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Russell's and Frege's Definition of Number 
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#1
Mar805, 05:30 PM

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As you may know, Russell wrote "Definition of Number" inspired by Frege's earlier definition.
I have heard that he did this for a specific reason, but hours of research have proven to be hopeless. What was the confusion about definition of number that Russell needed to do this? Also, Im not so sure I truelly understand it... "The number of a class is the class of all those classes that are similar to it." thus "A number is anything which is the number of some class." Bertrand Russell (He is only talking about whole cardinal numbers here of course) I did have a spark somewhere in my brain ( ), but I wanna hear opinions of others as well. What do you think of this definition? How do you understand it? 


#2
Mar805, 06:10 PM

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I think it is utter trivial BS. I agree with Hausdorff, a real mathematician: What is of interest to us is not what numbers "are", but how they behave.
Russell's definition is like saying that a property is defined simply by collecting together all those things that have that property. E.g. he would define "green" as the set of all green things. so he defines the cardinal number "5" as the class of all those sets which admit a bijection with the set {1,2,3,4,5}. 


#3
Mar805, 07:49 PM

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I see where you are coming from. Russell and Frege are clearly essentialists.



#4
Mar805, 09:20 PM

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Russell's and Frege's Definition of Number
i suspect they are people who do not have to earn a living.
and i have verified this in the case of russell who was born a lord of some kind. i have enormous respect for russell's courageous advocacy of peace, especially during wartime, but little regard at all for his mathematical work, which does not impress me. 


#5
Mar905, 12:00 AM

P: 38

Yeah, Russell did not work to earn money.
I personally dont understand why he felt the need to publish this when it was already published by Frege?? 


#6
Mar905, 05:26 AM

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(1) Defining equivalence classes of sets. (i.e. sets are equivalent if they are the same size) (2) Selecting a single representative from each class (3) Defining a number to be that representative. Effectively, that's what it means to be a cardinal number today... but a more interesting construction is required: steps (2) and (3) in general can't be done in modern set theory. 


#7
Mar2205, 11:35 PM

P: 3

Yes, the class of classless classes...all this leads to Goedel, doesn't it?



#8
Apr1505, 05:05 PM

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#9
Apr1505, 05:14 PM

P: 47

Mathwonk, Russell may not have had to work for a living, but this in no way makes his contributions to set theory and the foundations of mathematics trivial. He may have been wrong, but his was still an important step. Also, some of us are interested as much in what an object is, as how it behaves (further, some of us believe how it behaves is very much related to what it is). To many his efforts may have seemd useless. But I am comforted somewhat by the efforts of some people to provide some rigorous foundation for the rest of the maths. Even if Gödel later showed that not all things could be proved in such a system. Frege and Russell were both brilliant men with great contributions under their belts.



#10
Apr1505, 09:16 PM

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you could be right. But in my opinion the substance of this definition of number is really due to Georg Cantor, before 1883, and hence before either Frege's 1884 paper, published when Russell was about 8 years old.
I.e. I prefer Cantors Contributions to the Founding of the Theory of Transfinite Numbers to Russells work, thats all. 


#11
Apr1505, 11:15 PM

P: 47

No, I agree, I am every bit as interested in Cantor's work with the continuum and the origin of transfinite numbers as I am with Russel's work. Cantor's work was part of what motivated me to persue pure, and not so much applied, mathematics in uni. I still consider his proof of the equivalence of the cardinality of the rationals with that of the naturals, as well as his proof of the uncountability of the continuum, as among the most intuitive and beautiful I have yet encountered. I was simply stating that Russell's work was important. Though we use the naturals, the rationals, etc. all the time, I believe some firm definition was still warranted, even though we all know pretty much what number is and how to operate on it. I liken it to seismic retrofitting of structures. Yes, the building stands as it is now, but it's nice to know it can withstand a little more severe an attack due to our work on it. I think his work towards some rigorous foundation for the naturals helped in this manner. It certainly helped Gödel arrive at his result.



#12
Jan3111, 07:22 PM

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#13
Feb111, 09:33 AM

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#14
Feb111, 02:15 PM

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As the simplest case, he defines the notion of "a set with one element" starting from axioms of logic, without making any reference to the idea of "the number one". The interesting philosophical problem is why these manmade things called numbers have any relation to the way the universe seems to behave. Personally, I have a deep suspicion that the correct answer to that is "well, actually, they don't, because the foundations of math as understood in 2011 are in no better state than the foundations of physics were in 1811". 


#15
Feb111, 07:32 PM

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#16
Feb111, 07:37 PM

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#17
Feb211, 09:40 AM

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#18
Feb211, 11:03 AM

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The definition of "X is a set with one element" is pretty simple  it is the conjunction of:
i.e. [itex]\left( \exists x: x \in X \right) \wedge \left( \forall x \in X: \forall y \in X: x = y \right)[/itex] (or some equivalent thereof) 


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