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单词 DefinitionOfWellOrderedSetAVariant
释义

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 X is a partially ordered setMathworldPlanetmath in which every nonempty subset of X has a least member.

To justify the alternative, we prove that every partially ordered set X in which every nonempty subset has a least member is total:

let xX and yX, xy. Now, {x,y} has a least member, a fortiori, x,y are comparable. Hence, X is totally orderedPlanetmathPlanetmath.

The alternative has the benefit of being a stronger statement in the sense that

(partialorder)(totalorder)

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 TheoryMathworldPlanetmath, 3rd millennium ed., Springer, 2002.
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