Is infinity an odd or even number?

My 6 year old wants to know if infinity is an odd or even number. His 38 year old father is keen to know too.

Solution 1:

In the context of transfinite ordinals, the usual definition is that an ordinal number $\alpha$ is even if it is a multiple of $2$, specifically: if there is another ordinal $\beta$ such that $2\cdot\beta=\alpha$. In other words, the order type $\alpha$ can be viewed as $\beta$ many pairs in sequence, or in other words, $\alpha$ is left-divisible by $2$. Otherwise, it is odd.

It is easy to prove from this definition by transfinite recursion that the ordinals come in an alternating even/odd pattern, and that every limit ordinal (and hence every infinite cardinal) is even. Many transfinite constructions proceed by doing something different on the even as opposed to the odd stages, just as with finite constructions.

The smallest infinite ordinal is $\omega$, which is even on this definition, since having $\omega$ many pairs in sequence is order-isomorphic to $\omega$, and so $2\cdot\omega=\omega$. Meanwhile, the next infinite ordinal is $\omega+1$, which is odd. The ordinal $\omega+2$ is even, since it is equal to $2\cdot(\omega+1)$, even though it is not $\beta+\beta$ for any $\beta$.

(Please note that $\alpha=2\cdot\beta$ is not at all the same as saying $\alpha=\beta+\beta$, since $\beta$ copies of $2$ is not the same order type as $2$ copies of $\beta$, a phenomenon at the heart of the non-commutativity of ordinal multiplication. )

To explain the idea to a child, I would focus on the principal idea: whether finite or infinite, a number is even when it can be divided into pairs. For finite sets, this is the same as the ability to divide the set into two sets of equal size, since one may consider the first element of each pair and the second element of each pair. In the infinite context, as others have noted, there are numerous concepts of infinity, each with its own concept of even and odd. In my experience with children, one of the easiest-to-grasp concepts of infinity is provided by the transfinite ordinals, since it can be viewed as a continuation of the usual counting manner of children, but proceeding into the transfinite:

$$1,2,3,\cdots,\omega,\omega+1,\omega+2,\cdots,\omega+\omega=\omega\cdot2,\omega\cdot 2+1,\cdots,\omega\cdot 3,\cdots,\omega^2,\omega^2+1,\cdots,\omega^2+\omega,\cdots\cdots$$

This concept of infinity is attractive to children, because they can learn to count into the infinite this way. Also, this concept of infinity has one of the most successful parity concepts, since one maintains the even/odd pattern into the transfinite. The smallest infinity $\omega$ is even, $\omega+1$ is odd, $\omega+2$ is even and so on. Every limit ordinal is even, and then it repeats even/odd up to the next limit ordinal.

See the Wikipedia entries on transfinite ordinals and ordinal arithmetic for more information about the ordinals.

Solution 2:

I suggest that you read the discussion at Is infinity a number? first (since of course you need to answer that question to answer this question). There are some senses in which infinity is a number, and there are some senses in which infinity is not a number, and it all depends on what exactly you mean by "number," which in turn depends on what applications you have in mind.

On the other hand, there is a useful sense in which infinity is even. To explain this we have to replace "numbers" with cardinalities of sets.

Definition: A set $S$ has even cardinality if it can be written as the disjoint union of two subsets $A, B$ which have the same cardinality.

In other words, we need to be able to divide $S$ into pairs. This definition reduces to the ordinary definition for finite sets, but an infinite set always has even cardinality. For example, the cardinality of the natural numbers $\mathbb{N}$ is even because we can pair up even numbers with odd numbers.

This definition of "even" came up in my answer to this question, where precisely the above property turned out to be relevant.

Solution 3:

Be aware there are many different notions of infinity in mathematics, so the answer to your query will depend on the particular notion of infinity that you have in mind, and how it interacts with the operations and relations of the extended "number" system. For example, if your notion of $\infty$ satisfies $\:1 +\infty = \infty\:$ then this may yield an obstruction to extending parity arithmetic.

