January 11th, 2017, 04:05 PM  #121 
Senior Member Joined: Jun 2014 From: USA Posts: 363 Thanks: 26 
I didn't see your above post when I did this, only your second to most recent. In the following I focused on making what I'm saying very clear and concise. The argument is fairly simple. It starts with defining, for any rational q, the set $A_q$: $A_q$ = { x : x + q is a rational in (0, 1] } = (q, 1q] $\cap$ $\mathbb{Q}$ As an example, if q = 0, then $A_0 = (0, 1] \cap \mathbb{Q}$. As q increases, each element of $A_q$ decreases. If q = 1, we see that there is no element of $A_1 = (1, 0] \cap \mathbb{Q}$ that is greater than 0. If $0 \leq q < 1$, then $A_q$ will contain at least one rational number that is greater than 0. The following statement is therefore true: For all $q \in [0, 1) \cap \mathbb{Q}$, there exists at least one rational number in $A_q$ that is greater than 0. So, I asked the question: Is there a p such that p is in all $A_q$ such that $q \in [0, 1) \cap \mathbb{Q}$? We cannot compute what rational number p is actually in all $A_q$ such that $q \in [0, 1) \cap \mathbb{Q}$. For each computable p that we try, we’ll note that p > 1q for an infinite number of $q \in [0, 1) \cap \mathbb{Q}$. Since p cannot be an element of $A_q$ if p > 1q, we conclude that p is not an element of all $A_q$ such that $q \in [0, 1) \cap \mathbb{Q}$. This allows for another true statement: There is no computable p such that p is in all $A_q$ such that $q \in [0, 1) \cap \mathbb{Q}$. I don’t find a contradiction in recognizing that all relevant $A_q$ contain a rational greater than 0 but no computable rational greater than 0 is actually in all relevant $A_q$. This is just a consequence of the rationals being dense in the reals. … Now Maschke, here is where we appear to differ: … However, if all relevant $A_q$ contain a rational greater than 0 but no rational greater than 0 is actually in all relevant $A_q$, we do in fact have a contradiction. 
January 11th, 2017, 04:19 PM  #122  
Senior Member Joined: Sep 2015 From: USA Posts: 1,863 Thanks: 971  Quote:
$0\leq q < 1 \Rightarrow A_q \text{ contains infinite rational numbers}$ there are infinite rationals in the tiniest nondegenerate interval.  
January 11th, 2017, 04:28 PM  #123  
Senior Member Joined: Aug 2012 Posts: 1,858 Thanks: 513  Quote:
But you've already proven that there's no $p$ in the intersection of the $A_q$'s so I myself don't even have to make this point. You've already completed the conversation. There is no $p$, you proved it. Quote:
You're simply having a hard time squaring your incorrect intuition with the math that shows you're wrong. When the math shows your intuition is wrong, believe the math. Last edited by Maschke; January 11th, 2017 at 04:53 PM.  
January 11th, 2017, 08:40 PM  #124  
Senior Member Joined: Jun 2014 From: USA Posts: 363 Thanks: 26 
Ok. I’ll assume my intuition has led me astray as you say. If I do that, then I have one last question for you (sorry). I assume I'm wrong, so there is no rational p such that p is an element of every set $A_q$ such that $q \in [0, 1) \cap \mathbb{Q}$ (see the copy of my definitions below if unfamiliar). From this, we know that 1 is not an element of: B = $\{0\} \cup \{ y \in \mathbb{Q} : y > 0$ and $\exists p$ such that $p$ is an element of every set $A_q$ such that $q \in [0, y) \cap \mathbb{Q} \}$ For any rational y > 0, it is possible to test whether or not there exists a p such that p is an element of every set $A_q$ such that $q \in [0, y) \cap \mathbb{Q}$. For example, if y = 0.5, we know that 0.25 is a suitable p because 0.25 is in every set $A_q$ such that $q \in [0, 0.5)$, so 0.5 is in B. For each rational y where 0 $\leq$ y < 1, we can compute a suitable p right? I believe that means B = $[0, 1) \cap \mathbb{Q}$, does it not... or wait, but it can't? If so, then by definition, isn't this a contradiction? What is B? EXPLAIN PLEASE!!! Quote:
 
January 11th, 2017, 09:05 PM  #125 
Senior Member Joined: Aug 2012 Posts: 1,858 Thanks: 513  
January 11th, 2017, 09:06 PM  #126 
Banned Camp Joined: Dec 2012 Posts: 1,028 Thanks: 24  Again ??? Think you have a Set "A" of Apples, some Red and some Yellows, ...and now you wanna make a Set "B" including one Tomato {0}. You can do that, but no way to put in bijection the new Set "B" with "A", you agree ? 
January 11th, 2017, 10:14 PM  #127  
Senior Member Joined: Aug 2012 Posts: 1,858 Thanks: 513  Quote:
This is also true in general, even if $A$ and $B$ are uncountable sets in bijection, but the proof I gave would have to be modified to cover that case.  
