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单词 NaturalNumbersAreWellordered
释义

natural numbers are well-ordered


In many proofs, one needs the following property of positive and nonnegative integers:

Theorem.  Any non-empty set of natural numbers contains a least number.

Proof.  Let A be an arbitrary non-empty subset of .  Denote

C={xxaaA}.

Then of course,  0C.  There exists surely an element c of C such that  c+1C,  since otherwise the inductionMathworldPlanetmath property would imply that  C=.  Because  c+1C,  there is a number a0 of the set A such that  a0<c+1.  On the other , we must have  ca0.  Consequently,  c=a0  and therefore

a0=caaA.

Hence, A has the least number a0.  Q.E.D.

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