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What is the measure of the set of sequences with convergent subsequences in the space of all real sequences?

Mathematics Asked by Adam Rubinson on February 13, 2021

Let $S$ be the set of all unbounded (infinite) real sequences. What is the measure of: the subset $A=$ {sequences that have a convergent subsequence}?

I have a weak understanding of the concept of ”measure”, and there are many different types of “measure”, but I guess I’m referring to probability measure or Lebesgue measure? I’m not sure, but I’m hoping it doesn’t matter. I think I’m referring to the same type of measure that people mean when they say the rationals is measure zero in $mathbb{R}$.

And I guess the answer is probably either $0$ or $1$ ? Hmm I think it is $1$. Because if I shoot countably many bullets at the real line (but with the holes (points) forming an unbounded set), it seems to me that it’s much more likely there will be a limit point than not. But maybe I am wrong

One Answer

Baker defined a "Lebesgue measure" on $mathbb{R}^infty$, the space of all real sequences which has the following characteristic property: the measure of infinite dimensional rectangle $a_i<x_i<b_i$ is the product $Pi_i(b_i-a_i)$ when it converges.

Consider the set of all sequences bounded by a constant $|x_i|<M$. Since any bounded sequence has a convergent subsequence it is contained in the OP set. Its Baker's measure is $infty$ whenever $M>1/2$ because the partial products are $(2M)^n$ and grow without bound when $ntoinfty$. Since the OP set contains these sets for every $M$ its measure is also $infty$. This should not be surprising. The condition that a sequence has a convergent subsequence does not impose any restrictions on any finite stretch of the sequence. And those stretches are enough to make the measure as large as one wants.

Answered by Conifold on February 13, 2021

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