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 August 6th, 2014, 10:08 AM #1 Senior Member   Joined: Jun 2014 From: USA Posts: 422 Thanks: 26 Messing Around - Relating Sets of Dyadics to Reals I haven't seen any new threads here for a while. As always, I promise this is Fields Medal material , but perhaps someone will find it interesting anyways. To start, in my hopeless quest to find a discrete uniform distribution over $\mathbb{N}$, I was thinking in terms of base 2 on the unit interval and came across the following: Let $D = \{d \in (0,1]: d \text{ is a dyadic rational } \}$ Let $\alpha(x) = \{ d \in D : d < x \}$ Conjecture: There exists $x,y \in (0,1]$ where $x \not= y$ and $\alpha(x) = \alpha(y)$. Can anyone give an example where this is true? I'm thinking no, though it better be true if $|\mathbb{R}_{(0,1]}| > |D|$, so the lack of a real-world example isn't going to deter me here. Assuming the conjecture is true, we can create a unique interval on (0,1] for each dyadic in $D$: Let $\beta(x) = d \rightarrow (d \in D \wedge \alpha(x) = \alpha(d))$. Each dyadic $d \in D$ then defines its own interval equal to $\{ x \in (0,1] : \beta(x) = d \}$. These intervals are interesting. Their measure would have to be 0, because the sum of the measures would equal $\infty$ otherwise, but at the same time we must assume that the set of elements in each interval has a cardinality equal to $2^{\aleph_0}$ and is continuous. Is this possible? (clearly, it is, but ) Let $z$ be a randomly selected element of (0,1] given a uniform distribution. As CRGreathouse pointed out in an earlier thread, we can do this by randomly choosing each bit $x_i$ of $0.x_1x_2x_3...$ (temporarily ignoring the patchable problem of each dyadic having two binary representations, except 1, and all other elements of (0,1] having only one representation). This is the paragraph where CRGreathouse will have to come save the day (ie... potential crankery warning)... That said, we are guaranteed to have $\beta(z)$ equal to a dyadic in (0,1]. All dyadics in $D$ had an equal probability of being equal to $\beta(z)$, having selected $z$ uniformly at random, so we must conclude that our dyadic $\beta(z)$ is selected uniformly at random as well. ... Might as well finish it. Let $\psi$ be a bijection from $D$ onto $\mathbb{N}$. Then we have a unique positive integer, $\psi(\beta(z))$, where again all positive integers had an equal chance of being equal to $\psi(\beta(z))$, so I conclude that $\psi(\beta(z))$ is also selected uniformly at random. Last edited by AplanisTophet; August 6th, 2014 at 10:16 AM.
August 6th, 2014, 10:49 AM   #2
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For those of you following along: "dyadic rational" is the name for rational numbers with a denominator (in lowest terms) which is a power of 2.

Quote:
 Originally Posted by AplanisTophet Let $D = \{d \in (0,1]: d \text{ is a dyadic rational } \}$ Let $\alpha(x) = \{ d \in D : d < x \}$ Conjecture: There exists $x,y \in (0,1]$ where $x \not= y$ and $\alpha(x) = \alpha(y)$.
The conjecture is false. There is a dyadic rational in (min(x,y), max(x,y)) unless x = y.

Quote:
 Originally Posted by AplanisTophet it better be true if $|\mathbb{R}_{(0,1]}| > |D|$
I have no idea why you think these two are related.

Quote:
 Originally Posted by AplanisTophet Let $z$ be a randomly selected element of (0,1] given a uniform distribution. As CRGreathouse pointed out in an earlier thread, we can do this by randomly choosing each bit $x_i$ of $0.x_1x_2x_3...$ (temporarily ignoring the patchable problem of each dyadic having two binary representations, except 1, and all other elements of (0,1] having only one representation).
Right... but be sure to specify that you want z to be selected uniformly at random (not just "a randomly selected element of (0, 1]" which could have some other distribution).

Quote:
 Originally Posted by AplanisTophet Assuming the conjecture is true
I skipped this part since it isn't.

