30 September 2009

Is zero even?

Did you know that there are actually things to say about whether zero is even or odd (from Wikipedia)? Obviously it is, but the math-ed folks have seriously looked at this.

I found this via a comment by John Thacker at The Volokh Conspiracy. There's a poll there; right now 2% of people have said 0 is odd, 51% even, 43% both, 4% neither. I can kind of understand what's going on with people saying "neither" (perhaps they're getting this from some elementary-school notions), but how is 0 odd?

My answer: yes, zero is even, because it's twice an integer.

(Or because the identity permutation on n letters is an element of the alternating group An -- I've been thinking about permutations a lot lately. But if you understand that, you probably are like me, think zero is even, and didn't even think there was anything to discuss.)

Incidentally, sometime recently -- I forget the context -- I saw something that referred to the Gaussian integer a+bi as "uneven" if and only if a and b had different parity.

24 comments:

Anonymous Rex said...

The sign of a permutation is actually pretty confusing: odd cycles are even, while even cycles are odd.

The comments on the Volokh Conspiracy are a goat rope. My favorite comment was "It would be odd if it were not even." A great find was the book McGraw-Hill’s Catholic High School Entrance Exams, 2ed. (I particularly enjoyed the line "even integer +/- even integer = even integer" just after defining 0 as not even.)

A book by David J. Benson (a mathematician) says at some point that "The left side is even and the right side is odd, so their common value is both even and odd, and hence zero." which would be frightening if he weren't talking about functions!

CarlBrannen said...

I can't believe how well you've stunned me. This is amazing.

I wonder if it has anything to do with factorization: "Since 1 is neither composite nor prime, doesn't that mean that 0 is neither even nor odd?"

Meanwhile, I've got my first paper accepted for publication, at a reasonably prestigious place, International Journal of Modern Physics D, on gravitation.

.mau. said...

I prefer to see it in this way:
"even minus even is even", but 0 = 2-2 and therefore is even.

Anna said...

Wow, I'm really surprised to see how much debate there is. It seems so obvious to me!

@Carl: we say 1 isn't prime just so the theorems are shorter. Otherwise, loads of them woud be "for every prime number larger than 1..." and the expression "prime larger than 1" would be used many times more often that the word "prime" alone.

Zero being odd doesn't cause any problems like one being prime would. On the contrary, if we didn't consider 0 even, we'd often have to say things like "all even numbers and zero".

Mark Dominus said...

I was also surprised to discover last year that there is so much uncertainty about this. My four-year-old asked me whether it was even or odd. Our formula for evenness has followed the example of "Is 14 even?" "Yes, because it is 7+7." "Is 13 even?" "No, because it is 6+6+1". So when she asked if zero was even I said of course, because it is 0+0. But the other adults in the room looked at me strangely and asked if I was sure. Fortunately this is an issue on which I don't have to hedge, or simplify; I just said yes, zero is even.

Infinity is much more complicated.

CarlBrannen said...

I just asked out company's illustrator, "is zero even or odd", and he said "can't it be neutral?" He had no difficulty identifying 2 as even. When I pointed out that, in fact, zero is even, he said, "you see, I was right".

I'm still trying to wrap my mind around an understanding of why someone would think that zero is anything other than the most even of all the even numbers. If you're going to be unsure about a number being even or odd, it should be something large like:
7^193161 + 17163131341663
(which is even).

Maybe it has something to do with the absence of a multiplicative inverse of zero. But I suspect that instead it has to do with zero and one being "special" and not included in the primes and composites.

By the way, Anna, probably in a senior class in abstract algebra, I was taught that 1 is not a prime because it is a "unit". Interestingly, Wikipedia says that until the 19th century, 1 was considered prime.

Mark Dominus said...

I asked Iris (age 5.3) today to see what she would say, and she said it was even. When I asked why, she said it was because 1 was odd, and even and odd numbers always alternate.

Usually in such cases I likes to probe a little deeper, perhaps by bringing up some complication that confuses the issue. "But what about xyz," one says. "Shouldn't that mean that 0 is odd?" But in this case I could think of no such argument.

I guess that's what's puzzling about the whole issue. None of us seems to have any good ideas about why anyone would think otherwise.

Pseudonym said...

The question of whether or not 1 is prime actually makes a certain amount of sense as a debate. I must admit that I couldn't think of a good reason why it shouldn't be prime until I discovered finite fields.

unapologetic said...

