
Real Analysis Real Analysis Math Forum 
 LinkBack  Thread Tools  Display Modes 
September 28th, 2015, 08:05 AM  #1 
Newbie Joined: Sep 2015 From: São Paulo, Brazil Posts: 2 Thanks: 1 
The following is an excerpt from Serge Lang's "Real and Functional Analysis". In the proof, the author defines $\left \{k...k_n \right \}$ as a subset of D. How does he know that D is big enough to contain a set of elements that can be indexed to n? I assume that, by n, the textbook means an index set with the same cardinality as the natural numbers. Last edited by skipjack; September 28th, 2015 at 09:55 AM. 
September 28th, 2015, 12:18 PM  #2 
Senior Member Joined: Aug 2012 Posts: 2,047 Thanks: 585 
n is a finite number used in the induction.

October 4th, 2015, 12:16 PM  #3 
Newbie Joined: Sep 2015 From: São Paulo, Brazil Posts: 2 Thanks: 1 
@maschke How come $\displaystyle n$ is finite if its being used in a denumeration? To denumerate a sequence means "to trace a function from $\displaystyle Z^+$ to the set of the sequence terms". $\displaystyle n$ should be at least as big as $\displaystyle Z^+$... 
October 4th, 2015, 07:55 PM  #4 
Senior Member Joined: Aug 2012 Posts: 2,047 Thanks: 585 
It's an induction. Step 1. D is infinite, therefore it's nonempty, therefore by the wellordering property of the positive integers, it has a smallest element. Call that k1. Note that Lang has left out that level of detail but it's important to realize that you need wellordering in order to know that the smallest element of D exists. Step 2. D \ {k1} is nonempty (because D is infinite and {k1} is finite) so it has a smallest element. Call it k2. Step 3. D \ {k1, k2} is nonempty (because D is infinite and {k1, k2} is finite) so it has a smallest element. Call it k3. Step 4. D \ {k1, k2, k3} is nonempty (because D is infinite and {k1, k2, k3} is finite) so it has a smallest element. Call it k4. etc. Does that make the structure of the proof more clear? It's an induction on n. You keep picking elements from D and adding them to the ksequence. At each step of the induction, {k1, k2, ..., kn} is a finite set, and D \ {k1, k2, ..., kn} is nonempty, so you can always pick k_(n+1). Last edited by Maschke; October 4th, 2015 at 08:02 PM. 
October 4th, 2015, 08:09 PM  #5 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,548 Thanks: 111 
If D is not denumerable, neither is Z+. Contradiction.

October 5th, 2015, 08:47 AM  #6 
Senior Member Joined: Aug 2012 Posts: 2,047 Thanks: 585  
October 5th, 2015, 09:29 AM  #7 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,548 Thanks: 111 
Begin enumerating Z+ by enumerating D.

October 5th, 2015, 01:01 PM  #8 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,548 Thanks: 111 
An infinite set of anything is countable by definition. How else would you know it's infinite?

October 5th, 2015, 07:38 PM  #9  
Senior Member Joined: Aug 2012 Posts: 2,047 Thanks: 585  Quote:
The theorem is proving the statement that every infinite subset of Z+ is countable. You can't assume the thing you're trying to prove. By the way I'm not familiar with this text but I know Lang's graduate algebra book. He can be terse. You need to mentally add in all the details he's skipping over. It pays to read Lang slowly. Last edited by Maschke; October 5th, 2015 at 07:45 PM.  
October 6th, 2015, 11:17 AM  #10 
Senior Member Joined: Mar 2015 From: New Jersey Posts: 1,548 Thanks: 111  Induction
Maschke, Thanks. You are clear and correct on all counts. But just for fun: Induction Principle: If Pn > (IMPLIES) Pn+1 and P1=0, all Pn true. A subset of D does not have a lowest member because I removed its lowest member. It's a postulate (well ordering). So the OP is true by postulate. But I really don't believe that. If you accept the postulate, anything you prove after that is a valid theorem. And if b is true whenever a is true, a > b even though there is no causality. Like I said, just for fun. 

Tags 
denumerability, denumerable, subset 
Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
need help on the proof of the inf on a subset!!  munjo5746  Real Analysis  2  June 7th, 2013 06:20 PM 
Subset of a Function g[a] subset g[b]  redgirl43  Applied Math  1  April 21st, 2013 06:20 AM 
Open Subset  fienefie  Real Analysis  3  October 9th, 2011 06:11 AM 
Countable Subset  Jimena29  Real Analysis  4  November 10th, 2009 02:09 PM 
Subset of S  sansar  Linear Algebra  1  March 16th, 2009 07:42 AM 