Talk:Well-ordering principle

From Wikipedia, the free encyclopedia
Jump to: navigation, search
WikiProject Mathematics (Rated Start-class, Mid-priority)
WikiProject Mathematics
This article is within the scope of WikiProject Mathematics, a collaborative effort to improve the coverage of Mathematics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.
Mathematics rating:
Start Class
Mid Priority
 Field: Foundations, logic, and set theory

Z vs N[edit]

In the algebraic definition, I think it's usual to talk about all of Z rather than just N, because it is Z which is an integral domain, and Z which amalgamates the relevant algebraic properties. The natural numbers themselves are well-ordered in a set-theoretic (and order-theoretic) sense, of course; but the property of interest is algebraic here, no?

Will this be understood as I've left it? --VKokielov 03:42, 11 September 2006 (UTC)

If I understand you correctly, you (or perhaps Birkhoff-MacLane, which I doubt) define the phrase
R is a well-ordered integral domain
as an abbreviation for
R contains a well-ordered subset, called the natural numbers, of which any subset always contains a least element.
This does not make sense to me. First, "always contains a least element" just repeats "well-ordered". Second, I assume that by "called the natural numbers" you mean "isomorphic to the natural numbers". Third: the rational numbers also have this property (namely, that they contain the well-ordered subset of the natural numbers).
You may mean the following:
The ring of integers is characterized among the integral domains by the fact that it can be linearly ordered in such a way that
  1. the order agrees with the operations (i.e., addition and multiplication are monotone)
  2. the positive elements are well-ordered.
Aleph4 11:38, 11 September 2006 (UTC)
Right -- so I've caught myself and fixed it. --VKokielov 22:57, 11 September 2006 (UTC)
Hmm, I don't get it (the "fixed" part that is). Every version of this article since 11 September 2006 says "every well-ordered integral domain is isomorphic to the integers" which is silly, since the integers are not well ordered (and no integral domain can therefore be well-ordered, prime fields being not ordered at all). I'm throwing out the phrase; you may put something back in that makes sense. Marc van Leeuwen (talk) 14:55, 4 March 2011 (UTC)

Comment on well-ordering principle[edit]

I am writing concerning the Well-ordering principle. It is stated, at the address <http://en.wikipedia.org/wiki/Well-ordering_principle>, that the proposition:

Any nonempty subset of the Natural must contain a least element.

May have to be an axiom. I do not see why the following proof would not work in most any framework, i.e. why there is a need for an axiom?

Let S be a nonempty subset of the Naturals. Then S contains some element, say n. Consider the set {1,2,3,…,n} If 1 is in S, then it must be the least element of S, and we are done. Otherwise, if 2 is in S, then we are done. Continue in this manner, until the smallest element of S is found. Note: If none of the elements 1,2,3,…,n-1 is in S, then n must be the smallest element of S and we arrived at this conclusion in a finite number of steps.

I do recognize that I am making an assumption:

Given any nonempty set, an element of the set may be chosen.

But, this assumption (axiom) is much more benign than the Axiom of Choice.

PLEASE, COULD ANYBODY COMMENT ON THIS?

Hajdupe 19:41, 8 May 2007 (UTC)

request for a layman example[edit]

this is to enable us laymen to understand this prionciple Sanjiv swarup (talk) 13:27, 23 September 2008 (UTC)

Does every math wikipedia article start with a shit first sentence?[edit]

Well-ordering is not a property of ordered sets. The reals are ordered, but not well-ordered.

It is indeed a property of ordered sets; the reals simply do not have this property. If you dislike the wording, you are always welcome to change it. — Carl (CBM · talk) 13:55, 21 December 2011 (UTC)