There would be many questions in stack exchange asking about the value at odd integer values, but in this question, I have done my own work but stuck at a point. I don't know many properties of the zeta function that would come to use here. Also, tell me if I am going in the wrong direction.
I had seen the well known formula(of Euler) that gives the value of $\zeta(2n)$, and I wondered if there was a formula for odd integers. I tried it today. I will skip some steps like integrals which can be evaluated easily. Here is my work:
Let $x$ and $y$ be real numbers with real part greater than or equal to $1$. So
\begin{align}
\begin{split}
\zeta(x+y)={}&\sum_{k\geq 1}\frac{1}{k^{x}k^{y}}\\
={}&\sum_{k\geq 1}\frac{1}{y+1}\int_{0}^{1}(x+r)k^{x+r-2}dr\\
={}&\sum_{k\geq 1}\int_{0}^{1}(x+r)k^{x+r+y-2}dr\\
={}&\sum_{k\geq 1}(x+r)\int_{0}^{1}k^{x+r+y-2}dr\\
={}&\int_{0}^{1}(x+r)\zeta(2-x-y-r)dr\\
\end{split}
\end{align}
Now let $x$ be an integer.
Substituting $2x$ instead of $x$ and $1$ instead of $y$ gives
\begin{align}
\begin{split}
\zeta(2x+1)={}&\int_{0}^{1}(2x+r)\zeta(1-2x-r)dr\\
={}&(x^{2}+rx)|_{r=0}^{r=1}-\int_{0}^{1}(x^{2}+rx)\zeta'(1-2x-r)dr\\
\end{split}
\end{align}
We know that
$$\zeta'(s)=\sum_{k\geq 1}\frac{-\log k}{k^s}$$
So
\begin{align}
\begin{split}
(x^{2}+rx)|_{r=0}^{r=1}-\int_{0}^{1}(x^{2}+rx)\zeta'(1-2x-r)dr={}&x+\int_{0}^{1}\sum_{k\geq 1}\frac{\log k}{k^{1-2x-r}}(x^{2}+rx)dr\\
={}&x+\sum_{k\geq 1}\log k\int_{0}^{1}\frac{x^{2}+rx}{k^{1-2x-r}}dr\\
={}&x+\sum_{k\geq 1}\frac{xk^{2x-1}((kx+k-x)\log(k)-k+1)}{\log k}
\end{split}
\end{align}
Now I don't know what to do. I don't think that there is a closed form of the sum. If there isn't any closed form of the sum, I would be satisfied with any value of the sum. Also, I want to ask did I make any mistake?
Update:I expanded the sum and it became more complicated:
$$(x+1)\zeta(-2x)-x\zeta(1-2x)-\frac{\zeta(-2x)}{2}+\frac{1}{2}\sum_{k\geq 1}\frac{k^{2x}(2x^{2}\log^{2}k-2x\log k+1)}{4x^{3}\log^{2}k}+\sum_{k\geq 1}\frac{1}{\log k}$$
Another question arises from here: I we expand the last sum(the big one, not the $\frac{1}{\log k}$ sum), a similar sum(added to some other addends related to the zeta function) comes out. What is the formula for the $n$th sum that comes out? I am not asking for the value of the $n$th sum, I am asking for the formula for $a_k$ where the $n$ sum is $\sum_{k\geq 1}a_k$.
2026-04-11 20:58:05.1775941085