Is infinity an odd or even number? (2011)

It's definitely an odd number. If it were even you could divide it into 2 parts which, by definition, is not possible.

You count up from 1 in steps of 2 and never stop.

Infinity is the process of this never-ending count.

there is a difference between infinity and not a number, NaN isn't event equal to itself, in nodejs you get:

  > Infinity + Infinity
  Infinity
  > Infinity - Infinity
  NaN
  > Infinity == Infinity
  true
  > NaN == NaN
  false

The fun thing: NaN to the power of zero (because technically the type of NaN is a ... number)

  > NaN ** 0   
  1
  > typeof(NaN)
  'number'

By the same logic, NaN is also even because

  > NaN + NaN
  NaN

but i think it's better not explain that to your six year old...

I think "countably infinite" makes no sense to you because you have a different idea of countable than a mathematician (disclaimer: not a mathematician).

A mathematician compares the size of two sets of stuff by pairing off items from each set, but this is not a mechanical process taking a finite or even unbounded amount of time: they just need to show such a mapping exists or that nonexistence would lead to a contradiction; they don't need to actually carry out the process mechanically. By definition (according to mathematicians), something is countable if it is the same size as the set of natural numbers {0, 1, 2, ...} or smaller, and "countably infinite" just means it is the same size as the naturals (and not smaller, which would make it finite).

A small minority of mathematicians hold the position that proof-by-contradiction is not good enough, and that you really do need to positively prove something. They are called intuitionists.

Presumably, an even smaller minority of mathematicians hold the position that this proof must (theoretically) be able to be carried out in a mechanical manner. They're some flavor of constructivists, but maybe they're better called programmers. <- This is where you are.

I’d disagree personally. The idea that you can add things to an infinite set, or multiply an infinite set are actually useful concepts. If you imagine the universe is infinite and as such has infinite stars in it (ω) you could discuss how some infinite universes have twice as much star density as our infinite universe (ω2). Or imagine taking a copy of our universe and adding a single star 10 light years from earth (ω+1). In a very real sense the second universe would have twice as much stuff in it as the first universe, even if you can countably map the two universes to each other.

Or maybe put another way, taking the idea that infinity is just infinity makes a lot of sense when you’re primarily considering non-infinite numbers. When you’re primarily considering the concept of infinity and what you can do with it mathematically though, using systems that let you describe infinity with more nuance makes a lot of sense.

The correct answer is:

- No, even if you include all the decimals which can be individually described in any notation whatsoever.

You can disregard all the arguments in the other comments about whether "decimals" includes fractions like 1/7 using decimal repeat notation, or irrationals described by a formula like sqrt(2), or transcendentals from mathematical definitions like pi and e.

Those are interesting and deep rabbit holes, but they don't change the answer to your question, because it is still "no" with all of those. Even with all possible definitions which can be written in any symbolic language. This is because the set of all possible definitions which can be written can be enumerated systematically in a list, and mapped 1:1 to all the integers.

- Yes, if you include all the other numbers in the range 0 to 1 which are not ones you can individually describe. Most real numbers in the range 0 to1 are actually these type of "individually undescribables". But I can't point out an individual one, of course.

The "real numbers" contain these. They are present due to a consequence of logic that keeps regular math simpler and more consistent than it would be otherwise.

(Aside: The question of whether 1/3 = 0.3(repeating), times 3 = 0.9(repeating), is equal to 1 is an example of choosing the simpler and more consistent logic. Of course 1/3 times 3 is 1 so 0.9(repeating) must be defined as equal to 1, or fractions wouldn't be consistent with decimals...)

But the rationals (fractions), algebraic numbers (solutions to polynomials with integer coefficients, such as square roots), computables (numbers you can define by any algorithm), and even some types of uncomputables (such as all Chaitin's constants for all enumeration rules), and all mathematically individually definable transcendental numbers like pi/4 and e/3 do not contain these.

It follows that all the "individually undescribables" in the real numbers can only, conceptually, be imagined as infinitely long decimals with no repeats and no pattern to the digits definable by a finite-length rule in any language. You obviously can't write one of them down, you can only conceptualise what one already written down might look like. (For example a spiral of digits of ever descreasing size would fit one in finite area.) And we can only reason about them as a set by logical construction.

If you were to pick a random real number uniformly (ie. fairly) from the range 0 to 1 by picking a sequence of random decimal digits, it would ɓe one of these infinitely long decimals with probability 1. Because simple random values from a continuous range are like this, perhaps this explains why they are actually a natural and not unreasonable concept.

So the answer depends on whether your meaning of "decimal numbers" means the "real numbers" in the continuous range 0 to 1, or just certain ways of writing numbers. From the other comments, evidently some people include all sort of things in their idea of "decimals" including 1/3 = 0.333...(repeating) and the exact value of pi/4 for example, not just finite strings of digits. While other people think of "decimals" as being only strings of digits you can write down, so they would not include the exact value of pi/4 for example. These two meanings of "decimal numbers" give different answers to your question.