiguous, too. Why is $-1$ not "whole"? Why is $0$ not a counting number? –  Jonas Meyer Aug 1 '13 at 3:57
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Indeed, Wikipedia confirms that "whole number" is even more ambiguous than "natural number" because it may not only take both meanings of "natural number", but in addition also the meaning of "integer". –  celtschk Aug 1 '13 at 17:04

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