January 11th, 2017, 11:06 PM  #128 
Banned Camp Joined: Dec 2012 Posts: 1,028 Thanks: 24 
In my opinion: be honest... going infimus ($\lim_{K\to\infty}1/K$ ) talk of function (or analitic extension of your Set problem), instead of Set: it differs from talking of infinite Set $\mathbb{Q}$, $\mathbb{N}$ or else (open on the right), since infimus add continuity property to the ?elements? so cannot be part of Set Theory because you cannot define the element $n$, than $n+1$, or in that case you can sometimes discard some $1/K^m$ with $m\in\mathbb{N} : m>1$ elements of the ?Set? in same operations... Set theory born when continuity die, of course your problem can (sometimes) lay on a continous function used to define the relation between Domain and CoDomain. You agree ? Note: ?xx? means impossible element or impossible set Last edited by complicatemodulus; January 11th, 2017 at 11:09 PM. 
January 12th, 2017, 02:17 PM  #129  
Senior Member Joined: Aug 2012 Posts: 1,858 Thanks: 513 
I'm going to respond to the argument you presented in #111, since it is the simplest and also the one you claimed was your final argument. It's perfectly clear and there's no reason to add anything else. Quote:
Ok. Rewriting this, and using the convention that all intervals are implicitly intersected with the rationals (just a notational convenience) you are saying that Let $A^* = \{ (q, ~1  q] : q \in [0, ~1)\}$. Agreed? Here is a picture (unfortunately not very precise) of a few of the $A_q$'s. You ask if there can be some $p \in \bigcap_{q \in [0, ~1)} A_q$. Agreed? Now it is obvious  and what's more, you have already PROVEN YOURSELF,that $0 \notin A^*$ so that $p$ can't be $0$. On the other hand if $p \in (0, ~1)$, then by choosing $1  q < p$ we have that $p \notin [0, ~1  q]$ so that $p \notin A_q$. Therefore in fact $ \bigcap_{q \in [0, ~1)} A_q = \emptyset$. Agreed? Ok. That's the math. That is literally the end of the story. However, you have some intuitions. Which I agree are not actually bad intuitions. They're fairly normal. The thing is, they happen to be wrong. You think that as $q$ gets very close to $1$, you'll "trap a single rational" in the interval $(0, ~1  q)$. Of course you can see very clearly  because you have actually worked it out for yourself  that this is not so. But you want to think there is some magic thingie, like a goat perhaps, that's infinitesimally stuck to $1$ and just to the right of it in such a way that it's in every $A_q$. Now this is a BELIEF, and an INTUITION, but you haven't got any math to show that it exists. That's because mathematically, no such goat exists. Actually I like the word barnacle because it acts like a barnacle stuck to the hull of a ship. It's inseparable from the hull but not part of it. Any larger circle around the ship would include both the hull and the barnacle. That's what you're THINKING. But thinking doesn't make it so. If you had a PROOF that such a thing existed, you'd be right that there's in contradiction in math. But you don't. You have two things: 1) A PROOF that no such barnacle exists, attached to and inseparable from and just to the right of $1$; and 2) An INTUITION that it does. When the proof contradicts the intuition, we must believe the proof. And over time, our intuitions get better! So the sooner you update your intuition, the better. What say you? And whatever it is, strive to make it clear. I did not yet look at your "one last question" in #124 yet. I only want to make sure that we are in complete agreement about the math so far. ps  Or are you trying to trap the barnacle just to the right of $0$? I may be a little unclear on your argument. You'd still be wrong, but the details of the refutation would differ. Last edited by Maschke; January 12th, 2017 at 02:37 PM.  
January 12th, 2017, 06:14 PM  #130  
Senior Member Joined: Aug 2012 Posts: 1,858 Thanks: 513  Quote:
I haven't understood this in detail yet, but I get the feeling you are showing that for every interval there's some point. But there's still no point that's in every interval. Just like every cat has paws, but there is no single paw that belongs to every cat. Is that about right? Or are you doing something different than that? In other words you're reversing quantifiers. $\forall x \exists y P(x,y)$ is not the same as $\exists y \forall x P(x,y)$. Last edited by Maschke; January 12th, 2017 at 06:45 PM.  

Tags 
fun, questions, rationals, simple 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
three simple questions on counting.  Dean1884  Algebra  6  September 25th, 2013 06:03 AM 
TWO SIMPLE QUESTIONS  CEL  Calculus  3  September 19th, 2012 02:16 AM 
Simple limits questions  msman88  Calculus  4  February 16th, 2012 11:03 PM 
Two simple trig questions  Aanders5  Algebra  1  March 26th, 2010 02:36 PM 
Simple Algebra Questions  Buddy!  Algebra  5  June 14th, 2009 10:50 AM 