How to define/What is a probability that random sequence of real numbers is convergent?

141 Views Asked by At

Inspired by Are half of all numbers odd?, idea of asymptotic density in this answer for odd numbers in set of all positive integers as $$\lim_{n \to \infty} \frac{|\{k \leq n, k \text{ is odd}\}|}{n},$$ (which is $1/2$), which can be viewed as a probability that random integer is odd (if we limit to numbers up to finite boundary $n$ and then going $n\to\infty$). In same spirit, I was wondering what is a probability that random sequence $\{a_k\}_{k=1}^\infty$ of real numbers is convergent? How do we even choose probability distribution (as in above case, it should be in some sense uniform)? Intuitively, I would expect the probability to be $0$ since there seems to be vastly more divergent sequences than convergent, but I was thinking the same about transcendental numbers, so... Also, not sure if this will be any helpful, but cardinality of convergent sequences is continuum, as this shows: What is the cardinality of the set of all sequences in $\mathbb{R}$ converging to a real number? ...

I wanted to try similar approach as above with odd integers, to that I would need to have some sequence of finite sets of sequences $A_k$ and $B_k$, where $A_k$ converges to set of all convergent sequences and $B_k$ converges to set of all sequences (and I am not even sure in what metric space that would be...), and then compute $\lim \frac{|A_k|}{|B_k|}$. Problem is that I fail to construct sequence that would satisfy even the finiteness property, not mentioning the convergence...

Is there a natural way to define probability of random sequence being convergent? And if so, what is it?