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Single Idea 13460

[filed under theme 4. Formal Logic / F. Set Theory ST / 6. Ordering in Sets ]

Full Idea

A total order 'well-orders' its field just in case any nonempty subset B of its field has an R-least member, that is, there is a b in B such that for any a in B different from b, b bears R to a. So less-than well-orders natural numbers, but not integers.

Gist of Idea

'Well-ordering' must have a least member, so it does the natural numbers but not the integers

Source

William D. Hart (The Evolution of Logic [2010], 1)

Book Ref

Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.23


A Reaction

The natural numbers have a starting point, but the integers are infinite in both directions. In plain English, an order is 'well-ordered' if there is a starting point.


The 10 ideas with the same theme [ordered sets, and using sets to describe orderings]:

Order rests on 'between' and 'separation' [Russell]
Order depends on transitive asymmetrical relations [Russell]
The ordered pair <x,y> is defined as the set {{x},{x,y}}, capturing function, not meaning [Gupta]
'Well-ordering' must have a least member, so it does the natural numbers but not the integers [Hart,WD]
A 'partial ordering' is irreflexive and transitive; the sets are ordered, but not the subsets [Hart,WD]
A partial ordering becomes 'total' if any two members of its field are comparable [Hart,WD]
Von Neumann defines α<β as α∈β [Hart,WD]
'Well-ordering' of a set is an irreflexive, transitive, and binary relation with a least element [Shapiro]
A collection is 'well-ordered' if there is a least element, and all of its successors can be identified [Lavine]
Ordinals play the central role in set theory, providing the model of well-ordering [Walicki]