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Re:How can some infinities be bigger than others? by paulddraper on Wednesday April 19, 2023 @11:10PM (#35635927)
Correct...except that there are the same number of points on a line as in a plane.

Aleph_0^2 = Aleph_0

Aleph_1^2 = Aleph_1

Re:How can some infinities be bigger than others? by yarg on Wednesday April 19, 2023 @11:19PM (#35635974)
How though (genuine question)?

I can see space filling curves through I^I, but I can't see them covering R^I.

How does a space-filling curve cover a plane without being filled with infinitesimal discontinuities?

Re:How can some infinities be bigger than others? by adgjlsfhk1 on Thursday April 20, 2023 @12:13AM (#35636314)
Hilbert curves
Re:How can some infinities be bigger than others? by yarg on Thursday April 20, 2023 @12:33AM (#35636444)
Yes, I've got that part - but that only gets me as far as R * I ^ (d - 1), not R ^ d.

(d being the dimensionality of the curve.)

As I think about this there's an unfilled infinitesimal R ^ (d - 1) space orthogonal to every point on the curve.

I don't see how it is that that does not manifest as discontinuities in the filled space.

Re:How can some infinities be bigger than others? by Skeime on Thursday April 20, 2023 @04:55AM (#35637741)
You can get from R^d to I^d (with I the open unit interval) easily by applying x -> (arctan(x) + pi/2)/pi and y -> tan(pi * y - pi/2) to each coordinate. There are also ways to go from R to the closed unit interval. You can then combine this with space-filling curves for I^d.
Re:How can some infinities be bigger than others? by yarg on Thursday April 20, 2023 @05:09AM (#35637805)
You can even go from R to I^d.

The problem that I'm having is in getting from R to R^d, which I still don't think is possible - look at the comment where I mention "surjections" and "bijections".

Re:How can some infinities be bigger than others? by Skeime on Thursday April 20, 2023 @01:10PM (#35643030)
By applying the map from I^d to R^d (using the tan-arctan-construction above in each coordinate) after the map from R to R^d, you get a map from R to R^d.
Re:How can some infinities be bigger than others? by yarg on Thursday April 20, 2023 @10:42PM (#35649210)
I^n is just a countably infinite subset of R^n.
Re:How can some infinities be bigger than others? by Skeime on Saturday April 22, 2023 @07:44AM (#35665549)
No, I^n is also uncountable. In fact, the usual proof for the uncountability of R is really proof of the uncountability of I, and then the natural injection into R is used to extend it to R.
Re:How can some infinities be bigger than others? by ginnungagap on Thursday April 20, 2023 @12:20AM (#35636356)
Whether |R|=aleph_1 is independent of ZFC (but of course there are bijections both between R and R^2 and between aleph_1 and aleph_1^2, or between X and X^2 for any infinite set)
Re:How can some infinities be bigger than others? by yarg on Thursday April 20, 2023 @12:35AM (#35636459)
Re:How can some infinities be bigger than others? by ginnungagap on Thursday April 20, 2023 @04:55AM (#35637742)
First of all the first link explicitely constructs (in a very strong sense) an explicit bijection R->R^2 so I don't know how why you seem to think otherwise.

It is true that there is no continuous bijection R -> R^2. It is also true that this has nothing to with cardinality, which is only about the existence of bijections (and indeed is defined for arbitrary sets with no topological structure for which continuity makes no sense)

Re:How can some infinities be bigger than others? by yarg on Thursday April 20, 2023 @05:22AM (#35637875)
I don't think that it's actually formulating a bijection across the reals - the generator function looks to me to be generating the complete set of rationals.

> Represent each real number x∈[0,1) as a sequence of DIGITS, where each DIGIT is either in {0,…,8} or is of the form 10∗(10k−1)+i with k≥1 and i∈{0,…,8} (i.e., in {90,…,98;990,…,998;9990,…,9998;…}.

He's iterating to infinity, that sure as shit sounds countable to me.

Re:How can some infinities be bigger than others? by ginnungagap on Thursday April 20, 2023 @06:11AM (#35638100)
He's associating to each x in [0,1) an infinite sequence of natural numbers (just like decimal representations do), and there's uncountably many such sequences
Re:How can some infinities be bigger than others? by yarg on Thursday April 20, 2023 @06:34AM (#35638217)
I may have misread that one, but I still think the reverse mapping only gets as far as within arbitrary bounds.
Re:How can some infinities be bigger than others? by yarg on Thursday April 20, 2023 @06:27AM (#35638188)
But he's defining the numbers with a property that makes them countably infinite?
Re:How can some infinities be bigger than others? by yarg on Thursday April 20, 2023 @06:50AM (#35638285)
The part that seems problematic is "each real number" the reals exist across bounds, I'm not sure there can be an "each".

I think his iterator is generating infinitesimals of rational and not continuous nature.

Re:How can some infinities be bigger than others? by Skeime on Thursday April 20, 2023 @04:57AM (#35637749)
Because continuity is not required here. There are non-continuous bijections between R and R^2.
Re:How can some infinities be bigger than others? by yarg on Thursday April 20, 2023 @05:12AM (#35637822)
How is continuity not required?

If there are holes in the map does it really cover the whole space?

Re:How can some infinities be bigger than others? by Skeime on Thursday April 20, 2023 @01:06PM (#35642997)
I think you have misconceptions about what continuity means and I’m not even sure what you mean with holes.

As a simpler example: The map that multiplies each rational number by -1 and keeps irrational numbers as they are is bijective (it’s its own inverse), however it is nowhere continuous.