Suppose that $f,g$ are rational functions with integer coefficients such that $\sum\_{n=0}^{\infty}f(n)$ and $\sum\_{n=0}^{\infty}g(n)$ both converge. Is it decidable whether
$\sum\_{n=0}^{\infty}f(n)=\sum\_{n=0}^{\infty}g(n)$? If this problem is decidable, then what is the computational complexity of this problem? If this problem is undecidable, then what is the turing degree of this problem? I am also interested in answers concerning generalizations of this problem such as when we take double sums or when we include factorial functions and exponential functions as well. This question does not have much to do with my current research. Instead, I am mainly interested in this question out of curiosity.



Are sums of sequences decidable?
It seems pretty clear that this is an open problem, so I will do the job of (trying to) put this question out of its misery by summarizing a few of the remarks in the comments. This answer is community wiki.


Replacing $f(n)$ by $f(n)-g(n)$, one may as well ask whether the sum is zero or not. 


 **Variations:** Suppose one makes the restriction that $f(n)$ is an *even* function, or alternatively (and almost equivalently) consider summations from $-\infty$ to $\infty$. Then the degree of $f(n)$ must be at most $-2$, and one has (taking $R$ through a sequence of half integers and $C\_R$ the corresponding circle centered at zero):


$$0 = \lim\_{R \rightarrow \infty} \frac{1}{2 \pi i} \oint\_{C\_R} \frac{\pi \cos \pi z}{\sin \pi z} f(z) dz =
\sum\_{-\infty}^{\infty} f(n) + \sum\_{f(\alpha) = \infty} \pi \cot(\pi \alpha) \mathrm{Res}(f)(\alpha)$$ 
Literally speaking, this is only correct if $f(z)$ has simple poles, but the general case is no more difficult. It follows that determining whether the sum is equal to zero or not is the same as verifying an identity in
$$\overline{\mathbf{Q}}(e^{\pi \beta\_1}, \ldots, e^{\pi \beta\_n}),$$
for a fixed set of algebraic numbers $\beta\_i$. By Schanuel's conjecture, the only such relationships are the "obvious" ones, so in this case one *conjecturally* has an algorithm.


**Periods:** Passing now to the general (no longer even) case but specializing in a different direction: for certain special $f(n)$, the sum in question is a period (e.g. $\zeta(n)$ for $n \ge 2$ an integer). Kontsevich and Zagier raise the question whether determining the equality of two periods is decidable or not; the expectation is that it should be, but there are no real ideas in this direction. There certainly are no known general algorithms for proving equalities. 


**General Remarks:** By expanding into partial fractions, one has the general formula:
$$\sum f(n) = \sum\_{n=0}^{\infty} \sum\_{r,s} \frac{(-1)^r \cdot r! \cdot A\_{r,s}}{(n + \alpha\_s)^{r+1}}
 = \sum\_{r,s} A\_{r,s} \cdot \psi^{(r)}(\alpha\_i),$$ 
Where $\psi^{(r)}(z)$ is the Polygamma function, so $\psi^{(0)}(z) = \psi(z)$ is the logarithmic derivative of the Gamma function. Note that this only makes sense when one has convergence, which happens if and only if $\sum A\_{0,s} = 0$ (this also means one can replace $\psi(z)$ by $\psi(z) + \gamma$ if you wish). Explicitly, for those who worry about such things, the $r=0$ term can be written (since $\sum A\_{0,s} = 0$) as:
$$\sum\_{n=0}^{\infty} \sum\_{s} \frac{A\_{0,s}}{(n + \alpha\_s)}
= \sum\_{n=0}^{\infty} \sum\_{s} \frac{A\_{0,s}}{(n + \alpha\_s)} - \frac{A\_{0,s}}{n+1}
= \sum\_{s} A\_{0,s} (\psi(\alpha\_s) - \psi(1)) = \sum\_s A\_{0,s} \psi(\alpha\_s)$$


The functions $\psi^{(r)}(z)$ can also be thought of as Hurwitz zeta functions. The most general hope would be that the only relationships between values of polygamma functions at algebraic arguments are those which are occurring for "motivic" reasons. There's not much to motivate this beyond the related explanations (which are already conjectures) in the easier cases discussed above.


