Can <1=1 < 1 = 1 ?

Does there exists intervals I1,I2, I_1, I_2, \ldots , each of which are subintervals of [0,1] [ 0,1] , such that
1. the total length of these subintervals is strictly less than 1.
2. For each rational number r[0,1] r \in [0,1] , there exists a jj such that rIj r \in I_j .

Note by Calvin Lin
5 years, 2 months ago

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We can write the set of rational numbers in [0,1][0,1] as {m/n:0mn,n1}\{m/n : 0\leq m \leq n, n \geq 1\}. Now exclude the point 11 from all these sets and enclose each rational point with denominator nn by a sub interval of length 3n3^{-n}, lying completely in [0,1][0,1] (e.g. enclose the point m/nm/n by the sub-interval [m/n,m/n+3n][m/n, m/n + 3^{-n}]). The total length of all these sub-intervals is n=1n3n=34\sum_{n=1}^{\infty} n3^{-n}=\frac{3}{4}. Finally enclose the point 11 by the sub-interval [9/10,1][9/10,1]. Hence all rational points are enclosed and total length of the sub-intervals = 3/4+1/10<13/4+1/10 <1.

Abhishek Sinha - 5 years, 2 months ago

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This was really good.

A Former Brilliant Member - 5 years, 2 months ago

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Quick note that the total length is less than 17/2017/20 because of overlapping intervals.

Jonathan Wong - 5 years, 2 months ago

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Indeed, the rationals are measure-zero and countable (despite being dense in R\mathbb{R}); they are a null set. The total length of the required subintervals can be any ϵ>0\epsilon > 0.

Jonathan Wong - 5 years, 2 months ago

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That does not prove existence of sub intervals of the required properties.

Abhishek Sinha - 5 years, 2 months ago

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The proof of my initial statement eludes me. A proof that the total length is less than ϵ\epsilon for any ϵ>0\epsilon>0 is much easier (order the rationals and construct intervals around them with lengths of the terms of a converging infinite geometric series of your choice).

If anyone is able to prove my original statement (especially constructively), by all means, post it here. The "overlaps" of the intervals are bugging me quite a bit.

Jonathan Wong - 5 years, 2 months ago

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0.999999999999999.................................=1

Amritanshu Kumar - 5 years, 1 month ago

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