Here is a simple example that has some hope of being comprehensible to a 6-year-old. I will explain it in a language that is hopefully comprehensible to his 38-year-old father. Consider the ring of polynomial functions with integer coefficients, i.e $\rm\:\mathbb Z[x] = \{\: a_0 + a_1\ x\ +\:\cdots\: + a_n\: x^n\ :\ a_i \in \mathbb Z\:\}\:.\:$ If we consider these functions in a neighborhood of $\rm\:+\infty\:$ we obtain an ordered ring. Namely, define $\rm\ f(x) > g(x)\:$ if this holds true on some neighborhood $\rm\:(x_0,\:+\infty)\:$ of $\rm\:+\infty\:,\:$ i.e. if there is some $\rm\:x_0\:$ such that it holds true for all $\rm\:x > x_0\:,\:$ i.e. if it is "eventually" true. One easily checks that this is well-defined. Indeed, since polynomials have only a finite number of roots, they eventually have constant sign. Thus if $\rm\:f\ne g$ then eventually $\rm\:f-g > 0\:$ or $<0$ so eventually $\rm\:f>g\:$ or $\rm\:g>f\:$. In fact one easily deduces that this is equivalent to defining the sign of a polynomial to be the sign of its leading coefficient (the leading term eventually dominates lower-degree terms). This makes it clear that every polynomial is either positive, negative or zero, and the positive polynomials are closed under addition and multiplication (these are precisely the properties required in general to define a total order on a ring, compatible with the ring operations).

This ring $\rm\:\mathbb Z]x]\:$ has "infinite" elements, e.g. $\rm\:x > n\:$ for all integers $\rm\:n\:$ since $\rm\:x - n\:$ is eventually $> 0\:.\:$ Can we extend parity arithmetic from $\:\mathbb Z\:$ to such infinite elements? In fact we can, in two different ways. First, we can define $\rm\:x\:$ to be even. Since, by the Factor Theorem, $\rm\:f(x) = f(0) + x\ g(x)\:$ for some $\rm\:g(x)\in \mathbb Z[x]\:,\:$ this amounts to defining the parity of $\rm\:f(x)\:$ to be the parity of its constant coefficient $\rm\:f(0)\:.\:$ Alternatively we can define $\rm\:x\:$ to be odd. Again, by the Factor Theorem, we have $\rm\:f(x) = f(1) + (x-1)\ g(x)\:$ for some $\rm\:g(x)\in \mathbb Z[x]\:.\:$ Since $\rm\:x-1\:$ is even, this amounts to defining the parity of $\rm\:f(x)\:$ to be that of $\rm\:f(1)\:,\:$ i.e. the sum of its coefficients. Both definitions lead to a consistent parity arithmetic in the extension ring $\rm\:\mathbb Z[x]\:.\:$ But in general there is no compelling reason to decide which parity we should assign to the infinite element $\rm\:x\:.\:$

In contrast, there are "number" systems extending the integers where parity arithmetic has a unique extension. For example, the rational numbers (fractions) expressible with odd denominator have parity arithmetic given by defining the parity of $\rm\: m/(2\:n+1)\:$ to be the parity of $\rm\:m\:.\:$ Also the Gaussian integers $\ m + n\ i\:,\ m,n\in \mathbb Z\:,\ i = \sqrt{-1}\:,\: $ have parity arithmetic given by defining $\:i\:$ to be odd. On the other hand, there are also such number rings with no extension of parity, or with numerous possible extensions. For further discussion see my post here.

Also, as JDH mentioned, parity arithmetic extends in some sense to more exotic structures such as ordinals, which may or may not satisfy your definition of a "number". Based on a few decades teaching such concepts, I suspect that you'll have much more luck teaching a 6-year-old a concept of parity of polynomials vs. ordinals. Indeed, my experience is that many adult educated layfolks have difficulty comprehending ordinals (I've had hundreds of interactions with such adults based on my popular posts about Goodstein's Theorem, e.g. see my sci.math post of Dec 11 1995; update: now migrated here).

A nice introduction to the many different notions of infinity in mathematics is Rudy Rucker's book: Infinity and the Mind. Unlike many other popularizations, the author has expertise in the field, having completed a Ph.D. on a related topic. Moreover, Rucker has gone to great lengths to make the presentation faithful to the mathematics but still accessible to an educated layperson.

Solution 4:

JH Conway's Surreal Numbers have a well-defined notion of Omnific Integer which extends the definition of integer from finite numbers. I believe this splits infinite integers between odd and even numbers according to whether they are twice an integer or not, and such that Omnific Integers which differ by 1 always have opposite parities.

I would not recommend the theory to a 6-year-old, but Knuth's "Surreal Numbers" would be a good introduction for his father, and might give some interesting ideas on how to explore the idea of numbers with a child who is asking interesting questions.