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A-k-2-n-1-k-2-n-n-N-find-A-




Question Number 205794 by mnjuly1970 last updated on 30/Mar/24
           A = { (k/2^n ) ∣ 1≤ k ≤ 2^n  , n∈N }       find .  A^( −)  = ?
A={k2n1k2n,nN}find.A=?
Commented by A5T last updated on 30/Mar/24
A=(0,1]?   ⇒A^− =R\(0,1]  (if A^−  means complement of A)
A=(0,1]?A=R(0,1](ifAmeanscomplementofA)
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  _□
IwillassumekNandA¯tobetheclosureofA.Simplybecauseothermeaningsmakethequestiontootrivial.Butplease,nexttime,defineyourthingsLetAn:={k2nkN1k2n}A=nNAnWenoticethatA[0,1]Letx[0,1[andnN0x<102nx<2n02nx2nx<2nx+12nwithdenotingtheintegerpart.sotakingkn:=2nxforanyx[0,1]wehave02nxkn<1i.e.0xkn2n<12nInotherwordsx[0,1](an)AN:anxwhichmeansA¯[0,1]andofcoursesincewesawthatA[0,1]A¯Closure([0,1])=[0,1]WecanconcludeA¯=A◻
Commented by A5T last updated on 30/Mar/24
Is there any k(1≤k≤2^n ) such that (k/2^n )=0?
Isthereanyk(1k2n)suchthatk2n=0?
Commented by mnjuly1970 last updated on 30/Mar/24
bravo sir ..
bravosir..
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.
Ashouldbe(0,1]insteadtoshowtheexclusion.
Commented by TheHoneyCat last updated on 30/Mar/24
Nope.  althoug 0∉A  ((1/2^n ))  does and that goes to 0  so 0∈A^�
Nope.althoug0A(12n)doesandthatgoesto0so0A¯
Commented by A5T last updated on 30/Mar/24
I′m talking about A specifically, not A^−
ImtalkingaboutAspecifically,notA
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
Yea.
Yea.

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