March 6th, 2018, 10:29 AM  #31 
Senior Member Joined: Aug 2012 Posts: 1,780 Thanks: 482  I thought it was the OP who said that. The reals are defined first, and then limits are defined afterward. That invalidates OP's point.

March 6th, 2018, 10:43 AM  #32 
Math Team Joined: Dec 2013 From: Colombia Posts: 7,236 Thanks: 2412 Math Focus: Mainly analysis and algebra  No. The definition of limit works perfectly well with rationals. You only need real numbers for the limit itself. I believe that most places I see the definition written, no domain is specified (for $\epsilon$, which is the key quantity here).

March 6th, 2018, 10:53 AM  #33 
Senior Member Joined: Aug 2017 From: United Kingdom Posts: 128 Thanks: 39 Math Focus: Algebraic Number Theory, Arithmetic Geometry  Exactly. Not sure why you're bringing up the domain of $\epsilon$  that was never in question. The point is that the definition of a limit makes use of real numbers insofar as that the limits themselves are real numbers... In any case, a definition of real numbers referring to limits in this way is circular.
Last edited by cjem; March 6th, 2018 at 11:01 AM. 
March 6th, 2018, 11:10 AM  #34  
Senior Member Joined: Aug 2012 Posts: 1,780 Thanks: 482  Quote:
Referring back to the Wiki page on the Cauchy construction of the rationals, they define a rational Cauchy sequence as: "... for every rational ε > 0 ..." Aha! The Wiki author has thought this through correctly. To define the reals as Dedekind cuts, we don't need any epsilons, only sets of rationals. No problem. Having defined the reals that way, we can define Cauchy sequences using arbitrary epsilons. The domain of the epsilons is never important. But if we want to define the reals as equivalence classes of Cauchy sequences of rationals, we have to first carefully define Cauchy sequences using epsilons taken over the rationals. Then you can define the reals. And having done that, you can define limits of real sequences with epsilon being real. In the end none of this matters because the rationals are dense in the reals. So you get the same definition of limits whether epsilon is rational or real. But to be picky, if we are using the Cauchy sequence construction of the reals, we have to start by specifying that epsilon is rational. And Wiki got this detail right. Last edited by Maschke; March 6th, 2018 at 11:27 AM.  
March 6th, 2018, 12:52 PM  #35 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,301 Thanks: 94  Real Number
Definition: The infinite sequence of digits $\displaystyle \lim_{n \rightarrow \infty}.a_{1}a_{2}...a_{n}$ is a real number in [0,1). Two real numbers are the same iff each digit is the same. The algebraic limit of a real number exists but the real number is not unique. Unique real numbers: digital $\displaystyle \lim_{n \rightarrow \infty}$.49999.... digital $\displaystyle \lim_{n \rightarrow \infty}$.50000.... algebraic $\displaystyle \lim_{n \rightarrow \infty}$.49999....=1/2 BUT algebraic $\displaystyle \lim_{n \rightarrow \infty}$.49999....9m=1/2 for all m. Definition: algebraic $\displaystyle \lim_{n \rightarrow \infty}\frac{a_{1}}{10^{1}}+\frac{a_{2}}{10^{2}}+. ....+\frac{a_{n}}{10^{n}}$ Test: Are the real numbers .4999999.... and .50000000...... the same? Are their algebraic limits the same? It's really quite simple. 
March 6th, 2018, 03:31 PM  #36  
Senior Member Joined: Aug 2012 Posts: 1,780 Thanks: 482  Quote:
Also the phrase, "algebraic limit" is quite contradictory in this context. Analysis differs from algebra in that analysis involves limits and algebra doesn't. Now (to satisfy the pedants, of which I'm one) it's true that there are "limits" in category theory such as inverse limits and colimits, but these are not relevant to the current context. And ... I know you've been asked this before, but ... since you'd read some Rudin, and since you've tried to study the real numbers, why haven't you grokked anything? Last edited by Maschke; March 6th, 2018 at 03:36 PM.  
March 6th, 2018, 04:23 PM  #37  
Math Team Joined: Jan 2015 From: Alabama Posts: 2,970 Thanks: 807  Quote:
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Are you trying to make a distinction between "digital limit" and "algebraic limit"? If so what is that distinction. Quote:
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A series of the form a+ ab+ ab^3+ ...+ ab^n+ ... is called a "geometric series". It is fairly easy to show that such a series has sum [math]\frac{a}{1 b} as long as b is less than 1. Here, 1+ (1/10)+ (1/10)^2+ ...+ (1/10)^n+ .. has a= 1 and b= 1/10. The sum is 1/(1 1/10)= 1/(9/10)= 10/9. .09(10/9)= .1 so the entire sum is 0.4+ 0.4= 0.5= 1/2. I hope I don't have to prove that .50000... is also equa; to 1/2!  
March 7th, 2018, 05:24 AM  #38  
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,301 Thanks: 94  Quote:
The point was to distinguish between "limits," the failure to do so being the source of all the confusion.  
March 7th, 2018, 05:49 AM  #39 
Math Team Joined: Dec 2013 From: Colombia Posts: 7,236 Thanks: 2412 Math Focus: Mainly analysis and algebra  
March 10th, 2018, 04:41 PM  #40 
Math Team Joined: Dec 2013 From: Colombia Posts: 7,236 Thanks: 2412 Math Focus: Mainly analysis and algebra 
On the subject of irrational numbers not existing in the real world: Formula for $\pi$ discovered in hydrogen atoms Wikipedia article on the Wallis product $$\frac21 \frac23 \frac43 \frac45 \frac65 \frac67 \cdots = \frac\pi2$$ 

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0.999, 0.999..., 0.999...=1, arguments, fail, finitism, real numbers, usual 
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