Question Number 205794 by mnjuly1970 last updated on 30/Mar/24

Commented by A5T last updated on 30/Mar/24
![A=(0,1]? ⇒A^− =R\(0,1] (if A^− means complement of A)](https://www.tinkutara.com/question/Q205795.png)
Answered by TheHoneyCat last updated on 30/Mar/24
![I will assume k∈N and A^� to be the closure of A. Simply because other meanings make the question too trivial. But please, next time, define your things Let A_n :={(k/2^n )∣k∈N ∧1≤k≤2^n } A=∪_(n∈N) A_n We notice that A⊆[0,1] Let x∈[0,1[ and n∈N 0≤x<1 ⇒0≤2^n x<2^n ⇒0≤⌊2^n x⌋≤2^n x<⌊2^n x⌋+1≤2^n with ⌊•⌋ denoting the integer part. so taking k_n :=⌊2^n x⌋ for any x∈[0,1] we have 0≤2^n x−k_n <1 i.e. 0≤x−(k_n /2^n )<(1/2^n ) In other words ∀x∈[0,1] ∃(a_n )∈A^N : a_n →x which means A^� ⊇[0,1] and of course... since we saw that A⊆[0,1] A^� ⊆Closure([0,1])=[0,1] We can conclude A^� =A _□](https://www.tinkutara.com/question/Q205796.png)
Commented by A5T last updated on 30/Mar/24

Commented by mnjuly1970 last updated on 30/Mar/24

Commented by TheHoneyCat last updated on 30/Mar/24
no, k/x=0 => k=0x=0 and 0 is smaller than 1, so that's excuded...
Commented by TheHoneyCat last updated on 30/Mar/24
thanks
Commented by A5T last updated on 30/Mar/24
![A should be (0,1] instead to show the exclusion.](https://www.tinkutara.com/question/Q205802.png)
Commented by TheHoneyCat last updated on 30/Mar/24

Commented by A5T last updated on 30/Mar/24

Commented by TheHoneyCat last updated on 30/Mar/24
One way to see A is as the set of numbers in ]0,1] that have finite digits in basis 2. Saying Closure(A)=[0,1] means saying all numbers have an expression in basis two provided you allow infinite digits.
Note that if you change k/(2^n) to k/(10^n) you get the same question but in basis 10.
actually k/(b^n) gives you that in any basis b...
Commented by A5T last updated on 30/Mar/24
