# Are known transcendental numbers countable?

Do all known algorithms that generate infinitely many transcendental numbers like Gelfond-Schneider or Liouville only generate countably many? If uncountably many, is this set of measure zero?

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You shouldn't write "infinite transcental numbers" if you mean infinitely many transcendental numbers. "Infinite transcendtal numbers" means transcendental numbers each one of which, by itself, is infinite. Gelfond-Schneider doesn't generate any infinite numbers. –  Michael Hardy Dec 27 '12 at 4:06
If a computer can generate the numbers (in some listed order), then it is countable. –  Thomas Eding Dec 27 '12 at 5:55
Title needs fixing, all the known things are countable, transcendental or not. –  Arjang Dec 27 '12 at 5:59

The Liouville procedure generates continuum-many. As long as the gaps between successive $1$'s grow fast enough, we get a transcendental number.

Take any Liouville-type transcendental number, with $1$'s at positions $p_n$, and $0$'s elsewhere. We can modify this in $2^\omega$ many ways, by deciding for every $n$ whether or not to shift the $1$ at position $p_n$ to position $p_n+1$.

In the above example, a very loose notion of "algorithm" was used. If you mean algorithm in a formal sense, as in a Turing machine algorithm, then there are only countably many Turing machines, and hence only countably many algorithmically specifiable numbers.

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Even in a much looser sense, there are only countably many finite strings in a given finite alphabet, so there are at most countably many numbers those strings can uniquely define (given an unambiguous way of deciding whether a string defines a unique number). –  Robert Israel Dec 27 '12 at 4:48
I guess I've been careless in my use of the term "algorithm." But if I may bother you. Your uncountable set of Liouville-type numbers is measure zero right? Can you give a similarly nifty reason why that should be so? –  user54358 Dec 28 '12 at 5:37
Yes, it is of measure $0$. Nifty reason, no, a standard calculation. Robert Israel has given something that could be called a construction of a set of transcendentals that definitely has non-zero measure. –  André Nicolas Dec 28 '12 at 6:39

Here's a "method" to generate all transcendental numbers in the interval $(0,1)$. Of course such a thing can't be an actual algorithm, it must involve countably many arbitrary choices. Let $\{a_n\}_{n=1}^\infty$ be a sequence that runs through all algebraic numbers in $(0,1)$ (such things are easy enough to generate explicitly, but for brevity I'll just assume it's given). For each $n$ choose a positive integer $m_n$ such that $m_{n+1} > m_n$. Now take any sequence of positive integers $b_n$ with the restriction that if $m_j = n$ for some $j$ and the simple continued fraction representation of $a_j$ is $1/(c_1 + 1/(c_2 + \ldots ))$ (with at least $n$ elements), the n-tuples $[c_1, \ldots, c_n]$ and $[b_1, \ldots, b_n]$ are different. Thus there is at most one possible excluded value for each $b_n$. Finally, take $x$ to be the number with continued fraction representation $1/(b_1 + 1/(b_2 + \ldots)$. It is easy to see that $x$ will be a transcendental number in $(0,1)$ and that any such $x$ can be generated for an appropriate choice of the sequences $m_n$ and $b_n$.

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Correct me if I'm wrong. This is a diagonal argument right? Can I just stick with decimal representation for my countable list of algebraic numbers and just flip the numbers digit by digit to get my transcendental number. –  user54358 Dec 28 '12 at 5:12
You know what feels funny... even if I did produce my transcendental number, the only information I have about my transcendental number is that it is not on my list of algebraic numbers. –  user54358 Dec 28 '12 at 5:28
The main reason for using continued fractions here is to avoid ambiguity with non-uniqueness of decimal representation. There are other ways to do that, of course –  Robert Israel Dec 28 '12 at 5:46
What sort of extra property do you want to your transcendental number to have? That may be possible to enforce by adding further restrictions in the process. –  Robert Israel Dec 28 '12 at 7:44