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单词 BijectionBetweenClosedAndOpenInterval
释义

bijection between closed and open interval


For mapping the end points of the closed unit interval  [0, 1]  and its inner points bijectively onto the corresponding open unit interval  (0, 1),  one has to discern suitable denumerable subsets in both sets:

[0, 1]={0, 1, 1/2, 1/3, 1/4,}S,
(0, 1)={1/2, 1/3, 1/4,}S,

where

S:=[0, 1]{0, 1, 1/2, 1/3, 1/4,}.

Then the mapping f from  [0, 1]  to  (0, 1)  defined by

f(x):={1/2forx=0,1/(n+2)forx=1/n(n=1, 2, 3,),x  forxS

is apparently a bijection.  This means the equicardinality of both intervals.

Note that the bijection is neither monotonic (e.g. 012,  1214, 113) nor continuousMathworldPlanetmathPlanetmath.  Generally, there does not exist any continuous surjectivePlanetmathPlanetmath mapping [0, 1](0, 1),  since by the intermediate value theorem a continuous function maps a closed interval to a closed interval.

References

  • 1 S. Lipschutz: Set theoryMathworldPlanetmath.  Schaum Publishing Co., New York (1964).
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更新时间:2025/5/4 8:03:44