May 24th, 2015, 01:01 PM  #11 
Member Joined: Jan 2014 Posts: 86 Thanks: 4 
The partial list written is incomplete, the imagined "complete" list will include that number. The contradiction is that we postulate completeness but examine incompleteness.
Last edited by skipjack; May 29th, 2015 at 04:45 PM. 
May 24th, 2015, 01:06 PM  #12 
Senior Member Joined: Dec 2007 Posts: 687 Thanks: 47  Think a little bit more, do not draw fast conclusions. The assumption of completeness is fragile in the first example, in the second example the keystep is that the diagonal biconditional shows the list is non completable. All numbers can be represented in binary form, so this argument can be treated to show the list of Reals is not completable, since the number in the diagonal (in binary form to ease the argument) would need to be on the list, but you see that you create a new number changing 0 for 1, or 1 for 0, precisely in the (n,n) position.
Last edited by skipjack; May 29th, 2015 at 04:45 PM. 
May 24th, 2015, 01:10 PM  #13 
Senior Member Joined: Dec 2007 Posts: 687 Thanks: 47 
Another way to look at this: the decimal expansion of any real number can be seen as a infinite set of natural numbers, so the assumption that the Reals are countable is equivalent to the assumption of the power set of the naturals being countable. This I have show is not possible.

May 24th, 2015, 01:44 PM  #14 
Member Joined: Jan 2014 Posts: 86 Thanks: 4 
What does it mean for an infinite list to be completable?

May 24th, 2015, 01:47 PM  #15 
Senior Member Joined: Dec 2007 Posts: 687 Thanks: 47  
May 24th, 2015, 01:57 PM  #16 
Senior Member Joined: Dec 2007 Posts: 687 Thanks: 47 
Tau, what is the notion of a collection of objects being countable? This is the first fundamental step. Normally we use the notion: $\displaystyle S\text{ is countable iff }\forall\sigma\in S\exists f\text{ such that } f: S\mapsto\mathbb{N}\text{ is not surjective}.$ 
May 24th, 2015, 02:18 PM  #17 
Member Joined: Jan 2014 Posts: 86 Thanks: 4  That list has infinite cardinality and an infinite interval. The numbers on the number line are intervals, or differentials from zero and not points. This simple fact alone shows a contradiction between a set of numbers and their position on the number line. I can rewrite your list as sums of all rationals so the notion that it is a set only of the even numbers thus is false. Similarly an infinite set is always complete.
Last edited by Tau; May 24th, 2015 at 02:21 PM. 
May 24th, 2015, 02:19 PM  #18  
Senior Member Joined: Dec 2007 Posts: 687 Thanks: 47  Quote:
 
May 24th, 2015, 02:23 PM  #19  
Senior Member Joined: Dec 2007 Posts: 687 Thanks: 47  Quote:
 
May 24th, 2015, 02:59 PM  #20 
Member Joined: Jan 2014 Posts: 86 Thanks: 4  I've thought about it and the simple solution is that nobody is going to come for a room because everybody has one already.


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