Wednesday, December 26, 2018

general topology - What is the order type of [0,1)?



My question is pretty simple, but I want to make sure I'm understanding it correctly, because otherwise my misconception could go on a while.




What is the "order type" of [0,1)?



I know the set has a smallest element, does not have a largest element, and for any two elements in the set x < y we can find another element z so that x < z < y.



Are these the properties referred to by the phrase "order type of [0,1)"? Is there anything I am missing?



This was inspired by a homework problem from Munkres #12 from section 24 needing to show that [a,c) has the same order type as [0,1) iff both [a,b) and [b,c) have the same order type as [0,1).



Thanks!


Answer




Since the open interval (0,1) is isomorphic to the real line, its order type is λ, the order type of the real line. The half-open interval [0,1) therefore has order type 1+λ; similarly, the order type of the half-open interval (0,1] is λ+1, and the order type of the closed interval [0,1] is 1+λ+1.




I know the set has a smallest element, does not have a largest element, and for any two elements in the set x < y we can find another element z so that x < z < y.

Are these the properties referred to by the phrase "order type of [0,1)"?




No. The set Q[0,1) of all rational numbers in [0,1) has all the properties you listed, but is not isomorphic to [0,1) (different cardinalities), so it does not have the same order type; its order type is 1+η.


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