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Background

In the following question, User1 asks whether an infinite sum of irrational numbers can be rational. Multiple answers1 indicate the answer to this question is 'yes'. For instance, Rasmus Erlemann notes that

$$\tan \frac{\pi}4=\sum_{n=0}^\infty \frac{(-1)^n 2^{2n+2}(2^{2n+2}-1)B_{2n+2}}{(2n+2)!}\left(\frac{\pi}4\right)^{2n+1}=1, $$ by the Maclaurin series of $\tan(\cdot)$.

The Question

This question is asked in a similar spirit. I wonder whether there are any infinite series consisting solely of non-computable terms, that amount to a computable number. Examples of such non-computable numbers can found over here. They include Chaitin's constants.

To make the question more precise, define $$ C = \sum_{k=1}^{\infty} u_{k} . $$ Here, all $u_{k}$ are non-computable numbers. I am looking for explicit examples of infinite series such that $C$ is a computable number, and all $u_{k}$ are both positive and linearly independent over $\mathbb{Q}$.

Added

Although user TonyK answers my initial question correctly, I would like to add another restriction to make the question more interesting. I furthermore require that every sum of a finite subset of summands of the infinite series is also non-computable. This makes TonyK's example unworkable, since, for instance, the sum of the first two terms is computable: $$ \alpha u_{1} + (1-\alpha)u_{1} = u_{1}, $$ where $\alpha$ is non-computable and $u_{1}$ is a positive computable term.


Notes

[1] If you're interested in such series, I encourage you to also take a look at my recent answer. It involves a class of representations of numbers, called rational zeta series, that offers a fruitful way to approach this problem. For instance, the answer includes the series identity $$\sum_{n=1}^{\infty} \left[ \zeta(2n)-1 \right] = \frac{3}{4} . $$

Max Muller
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  • Isn't such a manipulation enough to show theoretically? If we have infinitely many $u_i$'s that are both positive and linearly independent over $\mathbb{Q}$, then let's say, that they sum to an uncomputable sum:
    $$\sum_{i=1}^{\infty} u_{i}=U$$ What I think is true but it's not a proof, that the following is true, and the final result also has the required properties: $$\sum_{i=1}^{\infty} \frac{C* u_{i}}{C}=U$$ $$\frac{1}{C} \sum_{i=1}^{\infty} C* u_{i}=U$$ $$\frac{1}{U} \sum_{i=1}^{\infty} C* u_{i}=C$$ $$\sum_{i=1}^{\infty} \frac{C* u_{i}}{U}=C$$
    – Dávid Laczkó Feb 21 '24 at 13:17

1 Answers1

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Let $\alpha\in(0,1)$ be an uncomputable number, and let $\sum u_k$ be any convergent series of positive computable terms linearly independent over $\Bbb Q$. Then the series $$\alpha u_1+(1-\alpha)u_1+\alpha u_2+(1-\alpha)u_2+\cdots$$ satisfies your conditions (because any linear dependency over $\Bbb Q$ would give us a way to compute the uncomputable $\alpha$).

You asked for an explicit example, so take for instance $u_k=p_k^{-3/2}$, where $p^k$ is the $p$th prime.

TonyK
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  • Hmmm, although this answer is probably correct, it somewhat feels like a trick. I'd prefer an answer in which the terms don't cancel out the non-computable numbers. I'm not sure, however, how this restriction ought to be incorporated within the question. Do you have any suggestions? – Max Muller Feb 21 '24 at 12:28
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    I can't read your mind, Max! – TonyK Feb 21 '24 at 12:31
  • I've added a restriction. I hope you don't take offence at this -- I'm grateful for the effort you've put in your answer, and for the opportunity it has provided to formulate a more focussed question. – Max Muller Feb 21 '24 at 12:46
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    @MaxMuller It doesn't really matter whether TonyK takes offence, it's against site norms to change a question in a way that invalidates existing answers. When you have a question that allowed an answer you didn't expect, the thing to do would be to post a new question and leave your original one (unless no one's posted an answer yet). Also, based on the title alone I came to give a worse version of TonyK's answer so if you are going to leave this question changed, it would be nice to at least change the title accordingly. – Mark S. Feb 21 '24 at 14:12