Let me play devil's advocate, Pseudonym... what's you're "good reason" now that you've heard about finite fields?

unapologetic said...

Gah. should be "your".. 9:00 is too early to be up.

CarlBrannen said...

unapologetic, it's presumably because for any prime, there is a field with that number of elements. But there is no field with 1 element because you need to have 0 and 1, so the least size is 2.

'"But what about xyz," one says. "Shouldn't that mean that 0 is odd?" But in this case I could think of no such argument.'

This is exactly right. I can think of no reason why 0 should be odd.

However, here's an argument: Human use of math is based on the desire for justice, hence our concentration on equality. If you have an even number of cupcakes, you can divide them equally so that Alice and Bob get the same number. But with zero cupcakes, there's nothing to divide.

unapologetic said...

Carl, that's exactly what I was hoping someone would say. And then I could point out that "but the only reason there's no 'field with 1 element' is that we make an arbitrary insistence that the additive and multiplicative identities of a ring be distinct".

In fact, there are many, many reasons why the field with one element should exist. Just because the undergraduate notion of sets-with-structure doesn't capture it doesn't mean it's not there.

JD said...

Obviously, zero is even. But then again, so is every number, from our lovely 2n (with n being integer) to an irrational complex.

Proof:

Let f(x) = C, where C is the number whose evenness you want to prove. f(x) is obviously an even function, and since you can't have even = odd, C is therefore even.


Note: the above is a mathematician detection tool. Anybody who reads it and cringes is a mathematician. :)

Pseudonym said...

unapologetic, in a sense, I guess you could say that the axiom that 1 is not prime is the same as the axiom that in all fields, 0≠1, or any equivalent axiom (e.g. the axiom that in any field, x/0 is not defined for any x).

But this is beside the point.

Caution: Nonstandard terminology follows.

Let p be a prime. Define the Galois field of order p GF(p) as the ring of p-adic integers with a suitable division operation.

If 1 were a prime, then it would not be the case that GF(p) is a field for all prime p. Even if you agree that there is a finite field with one element, it's not a Galois field.

Then the axiom that 0≠1 merely implies that all finite fields are Galois fields and vice versa. Or, to put it another way, if you do not admit the axiom 0≠1 (or equivalent), then all this would show is that there is a finite field which is not a Galois field.

And finally, all I said is that this was the first comprehensive argument that I encountered as to why 1 should not be placed in the category "prime number". This is an extremely modest claim.

plutoman said...

Interesting. Remember my mum teaching her primary school kids about rounding numbers to the nearest hundred. There was a bit of debate about how to round the number 30. Mum thought the answer should be 100 because "zero isn't a hundred, so the nearest hundred is 100".

Anonymous said...

One reason for saying that zero is not even, without claiming it is odd, might be the following.

"Is zero positive?"
"No."
"Is zero negative?"
"No."

Similarly

"Is zero even?"
"No."
"Is zero odd?"
"No."

Pseudonym said...

plutoman, I found that comment interesting because rounding is a more difficult problem than most people appreciate. How you round depends on what kind of error you're trying to minimise (absolute vs relative error, while also avoiding systematic error).

If you had to round 40 to the nearest power of 10, you should pick 1 if you're minimising absolute error, and 100 if you're minimising relative error.

unapologetic said...

Pseudonym: wait what?

choices: 1, 100

absolute error: 39, 60

relative error: 39/40, 3/2

1 minimizes both absolute and relative error.

Anonymous said...

At an exam one student wrote that "scientists are still arguing whether 0 is even or odd." No questions asked, he was given an F and was later exmatriculated from the faculty.

Pseudonym said...

unapologetic, sorry, I made a couple of mistakes there; I blame lack of caffeine.

You should round 40 to 100 rather than 10 (I incorrectly said 1) to minimise the relative difference, not relative error. The relative difference between 10 and 40 is (40-10)/40 = 0.75, and the relative difference between 40 and 100 is (100-40)/100 = 0.6.

Xi_Heather said...

I think 0 being odd comes if you don't think it's even. Zero is a strange number, so 0=0+0 doesn't feel like "proof" that it's even to everyone [you can put 0 objects in two piles of 0, but that just feels weird and doesn't fly with everyone].

If your definition of odd is "not even" rather than something along the lines of 2K+1, then if 0 isn't even it must be odd.

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