Even though you defined the number, you can't really do usual stuff with it: e.g. you can't compare it to other numbers.
IMO, these kind of numbers are no more "real" than infinitesimals: https://en.wikipedia.org/wiki/Hyperreal_number
IMO, these kind of numbers are no more "real" than infinitesimals: https://en.wikipedia.org/wiki/Hyperreal_number
The way you defined that number makes it a perfectly valid element of the set R, as described by, say, the axiomatic definition here:
https://en.wikipedia.org/wiki/Real_number#Axiomatic_approach
Whether it's easy or hard or computationally intractable to compare it to other numbers, that's a totally different question unrelated to its definition.
Plus, you can actually empirically compute a finite set of initial digits (a specific Turing machine can be analyzed to see if it terminates or not), so you can compare this number with one that's constructed by flipping its digits, or with pi, etc.
In constructive mathematics, there is a perfectly well-defined set of real numbers. The usual diagonalization proof that this set is not a countable set applies.
Either way, my point above was that this entire branch is not "mainstream math" by any means, AFAIK
I just wanted to clarify that intuitionistic and constructive mathematics don't have any problems with infinite constructions either. Finitism and ultrafinitism do, but they're not what's usually called "constructive mathematics".
There are at least three orthogonal axes which you can classify mathematical schools of thought in:
* Is the law of excluded middle accepted? ("Any statement is either true or not true.")
* Are infinite sets accepted? (They are not in finitism, but they are in constructive mathematics and of course in ordinary mathematics.)
* Can constructions implicitly refer to the result of what is being constructed? Is the powerclass of a set again a set? (Yes in ordinary mathematics and in constructive mathematics, no in predicative mathematics.)
Well, the fact that the number is not computable means that there will exist an index i, for which you will not be able to compute a_i (no matter how hard you try). In other words, you will not be able to analyze the Turing Machine i, i.e. it will not be possible to prove the termination or non-termination of the Turing Machine i.
So this specific digit will be a mystery forever, and you would not be able to compare it to anything.