definition of well ordered set, a variant
A well-ordered set is normally defined as a totally ordered set in which every nonempty subset has a least member, as the parent object does.
It is possible to define well-ordered sets as follows:
a well-ordered set is a partially ordered set in which every nonempty subset of has a least member.
To justify the alternative, we prove that every partially ordered set in which every nonempty subset has a least member is total:
let and , . Now, has a least member, a fortiori, are comparable. Hence, is totally ordered.
The alternative has the benefit of being a stronger statement in the sense that
given that every nonempty subset has a least member.
References
- 1 Schechter, E., Handbook of Analysis and Its Foundations, 1st ed., Academic Press, 1997.
- 2 Jech, T., Set Theory
, 3rd millennium ed., Springer, 2002.