August 6th, 2014, 01:56 PM   #3
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Quote:
 Originally Posted by AplanisTophet Can anyone give an example where [the conjecture from the op] is true? I'm thinking no, though it better be true if $|\mathbb{R}_{(0,1]}| > |D|$, so the lack of a real-world example isn't going to deter me here.
Quote:
 Originally Posted by CRGreathouse I have no idea why you think these two are related.
Well, let $\gamma(x) = \alpha(x) \setminus \bigcup_{y \in (0,x)} \alpha(y)$

The set $\gamma(x)$ will then contain at least one element if the conjecture is false (and $x \leq 1$). Further, the collection $\{ \gamma(x) : x \in (0,1] \}$ must be pairwise disjoint.

Let $\delta(d) = x \rightarrow d \in \gamma(x)$

Then, $\delta$ is a function from (a subset of) $D$ onto $\mathbb{R}_{(0,1]}$, and the cardinality of $D$ must be equal to or greater than the cardinality of $\mathbb{R}_{(0,1]}$.

Last edited by AplanisTophet; August 6th, 2014 at 02:17 PM.

August 7th, 2014, 05:45 AM   #4
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Quote:
 Originally Posted by AplanisTophet Well, let $\gamma(x) = \alpha(x) \setminus \bigcup_{y \in (0,x)} \alpha(y)$
Let's simplify that. $\alpha(x)=(0,x)\cap D,$ so
$$\gamma(x)=\left[(0,x)\setminus\bigcup_{y<x}(0,y)\right]\cap D=\{z:\ z<x\wedge\forall y<x,\ z>y\}\cap D=\emptyset\cap D=\emptyset$$

Quote:
 Originally Posted by AplanisTophet The set $\gamma(x)$ will then contain at least one element if the conjecture is false (and $x \leq 1$).
No, the conjecture is false and $\gamma(x)$ is uniformly empty.

August 7th, 2014, 07:55 AM   #5
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Quote:
 Originally Posted by CRGreathouse No, the conjecture is false and $\gamma(x)$ is uniformly empty.
I agree that $\gamma(x)$ is uniformly empty as implied by the op, but then I don't see how the conjecture is not true if that is the case.

Specifically, if the conjecture is false, we have $y < x \rightarrow \alpha(y) \subset \alpha(x)$. This should hold true for every possible value of $y < x$. It doesn't though, so at what point do we hit a $y$ where it doesn't?

August 7th, 2014, 08:08 AM   #6
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Quote:
 Originally Posted by AplanisTophet I agree that $\gamma(x)$ is uniformly empty as implied by the op, but then I don't see how the conjecture is not true if that is the case.
I'm sorry for your confusion, but I don't understand it.

Quote:
 Originally Posted by AplanisTophet Specifically, if the conjecture is false, we have $y < x \rightarrow \alpha(y) \subset \alpha(x)$.
I think you mean $0<y<x<1\rightarrow\alpha(y)\subsetneq\alpha(x).$ Right?

Expanding, this is $0<y<x<1\rightarrow(0,y)\cap D\subsetneq(0,x)\cap D.$ In other words, $(y,x)\cap D\ne\emptyset$ when $0<y<x<1$ ("there is some dyadic rational between y and x when y < x"). This is pretty obvious, yes? It's a standard undergraduate exercise to prove this for $\mathbb{Q}$ in place of D, and this version is no harder.

August 7th, 2014, 08:27 AM   #7
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Quote:
 Originally Posted by CRGreathouse Expanding, this is $0 It's only obvious when$y$is not thought of as approaching$x$infinitesimally. I compare this to standing in a field and declaring the world is flat based on what is observable from a point in the field. If$\gamma(x)$is empty and$y$never equals$x$, then there must exist a$y$and$x$where$\alpha(y) = \alpha(x)$. I don't see any way around this.  August 7th, 2014, 08:57 AM #8 Global Moderator Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms That I may properly understand you: you're claiming that there are real numbers$y
August 7th, 2014, 10:45 AM   #9
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Quote:
 Originally Posted by CRGreathouse That I may properly understand you: you're claiming that there are real numbers $y No. We'll have to leave that alone or we'll get stuck on this. We know that as$x$increases,$\alpha(x)$changes (grows, I'll say). Let$dx$be an infinitesimal change in$x$. Does$\alpha(x) = \alpha(x + dx)$? If$x \in D$, the answer is no, it doesn't. If$x \not\in D\$, then I believe the answer is yes.

Now what say you?

August 7th, 2014, 11:32 AM   #10
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Quote:
 Originally Posted by AplanisTophet Now what say you?
The only real infinitesimal is 0.

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